MC68000

Another great doc for the budding assembly langage programmers.
Parts 1-10 of of the 68000 Machine Language Course
Brought to you by Sewer Software

68000 MACHINE LANGUAGE COURSE PART I by Mark van de Boer

As the title already says this is the first part of an 68000
assembly language programming course. This course is intended for
people who already have a little experience with programming in
assembly language on microprocessors like the 6502 (6510 is in
fact the same) and the 68xx (like 6800, 6801, 6805, 6809) series.
In this course these two microprocessor-families will be
referenced by their most famous members, the 6502 and the 6809. At
this time it is not exactly known how many articles this course
will have. I think it will be about six articles long.
Now I will describe some features of the 68000. The 68000 is a
sixteen-bit microprocessor. This means that an opcode is always
sixteen bits (On the 6502 and 6809 an opcode is 8 bits, therefore
they are called 8-bit microprocessors). The databus of the 68000
is 16 bits wide, this means that 16 bits can be transferred at
once by the 68000 (The 6502 and 6809 both have a databus that is
eight bits wide, so they can transfer 8 bits at once).


Another important feature of the 68000 is its impressive set of
registers. First there are the eight data registers, numbered D0-
D7. They are 32 bits wide and can be used for operations on 8-bit,
16-bit and 32-bit quantities. Data registers can be compared with
the A-register (Accumulator) on the 6502 and 6809, their
function same, but the use of the data registers is much more
convenient. Second, there are the eight address registers,
numbered from A0-A7. They are 32 bits wide as well and their only
use is in addressing memory. However, the upper 8 bits are
ignored by the 68000 since its address bus is 'only' 24 bits
wide, meaning that the 68000 can address up to 16 megabytes of
memory. Register A7 has a special function; it is called the
system stackpointer. This means that if you execute a JSR
instruction, some data will be saved on the address contained in
this register. By the way, you can use every address register
very easily as a stackpointer. The third class consists of one
register, the PC (program counter). This register always contains
the address of the instruction to be executed next. Of course,
the upper eight bits of the PC are also ignored. The fourth class
consists of one 16 bit register, the status register, called SR.


This register is built up like this:

-------------------------------------------------------------
| T | | S | | I0| I1| I2| | | | X | N | Z | V | C |
-------------------------------------------------------------
system-byte | user-byte

The upper 8 bits are called the system byte. This byte contains
information that is important to the system. Normally you can not
change this byte if you run an application. Bit 15 is called the
trace-bit. If this bit is set, every time after executing an
instruction the 68000 will generate an exception (This is called
an interrupt on the 6502 and 6809). This is especially useful when
debugging programs. Bit 13 is called the supervisor bit. When this
bit is set the 68000 is in supervisor mode; when this bit is
cleared, however, the 68000 is in user mode. When executing in
supervisor mode, the 68000 can execute the so called privileged
instructions, which are not available in user mode. For example,
it is illegal trying to change the upper 8 bits of the SR when in
user mode. Bits 8, 9 and 10 are called the interrupt mask. In
total they can contain eight different values ranging from zero to
seven. For instance, if bits 8 and 10 are set and bit 9 is
cleared, the value of the interrupt mask is 5. This means that
only interrupts with a level of 5 and higher are recognized by the
68000 and interrupts with a level lower than 5 are ignored.
Interrupts of level 7 can be considered as non maskable interrupts
(compare this to the NMI on the 6502 and 6809). The lower 8 bits
are called the conditioncode register, CCR (this can be
compared to the CC of the 6502 and 6809). The CCR contains 5 bits,
which contain useful data. Bit 0 is the carry-flag (C), bit 1 is
the overflow-flag (V), bit 2 is the zero-flag (Z), bit 3 is the
negative-flag (N). The meanings of these bits are exactly the
same as on the 6502 and 6809. Then there is bit 4 which is called
the extend-flag (X). It is nearly exactly the same as the carry-
flag, but is not affected by every instruction that affects the
carry-flag. This feature of the extend-flag is especially useful
when using multiple precision arithmetic, e.g. adding 64-bit
numbers.


Another feature of the 68000 is its ability to access three data
formats: byte (8 bits), word (16 bits) and longword (32 bits). You
can indicate this with a suffix in the mnemonic field. The
suffixes are .b for byte, .w for word and .l for longword. E.g.
asr.b d0 , asr.w d0 , asr.l d0. These instructions shift
data register d0 one place to the right.
I think this is enough new stuff for today. Next time I will
explain the addressing modes of the 68000. If you have any
comments or questions on this article, please write to the
correspondence address and I'll take your notes into account.

A good Motorola MC 68000 book is:
The Motorola 68000 programming guide, which unfortunately is not
available in the stores.
Further there are a number of books on the 68000. I would like to
mention the book written by Lance Leventhal & Gerry Kane, which I
think gives good value for its money. Another good book is Steve
Williams' "Programming the 68000".



68000 ASSEMBLY LANGUAGE COURSE PART II by Mark van den Boer

Welcome to part II of this course. In part I, some fundamentals
of the 68000 were shown. In this part, I will show you which
addressing modes the 68000 uses. First an example of addressing
modes which the 6502 and 6809 use:

lda #10 * immediate addressing
lda $10 * zero-page (6502) or direct-page addressing (6809)
inx * inherent (6502)
inca * inherent (6809)

Now, what does a line of code in 68000 assembler look like?

LABEL OPCODE OPER1,OPER2 COMMENT

The meanings of these fields are:
label: A name given to this line of code. Some assemblers
require a : to follow the label-name. This field is
optional.


opcode: This field specifies the operation you wish to perform.
It is the only field that isn't optional. Depending on
the opcode the 68000 expects 0, 1 or 2 operands.
oper1: The first operand to appear with the opcode. The
appearance of this field depends on the specified
opcode.
oper2: The second operand to appear with the opcode. The
appearance of this field depends (surprise, surprise) on
the specified opcode.
comment: Another optional field which is used for commenting all
tricks people put in their programs. Most assemblers
require a * as the first character of the comment field.
The comment field is optional.

Now what does an addressing mode do? An addressing mode specifies
on which data the opcode (instruction) must operate.
The 68000 has a total of 14 addressing modes, all of which now
will explained. As examples in all addressing modes I will use
the MOVE instruction. MOVE can have the .b, .w and .l suffixes as
mentioned in part I of the course. The MOVE instruction moves the
data specified by oper1 to the place specified by oper2.

Example: MOVE.B $1,$2 performs exactly the same operation as the
following 6502 and 6809 code: LDA $1 STA $2.

The addressing modes:

1. Inherent addressing

In this addressing mode there are no operands since they are
already supplied by the opcode.
E.g.: RESET * reset all peripherals
RESET is an 68000 instruction which is used to reset all the
peripherals.

2. DATA REGISTER DIRECT ADDRESSING

Assembler syntax: Dn (n can range from 0 to 7)
In this addressing mode a data register contains the operand.
E.g.:




Instruction Before After
MOVE.B D1,D0 d0=ffffffff d0=ffffff67
d1=01234567 d1=01234567
MOVE.W D1,D0 d0=ffffffff d0=ffff4567
d1=01234567 d1=01234567
MOVE.L D1,D0 d0=ffffffff d0=01234567
d1=01234567 d1=01234567

As you might have noticed, an instruction with .b as a suffix
only changes the lowest 8 bits of the destination, and
instructions with .w as a suffix only change the lowest 16 bits
of the destination. Instructions with .l as a suffix change all
32 bits of the destination.

3. ADDRESS REGISTER DIRECT ADDRESSING

Assembler syntax: An (n can range from 0 to 7)
In this addressing mode an address register contains the operand.
Byte operators (those with .b suffix) are not allowed in this
addressing mode. When using the address register as a destination
and it is a word operation (suffix is .w), the word is sign-
extended into a longword. This means that during a wordtransfer
into a data register the upper 16 bits are filled with the value
of the most-significant bit (this is bit 15) of the word. An
example below will show you how it's done.
E.g.:

Instruction Before After
MOVE.W A1,D0 d0=ffffffff d0=ffff4567
a1=01234567 a1=01234567
MOVE.W D0,A1 d0=01234567 d0=01234567
a1=ffffffff a1=00004567 <- extend!!
MOVE.W D0,A1 d0=0000ffff d0=0000ffff
a1=00000000 a1=ffffffff <- extend!!
MOVE.L A1,D0 d0=ffffffff d0=01234567
a1=01234567 a1=01234567

4. ADDRESS REGISTER INDIRECT ADDRESSING

Assembler syntax: (An) (n between 0 and 7)
In this addressing mode, the address register contains the
address of the operand. In assembler this is being denotated by
putting parentheses around an address registers name, e.g. (a0).
The contents of a0 points to the address where the data has to be
fetched from. When using word (.w) or longword (.l) addressing it
is absolutely necessary that the address register contains an
even number (I will explain the reason for this in a forthcoming
article).
E.g.:

Instruction Before After
MOVE.L (A1),D0 d0=ffffffff d0=01234567
a1=00001000 a1=00001000
address $1000 contains 01234567
MOVE.L D0,(A1) d0=76543210 d0=76543210
a1=00001000 a1=00001000
address $1000 now contains 76543210

5. ADDRESS REGISTER INDIRECT ADDRESSING WITH POST-INCREMENT

Assembler syntax: (An)+ (n between 0 and 7)
This addressing mode resembles the address register indirect
addressing mode. The only difference is that after having fetched
or stored the data, the address register is incremented. The
increment depends on the suffix used in the opcode. If the suffix
is .b then the address register will be incremented by one. If
the suffix is .w then the address register will be incremented by
two (one word is two bytes). If the suffix is .l then the address
register will be incremented by four (one longword is four
bytes). In assembler this addressing mode is denotated by putting
the address register within parentheses followed by a + sign. For
example: (a7)+
E.g.:

Instruction Before After
MOVE.L (A1)+,D0 d0=ffffffff d0=01234567
a1=00001000 a1=00001004
address $1000 contains 01234567
MOVE.W (A1)+,D0 d0=ffffffff d0=ffff0123
a1=00001000 a1=00001002
address $1000 contains 01234567
MOVE.B (A1)+,D0 d0=ffffffff d0=ffffff01
a1=00001000 a1=00001001
address $1000 contains 01234567
MOVE.L D0,(A1)+ d0=76543210 d0=76543210
a1=00001000 a1=00001004
address $1000 now contains 76543210

6. ADDRESS REGISTER INDIRECT ADDRESSING WITH PRE-DECREMENT

Assembler syntax: -(An) (n between 0 and 7)
This addressing mode resembles the address register indirect
addressing mode. The only difference is that after before
fetching or storing the data, the address register is decre-
mented. The decrement depends on the suffix used in the opcode.
If the suffix is .b then the address register will be decremented
by one. If the suffix is .w then the address register will be
decremented by two (one word is two bytes). If the suffix is .l
then the address register will be decremented by four (one
longword is four bytes). In assembler this addressing mode is
denotated by putting the address register within parentheses
preceded by a - sign. For example: -(a7)
E.g.:







Instruction Before After
MOVE.L -(A1),D0 d0=ffffffff d0=01234567
a1=00001004 a1=00001000
address $1000 contains 01234567
MOVE.W -(A1),D0 d0=ffffffff d0=ffff4567
a1=00001004 a1=00001002
address $1000 contains 01234567
MOVE.B -(A1),D0 d0=ffffffff d0=ffffff67
a1=00001004 a1=00001003
address $1000 contains 01234567
MOVE.L D0,-(A1) d0=76543210 d0=76543210
a1=00001004 a1=00001000
address $1000 now contains 76543210

7. ADDRESS REGISTER INDIRECT ADDRESSING WITH DISPLACEMENT

Assembler syntax: w(An) (w stands for word displacement)
This addressing is also rather similar to address register
indirect addressing. The only difference lies in the fact that
before fetching or moving the data a 16-bit signed displacement
is added to the contents of the address register (the address
register itself does not change). In assembler this addressing
mode is denotated by enclosing the address register name in
parentheses preceded by a 16-bit constant. For example: 8(a6)
denotes the memory location whose address is the contents of a6
plus 8. This addressing method comes in very handy when passing
parameters to subroutines.
By the way, did you ever wonder why the ATARI ST has a resolution
of 640 by 400 pixels?
Here's one reason: 640*400=256000 bits=32000 bytes. Since 32000
bytes can be addressed using 16 bits, the address register
indirect with displacement is an easy way to address the screen.
E.g.:

Instruction Before After
MOVE.L 8(A1),D0 d0=ffffffff d0=01234567
a1=00001000 a1=00001000
address $1008 contains 01234567
MOVE.L D0,-6(A1) d0=76543210 d0=76543210
a1=00001006 a1=00001006
address $1000 now contains 76543210



8. ADDRESS REGISTER INDIRECT ADDRESSING WITH INDEX

Assembler syntax: b(An,Rn.w) or b(An,Rn.l)
( b stands for byte, w and l for word and longword and R for
register).
This addressing mode makes it possible to add a variable index
(contained in an address or data register) to an address register
and also an eight bit signed displacement. The variable index
may be either word or longword. Both the index and displacement
are sign extended before they are added to the address register.
E.g.:

Instruction Before After
MOVE.L 8(A1,A0.L),D0 d0=ffffffff d0=01234567
a1=00001000 a1=00001000
a0=00078000 a0=00078000
address $79008 contains 01234567
MOVE.L 8(A1,A0.W),D0 d0=ffffffff d0=01234567
a1=00001000 a1=00001000
a0=00078000 a0=00078000
*** a0.w=8000 -> sign-extend gives ffff8000 ***
address $ffff8008 contains 01234567
MOVE.W 8(A1,D0.L),D0 d0=0001fffe d0=00010123
a1=00001000 a1=00001000
*** 00001000 (contents of a1)
0001fffe (contents of d0.l)
00000008 (sign-extended byte displacement)
---------
00021006
address $21006 contains 01234567
MOVE.L 8(A1,D0.W),D0 d0=0001fffe d0=01234567
a1=00001000 a1=00001000
*** 00001000 (contents of a1)
fffffffe (sign-extended contents of d0.w)
00000008 (sign-extended byte displacement)
---------
00001006
address $1006 contains 01234567

9. ABSOLUTE SHORT ADDRESSING

Assembler syntax: x (x is a 16 bit constant)
With absolute short addressing it is only possible to specify a
16 bit constant. At execution time the 68000 sign extends the
word into a long address, meaning that only addresses 0 to 7fff
and ffff8000 to ffffffff can be addressed using this form. This
addressing mode can be compared with zero-page addressing on the
6502 and direct-page addressing on the 6809.Like on the 6502 and
6809 this mode is faster than any other mode. This addressing
mode can be compared with zero-page addressing on the 6502 and
direct-page addressing on the 6809.
By the way, on the Atari ST, the lower 32 K of memory can only be
accessed in supervisor-mode (the S-bit in SR is set, see part I).
E.g.:

Instruction Before After
MOVE.L $1234,D0 d0=ffffffff d0=01234567
address 1234 contains 01234567
( the $ sign is used to denote a hex digit)
MOVE.L $8000,D0 d0=ffffffff d0=76543210
address $ffff8000 contains 76543210





10. ABSOLUTE LONG ADDRESSING

Assembler syntax: l (l is 32 bit constant)
With this addressing mode a long address is supplied. It is very
similar to absolute short addressing.
E.g.:
Instruction Before After
MOVE.L $12345678,D0 d0=ffffffff d0=01234567
address $00345678 contains 01234567
Note that since the address bus is only 24 bits wide the upper
byte of the address is ignored by the 68000.

11. PROGRAM COUNTER WITH DISPLACEMENT

Assembler syntax: x(PC) (x is a 16 bit constant)
This addressing mode is in fact the same as address register
indirect with displacement. The only difference is that the
address register is replaced with the PC (the PC is in fact also
an address register).
E.g.:


Instruction Before After
MOVE.L 8(PC),D0 d0=ffffffff d0=01234567
pc=00001000 pc=00001000
address $1008 contains 01234567

12. PROGRAM COUNTER WITH INDEX

Assembler syntax: b(PC,Rn.L) or b(PC,Rn.w) (b is 8 bits)
This mode is in fact the same address register indirect
addressing with index.
E.g.:

Instruction Before After
MOVE.L 8(PC,A0.L),D0 d0=ffffffff d0=01234567
pc=00001000 pc=00001000
a0=00078000 a0=00078000
address $79008 contains 01234567
MOVE.L 8(PC,A0.W),D0 d0=ffffffff d0=01234567
pc=00001000 pc=00001000
a0=00078000 a0=00078000
*** a0.w=8000 -> sign-extend gives ffff8000 ***
address $ffff8008 contains 01234567
MOVE.W 8(PC,D0.L),D0 d0=0001fffe d0=00010123
pc=00001000 pc=00001000
*** 00001000 (contents of pc)
0001fffe (contents of d0.l)
00000008 (sign-extended byte displacement)
---------
00021006
address $21006 contains 01234567
MOVE.L 8(PC,D0.W),D0 d0=0001fffe d0=01234567
pc=00001000 pc=00001000
*** 00001000 (contents of pc)
fffffffe (sign-extended contents of d0.w)
00000008 (sign-extended byte displacement)
---------
00001006
address $1006 contains 01234567

13. IMMEDIATE ADDRESSING

Assembler syntax: #x (x is byte, word or longword)
The data for the operation is the value x. Programmers of the
6502 and 6809 will recognize this addressing mode. For example
(6502 and 6809) LDA #$21.
E.g.:

Instruction Before After
MOVE.L #$A03B4C11,D0 d0=00000000 d0=a03b4c11

14. STATUS REGISTER ADDRESSING

Assembler syntax: SR or CCR
This mode is used to control the contents of this register. See
part I of this course for the individual meanings of the bits
contained in this register. Changes to the SR can only be made
when in user-mode. Changes to the CCR can be made in any mode.
E.g.:

Instruction Before After
MOVE.W SR,D0 d0=87654321 d0=87652700
sr=2700 sr=2700
MOVE.W #$0500,SR sr=2700 sr=0500
Notice that the 68000 was in supervisor mode before executing the
instruction but after completion it is in user mode!! This
operation isn't possible the other way around.
To conclude this part, I will give you a summary of the
addressing modes of the 68000.

SYNTAX NAME
-----------------------------------
Dn | Data register direct
An | Address register direct
(An) | Address register indirect
(An)+ | Address register indirect with post-increment
-(An) | Address register indirect with pre-decrement
w(An) | Address register with displacement
b(An,Rn) | Address register with index
w | Absolute short
l | Absolute long
w(PC) | PC with displacement
b(PC,Rn) | PC with index
#x | Immediate
SR or CCR | Status register




b is a byte constant
w is a word constant
l is a long constant
x any of b, l or w
n is a register number ranging from 0 to 7
R is a register specifier, either A or D

If you have any comments on these courses, please let me know!



68000 MACHINE LANGUAGE COURSE PART III by Mark van den Boer

In part I & II the basics of programming the 68000 have been
introduced. In the following parts of this course I will discuss
the instructions of the 68000. To understand this part and the
coming ones I suggest you keep a printed copy of part II (the
addressing modes) in close range, so you can look things up.
Before discussing the individual instructions of the 68000 I will
divide its instructions into eight classes.
These classes are:
- Data movement operations.
- Integer arithmetic operations (on two's complement binary
numbers.
- Logical operations.
- Shift and rotate operations.
- Bit manipulation operations.
- Binary Coded Decimal (BCD) operations.
- Program control instructions.
- System control operations.



In this part and the following parts all classes will be
discussed. This approach is somewhat different of the one chosen
by other authors (the alphabetic order). For an explanation of
the source and destination addressing modes I refer to the table
at the end of part II.
To specify the operands of the instructions the following
conventions are used:

Rn stands for either a data- or addressregister
An stands for an address register
Dn stands for a data register
<ea> stands for an effective address operand. This means that
after the operand after evaluation must yield an address.
# stands for immediate data

All other symbols which appear in either field of the
instruction's assembler syntax will be explained.
One suggestion before the explanation starts: Since 68000
instructions may operate on bytes, words and longwords, it is
wise to always specify which size you want to manipulate. This
way will considerably reduce the amount of very hard to find bugs
in your program. Since assemblers take defaults when no size is
specified you can get very odd results. E.g. BSET (instruction
will be explained later) operates only on bytes and longwords.
The assembler makes the default for you. Suppose you want a word-
operation to be performed and you don't specify the size,
thinking that the default is nearly always word. Well in this
case the default isn't word, so the assembler thinks your code is
alright, but it doesn't do what you want.

Data movement operations

Instruction: EXG
Syntax: EXG Rn,Rn
Data sizes: Long
Condition codes affected: None
Addressing modes:
source: Dn
An
destination:
Dn
An
Function: Exchange the contents of two registers

Example:
Instruction Before After
EXG a0,d0 d0=11111111 d0=22222222
a0=22222222 a0=11111111

Instruction: LEA
Syntax: LEA <ea>,An
Data sizes: Long
Condition code affected: None
Addressing modes:
Source:
(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
Destination:
An
Function: Load an address register with a value. LEA stands
for Load Effective Address.
Example:
Instruction Before After
LEA (a1),a0 a0=00000000 a0=12345678
a1=12345678 a1=12345678
LEA 9(a0),a0 a0=00111111 a0=00111119
LEA 5(a0,d0.w) a0=10000100 a0=10001111
d0=0000100b d0=0000100b
LEA $12345678,a0 a0=00000000 a0=12345678

Instruction: LINK
Syntax: LINK An,#
Data sizes: None
Condition codes affected: None
Addressing modes:
source: An
destination:
#w





Function: Create temporary space on stack. The source
(address register) is pushed onto the stack, the
stack-pointer (a7) is copied into the source and
the destination is added to the stack-pointer. The
destination-operand is usually negative because
the 68000-stack grows downward in memory. This
instruction is used very much by high-level
languages such as C and PASCAL. This instruction
has the advantage that local variables in a
subroutine can be accessed through the source-
address register.
E.g.: Every C-compiler on the Atari St uses this
instruction at the beginning of every subroutine.
Example:
Instruction Before After
LINK a6,#-4 a6=12345678 a6=44444444
a7=44444448 a7=44444440
Let's analyse: First a6 is pushed onto the stack: a7=44444444.
Then the stack-pointer is copied into a6. Then the
destination-operand is added to a7.


Instruction: UNLK
Syntax: UNLK source
Data sizes: None
Condition codes affected: None
Addressing modes:
source: An
Function: The opposite of LINK. Also used by all C-compilers
at the end of functions.
Example:
For values see LINK
Instruction Before After
UNLK a6 a6=44444444 a6=12345678
a7=???????? a7=44444448
First a6 is copied into a7: a7=44444444. Then a6
is pulled from the stack, which yields the final
values of a6 and a7

Instruction: MOVE
Syntax: MOVE <ea>,<ea>
Data sizes: Byte, Word, Long
Condition codes affected: N,Z (conditionally set/cleared),
V,C (always cleared)
Addressing modes:
source: Dn
An
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
#x
destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Move the contents of the source to the
destination. This instruction is the most
frequently used 68000-instruction and is very
powerful. Note that the PC-relative addressing
modes are not allowed as destination-operand. This
seems illogical, but there's a good reason for it.
In the view of the designers at Motorola it is bad
practice to have self-modifying code in a program.
Example:
See part II of this course for examples on the MOVE-instruction.

Instruction: MOVE to CCR
Syntax: MOVE <ea>,CCR
Data sizes: Byte
Condition codes affected: All as a direct result of the
instruction.






Addressing modes:
source: Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
#x
Function: Set some condition codes. With this instruction you can
clear/set the N,Z,V,C and X flag.
Example: See part II

Instruction: MOVE to SR
Syntax: MOVE <ea>,SR
Data sizes: Word
Condition codes affected: All as a direct result of the
instruction. This instruction is priviliged .

Addressing modes:
source: Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
#x
Function: Set condition codes.
Example: See part II

Instruction: MOVE from SR
Syntax: MOVE SR,<ea>
Data sizes: Word
Condition codes affected: None.
Addressing modes:
source: SR

destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Request condition codes. This function is not
priviliged on the 68000 and 68008, but is priviliged on
all other members of the 68000-family.
Example: See part II

Instruction: MOVE to/from USP
Syntax: MOVE USP,An or MOVE An,USP
Data sizes: Long
Condition codes affected: None.
Addressing modes: see Syntax



Function: This instruction is priviliged. The instruction exists
because the 68000 has in fact two stack-pointers. They
are both contained in a7. This is possible because the
68000 has two modes of operation: user- and supervisor-
mode. When in usermode it is impossible to access the
supervisor stackpointer. It is rather logical that
there are in fact two stackpointers. If there had been
only one it would have been possible to mess up the
stack in such a way that the system would crash.
However, the supervisor must have the ability to access
the user-stackpointer (for parameter-passing etc.).
Example:
Instruction Before After
MOVE USP,a6 USP=12345678 USP=12345678
a6=33333333 a6=12345678

Instruction: MOVEA
Syntax: MOVEA <ea>,An
Data sizes: Word, Long
Condition codes affected: None


Addressing modes:
source: Dn
An
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
#x
destination:
An







Function: Load an address register with a value. If the operation
is word-sized then the address is sign-extended. Most
assemblers will accept MOVE <ea>,An as well. However, I
think it is better to write MOVEA instead of just MOVE.
When you intend to move a value into an address
register but you forget to specify an address register
and specify something else, the assembler will not
generate an error. E.g. you want to move $12345678 into
a0. MOVE.L #$12345678,d0 will not generate an error,
while MOVEA.L #$12345678,d0 will generate an error.
Example: See part II

Instruction: MOVEM
Syntax: MOVEM register list,<ea> or
MOVEM <ea>,register list
register list is a set of registers. Suppose you
want to specify all data-register. You could
write: d0-d7 or d0/d1/d2/d3/d4/d5/d6/d7 or d0-
d3/d4/d5-d7. A number of registers in a row can be
denotated with a minus ('-') between them. Single
registers can be specified with a slash ('/')
between them.
Data sizes: Word or Long
Condition codes affected: None
Addressing modes:
destination:
(An)
-(An)
w(An)
b(An,Rn)
w
l
source: (An)
(An)+
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)




Example:
Instruction: Before After
MOVEM.W $1000,d0/d3 $1000=$1234 d0=$1234
$1002=$5678 d1=$5678
MOVEM.L d0-d7/a0-a6,-(a7) will push all registers onto the stack.
MOVEM.L (a7)+,d0-d7/a0-a6 will pull all registers from the stack.

Instruction: MOVEP
Syntax: MOVEP Dn,w(An) or MOVEP w(An),Dn
Data sizes: Word, Long
Condition codes affected: None
Function: Transfer data from or to a peripheral. This instruction
has been specially designed for communication with all
devices which have been originally designed for 8-bit
micro-processors. This way the 68000 can communicate
with these devices. E.g. in the Atari are two ACIA's
present. ACIA's are designed for 8-bit computers. I
won't explain the operation of this instruction here
for it is rather seldom used, especially by beginners.
If you want to know all about this instruction I
suggest you contact the writer of this article.

Instruction: MOVEQ
Syntax: MOVEQ #,Dn
Data sizes: Long
Condition codes affected: See move.
Function: This instruction provides an efficient way (both in
space and time) to set a data register to a value).
Values range from -128 to +127 decimal. Most assemblers
will generate MOVEQ-instructions where possible.
Example:
Instruction Before After
MOVEQ #$34,d0 d0=ffffffff d0=00000034

Instruction: PEA
Syntax: PEA <ea>
Data sizes: Long
Condition codes affected: None






Addressing modes:
source: (An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
Function: Push an address onto the stack. This instruction is
often used to write position independent code. It has
nearly the same function as MOVE.L <ea>,-(a7).
Example:
Instruction Before After
PEA (a6) a6=12345678 a6=12345678
a7=44444444 a7=44444448

Instruction: SWAP
Syntax: SWAP Dn
Data sizes: None
Condition codes affected: V, C: always cleared
Z set if the result is 0
N set if bit 31 of the result is 1
Function: exchange upper and lower half of a data register. This
instruction is often used in conjunction with the
divide operation (will be explained further)
Example:
Instruction Before After
SWAP d0 d0=12345678 d0=56781234

Next time I will discuss the integer arithmetic operations. By
the way: I'm beginning to believe that this course is the best
there is, since I don't get any comments!!! If you read this and
react you can earn yourself a surprise.



MC 68000 MACHINE LANGUAGE COURSE PART IV by Mark van den Boer

What a pity!! You missed the mega-surprise of part 3. Next time
better luck! I am gonna take that holiday to Hawaii myself!

This time I will discuss the Integer Arithmetic Instructions. The
syntax used is of course the same as in part 3, so when in doubt
refer to part 3. This class of instructions is used to do
mathematical calculations. This group is very often used by
assembly language programmers, especially the instructions for
adding and subtracting.


Integer Arithmetic Instructions


Instruction: ADD
Syntax: ADD Dn,<ea>
ADD <ea>,Dn
Data sizes: byte, word, long


Condition codes affected:
X, C set by carry out of the most significant bit
N set if the result was negative, cleared otherwise
Z set if the result was zero, cleared otherwise
V set if the result overflowed, cleared otherwise
Addressing modes:
Source: (destination is Dn)
Dn
An
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)




Destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Add source to destination and put the result in
destination.
Examples:
Instruction Before After
ADD.W d0,d1 d0=00000011 d0=00000011
d1=0000FFFA d1=0000000B
XNZVC=00000 XNZVC=11001
ADD.L (a0),d0 d0=00000022 d0=00000027
a0=12345678 a0=12345678
12345678 contains 5
XNZVC=00000 XNZVC=00000


Instruction: ADDA
Syntax: ADDA <ea>,An
Data sizes: word, long
Condition codes affected: None
Addressing modes:
Source:
Dn
An
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
#
Destination:
An


Function: Add a value to an address register. This operation does
not change any of the condition code values. Note that
most operations that have an address register as a
destination does not change the condition codes.
Example:
Instruction Before After
ADDA.L a0,a0 a0=00000002 a0=00000004
Notice that this instruction has the same effect as multiplying
the address register with two (if this was possible).


Instruction: ADDI
Syntax: ADDI #,<ea>
This instruction has exactly the same characteristics as the ADD
instruction, except that the source can only be a constant.

Instruction: ADDQ
Syntax: ADDQ #,<ea>
Same story as for ADDI, except that the immediate values in the
source field can only range from 1 to 8. Q stands for Quick,
since this instruction is the fastest way to add a number from 1
to 8 to a destination operand.
A note on ADD, ADDI, ADDQ:
Most assemblers accept the following instruction: ADD #1,Dn
and will translate it automatically to ADDQ #1,Dn thus saving
a few bytes of object code and some clock cycles execution time.


Instruction: ADDX
Syntax: ADDX Dn,Dn
ADDX -(An),-(An)
Data sizes: byte, word, long
Condition codes affected: see ADD
Function: Add X-bit and source to destination and store the
result in destination. This instruction is used for
multiple precision operations and is therefore only
available with the two addressing modes mentioned.
Example:
Instruction Before After
ADDX.B -(a0),-(a1) a0=10001001 a0=10001000
a1=10002001 a1=10002000
10001000 contains AA the same
10002000 contains 5A 10002000 contains 4
X=0 X=1
ADDX.B -(a0),-(a1) a0=10001000 a0=10000fff
a1=10002000 a1-10001fff
10000fff contains 0 the same
10001fff contains 0 10001fff contains 1
X=1 X=0
In this example the word that begins at 10000fff is added to the
word that begins at 10001fff. If one should try to do this with
two ADD.W instruction an address error would occur since words
always must be aligned to even addresses. This instruction can be
compared to the ADC instruction of the 6502 and 6809.


Instruction: CLR
Syntax: CLR <ea>
Data sizes: byte, word, long
Condition codes affected:
N always cleared
Z always set
V always cleared
C always cleared


Addressing modes:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Set an effective address to zero. You will have noticed
that you can't CLR an address register. However, most
assemblers allow the programmer to CLR an address
register by substituting CLR a0 with SUB.L a0,a0 .
This instruction has exactly the same result.
Example:
Instruction Before After
CLR.W d0 d0=ffffffff d0=00000000
NZVC=1011 NZVC=0100




Instruction: CMP
Syntax: CMP <ea>,Dn
Data sizes: byte, word, long
Condition codes affected: NZVC (X is not affected)
Addressing modes (source):
Dn
An
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
Function: compare an effective address with a data register. In
fact all condition codes are set as if Dn-<ea> was
performed. So CMP is kind of a subtraction which only
affects the conditon codes.


Example:
Instruction Before After
CMP.L d0,d1 d0=00000001 d0=00000001
d1=00000002 d1=00000002
NZVC=1111 NZVC=0000


Instruction: CMPA
Syntax: CMPA <ea>,An
Data sizes: word, long
Function: This instruction differs only from CMP in that the
second operand is an address register and that byte
isn't allowed as a data size.


Instruction: CMPI
Syntax: CMPI #,Dn
Function: Yes, it is nearly exactly the same as compare but now
the first operand must be a constant.



Instruction: CMPM
Syntax: CMPM (An)+,(An)+
Function: Again, nearly exactly the same as CMP, but now both the
source and destination operand must be (An)+. This
instruction is used to compare areas of memory. For
those of you who have a working knowledge of C: strcmp
can be programmed easy with this instruction.

Note on all CMPx instructions.
Most assemblers accept instructions like:
CMP.W (a0)+,(a1)+
CMP.L #3,d0
Substitution of CMPM, CMPI and CMPA are automatically performed
by the assembler.


Instruction: DIVS
Syntax: DIVS <ea>,Dn
Data sizes: word



Condition codes affected:
N behaves normal; undefined on overflow
Z behaves normal; undefined on overflow
V behaves normal
C always cleared
Addressing modes (source):
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
#





Function: Guess what? This instruction performs a division
between two signed numbers. The
destination register is always a longword and the
source operand is always a word. After the division the
destination operand contains the result. The quotient
is always in the lower word and the remainder is always
in the high order word of the data register! This way a
modulo operation is also performed, you just SWAP the
data register and you have your result in the lower
word of the data register. Overflow occurs when you
attempt to divide a large number by a small number e.g.
ffffff divided by 1, the result doesn't fit in a word.
Another error occurs when attempting to divide by zero.
In this case the 68000 generates an exception and will
trap to a special routine which handles division by
zero erros. On the Atari you must set up this routine
yourself. E.g. FLOYD (a machine language monitor)
responds to a division by zero with the following
sentence "The answer is 42". Remember, don't panic when
you see such an answer.


Example:
Instruction Before After
DIVS #3,d0 d0=0000000B d0=00020003
NZVC=1111 NZVC=0000


Instruction: DIVU
Function: Nearly exactly the same as DIVS, only this time both
operands are assumed to be unsigned.


Instruction: EXT
Syntax: EXT Dn
Data sizes: word, long
Condition codes affected:
N behaves normal
Z behaves normal
V always cleared
C always cleared



Function: turn a byte into a word, or turn a word into a long.
This instruction provides a convenient way to turn a
word into a long and still have the same value for that
register. If the high order bit of the data register is
0, so the data register is positive, zeroes are padded
in, otherwise ones are padded in.
Example:
Instruction Before After
EXT.W d0 d0=000000ff d0=0000ffff
EXT.L d0 d0=ffff0000 d0=00000000


Instruction: MULS
Syntax: MULS <ea>,Dn
Data sizes: word
Condition codes affected:
N behaves normal
Z behaves normal
V always cleared
C always cleared


Addressing modes (source):
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
#
Function: Ah! another very handy instruction. This instruction
performs a multiplication of the source and destination
operand, putting the result in the destination operand.
Example:
Instruction Before After
MULS #3,d0 d0=0000000B d0=00000021
NZVC=1111 NZVC=0000



Instruction: MULU
Function: Nearly exactly the same as MULUS, only this time both
operands are assumed to be unsigned.


Instruction: NEG
Syntax: NEG <ea>
Data sizes: byte, word, long
Condition codes affected: XNZVC (all behave normal)
Addressing modes:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: negate an effective address operand. In a high level
language it would look like this: a = -a


Example:
Instruction Before After
NEG.L d0 d0=00000001 d0=ffffffff


Instruction: NEGX
Syntax: NEGX <ea>
Data sizes: byte, word, long
Condition codes affected: XNZVC (all behave normal)
Addressing modes:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: negate an effective address operand and add the X-bit
to the result. This is another instruction which
provides a way to handle multi-precision (e.g. 8-byte
integers).
Example:
Instruction Before After
NEGX.L d0 d0=00000001 d0=00000000
X=1 X=1


Instructions: SUB, SUBA, SUBI, SUBQ, SUBX
All these instruction perform subtractions. They only differ in
that way from from the ADD instructions, all other
characteristics are the same.


Instruction: TAS
Syntax: TAS <ea>
Data sizes: byte
Condition codes affected:
N evaluated before setting the byte
Z evaluated before setting the byte
V always cleared
C always cleared


Addressing modes:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: First test the operand and set the condition codes,
then set the high-order bit to 1. People who know what
semaphores (in programming of course...) are, immedia-
tely will love this instruction. For those who don't
know what semaphores are: M. Ben Ari has written a good
book on the subject called "Principles of Concurrent
Programming". Never, I repeat never, read a book on
this subject written by a certain Ir. E.H.H. Dijkstra
(not the famous Dijkstra, this Dijkstra will never be
famous).



Example:
Instruction Before After
TAS $436 $436=00 $436=80
NZVC=1111 NZVC=0100
TAS $436 $436=FF $436=FF
NZVC=1111 NZVC=1000


Instruction: TST
Syntax: TST <ea>
Data sizes: byte, word, long
Condition codes affected:
N behaves normal
Z behaves normal
V always cleared
C always cleared






Addressing modes:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: test an effective address operand. This instruction can
be seen as CMP <ea>,d0 where d0 is 0. TST is nearly
always followed by a branch instruction (more on these
later)



MACHINE LANGUAGE COURSE PART V by Mark van den Boer

Logical Operations

Instruction: AND
Syntax: AND <ea>,Dn
AND Dn,<ea>
Data sizes: byte, word, long
Condition codes affected:
X not affected
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V,C always cleared









Addressing modes:
Source: (destination is Dn)
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
Destination: (source is Dn)
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Perform a boolean bitwise AND from source to
destination.
Examples:
Instruction Before After
AND.B d0,d1 d0=33333333 d0=33333333
d1=ffffffff d1=ffffffcc
AND.W d0,d1 d0=33333333 d0=33333333
d1=ffffffff d1=ffffcccc
AND.L d0,d1 d0=33333333 d0=33333333
d1=ffffffff d1=cccccccc


Instruction: ANDI
Syntax: ANDI #,<ea>
Data sizes: byte, word, long
Condition codes affected:
X not affected
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V,C always cleared




Addressing modes:
Destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
CCR, SR
Function: Perform a boolean bitwise AND from source to
destination.
Examples:
Instruction Before After
ANDI.B #7,d0 d0=33333333 d0=33333330
ANDI.W #$ff00,SR SR=ffff SR=ff00





Instruction: EOR
Syntax: EOR Dn,<ea>
Data sizes: byte, word, long
Condition codes affected:
X not affected
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V,C always cleared
Addressing modes:
Destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l




Function: Perform a bitwise exclusive or from source to destina-
tion. You will notice that this instruction can only
take a data register as the source, unlike the AND and
OR instructions. I think the designers of the 68000
should have mode addressing modes more orthogonal.
Examples:
Instruction Before After
EOR.L d0,d1 d0=ffffffff d0=ffffffff
d1=f0f0f0f0 d1=0f0f0f0f


Instruction: EORI
Syntax: EORI #,<ea>
Data sizes: byte, word, long
Condition codes affected:
X not affected
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V,C always cleared



Addressing modes:
Destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
CCR, SR
Function: Perform a bitwise exclusive or from source to
destination.
Examples:
Instruction Before After
EORI.B #$ff,d0 d0=33333330 d0=333333cf
EORI.W #$00ff,SR SR=ff32 SR=ffcd





Instruction: NOT
Syntax: NOT <ea>
Data sizes: byte, word, long
Condition codes affected:
X not affected
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V,C always cleared
Addressing modes:
Destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Perform a bitwise not to the effective address operand.
Note that NOT.W d0 is the same as EORI.W #$ffff,d0.


Examples:
Instruction Before After
NOT.B #$ff,d0 d0=ffffff78 d0=ffffff87


Instruction: OR
Syntax: OR <ea>,Dn
OR Dn,<ea>
Data sizes: byte, word, long
Condition codes affected:
X not affected
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V,C always cleared








Addressing modes:
Source: (destination is Dn)
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
Destination: (source is Dn)
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Perform a boolean bitwise OR from source to destination.

Examples:
Instruction Before After
OR.B d0,d1 d0=ffffffff d0=ffffffff
d1=33333333 d1=333333ff
OR.W d0,d1 d0=ffffffff d0=ffffffff
d1=33333333 d1=3333ffff
OR.L d0,d1 d0=ffffffff d0=ffffffff
d1=33333333 d1=ffffffff


Instruction: ORI
Syntax: ORI #,<ea>
Data sizes: byte, word, long
Condition codes affected:
X not affected
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V,C always cleared




Addressing modes:
Destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
CCR, SR
Function: Perform a boolean bitwise OR from source to destina-
tion.
Examples:
Instruction Before After
ORI.B #7,d0 d0=33333330 d0=33333333
ORI.W #$00ff,SR SR=ff00 SR=ffff






ASSEMBLY LANGUAGE COURSE PART VI by Mark van den Boer

Shift and Rotate Operations

Instruction: ASL
Syntax: ASL #,Dn (the immediate operand always modulo 8)
ASL Dn,Dn (the first operand always modulo 8)
ASL <ea>
Data sizes: byte, word, long except for ASL <ea> which only
allows word and long as data sizes.
Condition codes affected:
X set to the last bit shifted out
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V set if the most significant bit is changed
during the operation
C see the X-bit





Addressing modes allowed with the ASL <ea> instruction:
Destination:
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Perform a shift left of the destination operand. This
instruction can be used as a fast form of multiplying an
operand with a power of two. On a processor like the
6502 this instruction is the only way to perform a
multiply operation. The lower bit of the destination is
always set to zero.
Examples:
Instruction Before After
ASL.L d0,d1 d0=33333333 d0=33333333
d1=00000005 d1=00000028
ASL.W $4ee $4ee=0009 $4ee=0012


Instruction: ASR
Syntax: ASR #,Dn (the immediate operand always modulo 8)
ASR Dn,Dn (the first operand always modulo 8)
ASR <ea>
Data sizes: byte, word, long except for ASR <ea> which only
allows word and long as data sizes.
Condition codes affected:
X set to the last bit shifted out
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V set if the most significant bit is changed
during the operation
C see the X-bit
Addressing modes allowed with the ASR <ea> instruction:
Destination:
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Perform a shift right of the destination operand. This
instruction can be used as a fast form of dividing an
operand with a power of two. On a processor like the
6502 this instruction is the only way to perform a
divide operation. The upper bit (sign bit) is always
repeated.
Examples:
Instruction Before After
ASR.L d0,d1 d0=33333333 d0=33333333
d1=00000005 d1=00000002
ASR.W $4ee $4ee=8009 $4ee=c004


Instruction: LSL
See the ASL instruction. The LSL instruction is exactly the same.
At the moment I haven't got the machine codes for the ASL and LSL
operations but I think that even the machine codes are the same.
E.g. on the 6809 both ASL and LSL exist but translate to the same
machine code.



Instruction: LSR
Syntax: LSR #,Dn (the immediate operand always modulo 8)
LSR Dn,Dn (the first operand always modulo 8)
LSR <ea>
Data sizes: byte, word, long except for LSR <ea> which only
allows word and long as data sizes.
Condition codes affected:
X set to the last bit shifted out
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V set if the most significant bit is changed
during the operation
C see the X-bit
Addressing modes allowed with the LSR <ea> instruction:
Destination:
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Perform a shift right of the destination operand. This
instruction differs from ASR in that the high order bit
is always cleared.
Examples:
Instruction Before After
LSR.L d0,d1 d0=33333333 d0=33333333
d1=00000005 d1=00000002
LSR.W $4ee $4ee=0009 $4ee=0004


Instruction: ROL
Syntax: ROL #,Dn (the immediate operand always modulo 8)
ROL Dn,Dn (the first operand always modulo 8)
ROL <ea>
Data sizes: byte, word, long
Condition codes affected:
X not affected
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V always cleared
C set to the last bit shifted out the operand

Addressing modes allowed with the ROL <ea> instruction:
Destination:
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Perform a bitwise rotate left of the destination
operand.
Examples:
Instruction Before After
ROL.L d0,d1 d0=00000001 d0=00000001
d1=88000001 d1=10000002 (C bit set)
ROL.W $4ee $4ee=8009 $4ee=0012






Instruction: ROR
Syntax: ROR #,Dn (the immediate operand always modulo 8)
ROR Dn,Dn (the first operand always modulo 8)
ROR <ea>
Data sizes: byte, word, long
Condition codes affected:
X not affected
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V always cleared
C set to the last bit shifted out the operand
Addressing modes allowed with the ROR <ea> instruction:
Destination:
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Perform a bitwise rotate right of the destination
operand.
Examples:
Instruction Before After
ROR.L d0,d1 d0=00000001 d0=00000001
d1=88000001 d1=c4000000 (C bit set)
ROR.W $4ee $4ee=8009 $4ee=c004


Instruction: ROXL
Syntax: ROXL #,Dn (the immediate operand always modulo 8)
ROXL Dn,Dn (the first operand always modulo 8)
ROXL <ea>
Data sizes: byte, word, long
Condition codes affected:
X set to the last bit shifted out the operand
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V always cleared
C set to the last bit shifted out the operand




Addressing modes allowed with the ROXL <ea> instruction:
Destination:
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Function: Perform a bitwise rotate left of the destination
operand. There is very little difference with the ROL
instruction. By the way, it is very handy to have a
wordprocessor with cut/paste and find/replace
facilities. All I did was cut out the complete ROL
instruction and replaced all ROL's by ROXL's.
Examples:
Instruction Before After
ROXL.L d0,d1 d0=00000001 d0=00000001
d1=88000001 d1=10000002
ROXL.W $4ee $4ee=8009 $4ee=0012


Instruction: ROXR
Syntax: ROXR #,Dn (the immediate operand always modulo 8)
ROXR Dn,Dn (the first operand always modulo 8)
ROXR <ea>
Data sizes: byte, word, long
Condition codes affected:
X set to the last bit shifted out the operand
N set to the most significant bit of the result
Z set if the result is zero, cleared otherwise
V always cleared
C set to the last bit shifted out the operand
Addressing modes allowed with the ROXR <ea> instruction:
Destination:
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l


Function: Perform a bitwise rotate right of the destination
operand. There is very little difference with the ROR
instruction. By the way, it is very handy to have a
wordprocessor with cut/paste and find/replace
facilities. All I did was cut out the complete ROXL
instruction and replaced all ROXL's by ROXR's.
Examples:
Instruction Before After
ROXR.L d0,d1 d0=00000001 d0=00000001
d1=88000001 d1=10000002
ROXR.W $4ee $4ee=8009 $4ee=0012



MC 68000 MACHINE LANGUAGE COURSE PART VII by Mark van den Boer

I would like to dedicate this part to Willeke, who gives Richard
sleepless nights and the inspiration to write even more exciting
issues of ST NEWS. I only saw Willeke on photograph, but she must
be a fine girl. In my opinion there are three qualities which a
girl must have, to qualify as a fine girl. These are: 1) like
Queensrche, 2) like ST NEWS (no, she doesn't have to like this
particular machine language course).
Now, you're all anxious to know the third quality, aren't you? If
you think you know the third one, send your answer to ST NEWS. A
bottle of wine will be raffled among the persons who gave the
right answer. There will be another bottle for the most original
answer!

Bit Manipulation instructions

Instruction: BTST
Syntax: BTST Dn,<ea> or BTST #,<ea>
Data sizes: only byte when <ea> is an address. Only long when
<ea> is a data register.

Condition codes affected:
X not affected
N not affected
Z set if the result is zero, cleared otherwise
V not affected
C not affected
Addressing modes allowed:
Destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
# (only when source is Dn)



Function: Test a single bit of an effective address operand. Bits
are numbered from 0 to 31, where 0 is the least
significant bit (you could use this instruction to test
if a number is odd). This instruction is useful when
specific bits of an operand have to be checked. E.g.
when reading joystick information one could test with a
single instruction whether the fire-button was pressed
or not. Compared to the 6502 and 6809 this instruction
(in fact all bit manipulation instructions) are a step
forward, since with these older processors one had to
put the data in a register first, then filter the bit
with an AND-operation and then the Z-bit in the status
register was at last set. Viva el 68000!! Since this
instruction has the rather odd property of only working
on byte and long operands it is important that you
remember what I wrote in a previous part about
specifying data sizes.





Examples:
Instruction Before After
BTST.B #5,$345678 $345678 $345678 contains
contains $78 $78
Z-bit is 1
BTST.L d0,d1 d0=0 d0=0
d1=$12345678 d1=$12345678
Z-bit is 0


Instruction: BCLR
Syntax: BTST Dn,<ea> or BTST #,<ea>
Data sizes: only byte when <ea> is an address. Only long when
<ea> is a data register.
Condition codes affected:
X not affected
N not affected
Z set if the result is zero, cleared otherwise
V not affected
C not affected


Addressing modes allowed:
Destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
# (only when source is Dn)
Function: Bit test and CLeaR. First tests the bit to be cleared
and sets the Z-bit accordingly, then clears the
specified bit.






Examples:
Instruction Before After
BCLR.B #5,$345678 $345678 $345678 contains
contains $78 $58
Z-bit is 1
BCLR.L d0,d1 d0=0 d0=0
d1=$12345678 d1=$12345678
Z-bit is 0


Instruction: BSET
Syntax: BSET Dn,<ea> or BSET #,<ea>
Data sizes: only byte when <ea> is an address. Only long when
<ea> is a data register.
Condition codes affected:
X not affected
N not affected
Z set if the result is zero, cleared otherwise
V not affected
C not affected


Addressing modes allowed:
Destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
# (only when source is Dn)
Function: Bit test and SET. First tests the bit to be set and sets
the Z-bit accordingly, then sets the specified bit. This
instruction and the BCLR instruction can be used as
alternatives to the TAS-instruction.





Examples:
Instruction Before After
BSET.B #5,$345678 $345678 $345678 contains
contains $78 $78
Z-bit is 1
BSET.L d0,d1 d0=0 d0=0
d1=$12345678 d1=$12345679
Z-bit is 0


Instruction: BCHG
Syntax: BCHG Dn,<ea> or BCHG #,<ea>
Data sizes: only byte when <ea> is an address. Only long when
<ea> is a data register.
Condition codes affected:
X not affected
N not affected
Z set if the result is zero, cleared otherwise
V not affected
C not affected


Addressing modes allowed:
Destination:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
# (only when source is Dn)
Function: Bit test and CHanGe. First tests the bit to be changed
and sets the Z-bit accordingly, then changes the
specified bit.






Examples:
Instruction Before After
BCHG.B #5,$345678 $345678 $345678 contains
contains $78 $58
Z-bit is 1
BCHG.L d0,d1 d0=0 d0=0
d1=$12345678 d1=$12345679
Z-bit is 0


Binary Coded Decimal (BCD) instructions

To understand this instructionclass we must first know what a BCD-
digit is. It is a representation of decimal digits in an array of
bytes (array may be of length 1 or greater). In every byte the
decimal number 0 to 99 can be represented. This is done as
follows: a byte can be divided into two four-bit parts, called
nibbles. In every nibble, one decimal digit is represented. This
implicates that the binary combination 1010 can never occur in BCD
representation, since it isn't in the decimal range from 0 to 9.


The BCD-representation is especially convenient when printing such
a digit, since it doesn't take much calculation to convert it to a
printable character. A disadvantage of the BCD-representation is
that one doesn't use the full storage capacity of a byte or word.
The 68000 has three special BCD-artithmetic instructions.


Instruction: ABCD
Syntax: ABCD Dn,Dn or ABCD -(An),-(An)
Data sizes: byte
Condition codes affected:
X set by carry out of most significant
BCD-nibble, cleared otherwise
N undefined
Z set if the result is zero, cleared otherwise
V undefined
C same as X-bit
Function: Add two BCD-digits. The predecremeting addressing mode
has been provided for computations with multiple
precision BCD-numbers. This implies that the most
significant BCD-numbers must be stored in the lower
memory addresses.
Examples:
Instruction Before After
ABCD.B d0,d1 d0=$53 d0=$53
d1=$32 d1=$85


Instruction: SBCD
Syntax: SBCD Dn,Dn or SBCD -(An),-(An)
Data sizes: byte
Condition codes affected:
X set by carry out of most significant
BCD-nibble, cleared otherwise
N undefined
Z set if the result is zero, cleared otherwise
V undefined
C same as X-bit
Function: Subtract two BCD-digits.
Examples:
Instruction Before After
ABCD.B d0,d1 d0=$53 d0=$53
d1=$32 d1=$21

Instruction: NBCD
Syntax: NBCD <ea>
Data sizes: byte
Addressing modes allowed:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
Condition codes affected:
X set by borrow out of most significant
BCD-nibble, cleared otherwise
N undefined
Z set if the result is zero, cleared otherwise
V undefined
C same as X-bit



Function: Negate a BCD-number. How it functions can be best
described with an example. Let's negate $23. The NBCD
operation yields $77. Now, how did we get this result?
It's easy, just subtract $23 from $99 and you've got it.
Examples:
Instruction Before After
NBCD.B d0 d0=$43 d0=$56


This is the end of part seven. Next time I will deal with all
program flow instruction, such as branches and jumps.



MC 68000 ASSEMBLY LANGUAGE COURSE PART VIII by Mark van den Boer

Program Control Instructions

This class of instructions enables a programmer to create loops
and IF-THEN-ELSE like decisions. That's why it's the most
important group of instructions and every programmer should have
a thorough knowledge of this group. This class of instructions
are specifically meant to affect the program counter.

Instructions: Bcc (cc stands for Condition Code)
Syntax: Bcc <address>
Data sizes: Byte or word. This implicates that the branch-
instructions can branch in an area of 32K. When
using a branch with a byte offset you can put a .S
suffix behind the instruction e.g. BEQ.S . When
using a branch with a word offset you can put a .W
suffix behind the instruction e.g. BEQ.W . Most
assemblers will determine if the short or word
form is needed. Also most assemblers will optimize
word-branches to byte-branches whenever possible.

Condition codes affected:
None
Function: Test a combination of the NZVC-flags in the status-
register and conditionally perform a branch to another
address. If the testing of the condition codes is true,
then the branch will be taken, in the other the
instruction immediately following the Bcc instruction
will be executed. A total of 15 possible variations of
this instruction are listed below.
BCC: where CC stands for Carry Clear. The branch is
taken if the C-bit is 0. This instruction is often
used in combination with shift and rotate
instructions.
BCS: where CS stands for Carry Set. The branch is taken
if the C-bit is 1. This instruction is the
counterpart of the BCC-instruction.
BEQ: where EQ stand for EQual. The branch is taken if
the Z-bit is 1. This instruction is often used
after a TST-instruction or CMP-instruction.
BNE: where NE stands for Not Equal. The branch is taken
if the Z-bit is 0. This instruction is the
counterpart of the BEQ-instruction.
BPL: where PL stands for PLus. The branch is taken if
the N-bit is 0. This instruction is often used
after a TST-instruction or CMP-instruction.
BMI: where MI stands for MInus. The branch is taken if
the N-bit is 1. This instruction is the
counterpart of the BPL-instruction.
BVC: where VC stands for oVerflow Clear. The branch is
taken if the V-bit is 0. This instruction is often
used after an Integer Arithmetic instruction like
ADD, SUB, MUL etc.
BVS: where VS stands for oVerflow Set. The branch is
taken if the V-bit is 1. This instruction is the
counterpart of the BVC-instruction.
BRA: where RA stands for bRanch Always. This
instruction is often used at the end of a loop to
go back to the beginning of the loop.
Branches often used after an arithmetic operation on
two's complement numbers.
BGE: where GE stands for Greater or Equal. This branch
is taken if the N and V-bits contain the same
value.

BGT: where GT stands for Greater Than. This branch is
taken in the following cases:
- N is 1, V is 1, Z is 0
- N is V is Z is 0
BLE: where LE stands for Lower or Equal. This branch is
taken in the following cases:
- Z is 1
- N and V-bits contain different values
BLT: where LT stands for Less Than. This branch is
taken if the N and V-bits contain different
values.
Brances often used after an arithmetic operation on
unsigned numbers.
BHI: where HI stands for HIgher. This branch is taken
if the N and V-bits contain the same value.
BLS: where LS stands for Lower or Same. This branch is
taken if the C and Z-bits contain different
values.
Example:
This shows a piece of a C-program and an equivalent
piece of a PASCAL-program which are translated into
assembler. (variabele is signed)
C:
if (variable == 1 || variable > 4) variable = 5;
else var *= 3;
PASCAL:
if (variable == 1) or (variable > 4)
then variable := 5
else variable := variable * 3

* Most assemblers will optimize the branch-instructions
* to the short forms
CMP.W #1,variable
BEQ L10000
CMP.W #4,variable
BLE L2
L10000:
MOVE.W #5,variable
BRA L3
L2:
MOVE.W variable,R0
MULS #3,R0
MOVE.W R0,variable
L3:
Instructions: DBcc (cc stands for Condition Code)
Syntax: DBcc Dn,<address>
Data sizes: byte or word. This implicates that the branch-
instructions can branch in an area of 32K. Dn is
considered to contain a word.
Condition codes affected:
None
Function:
The group of Decrement and Branch (DBcc) instructions
provide an efficient way of creating loops. They are
nearly always placed at the end of a loop. First the
condition is tested, then the dataregister is
decremented. The branch is taken in the following
cases:
- Dn is -1;
- The condition cc in DBcc is satisfied.
There are 16 possible variations of this instruction.
They all are nearly the same as the Bcc-instructions,
with two exceptions. These are:
DBF or DBRA:
This loop can only be terminated by count since
the other condition can never be satisfied.
DBT: Only performs a decrement on the dataregister and
never branches. To me this seems a pretty useless
instruction, which is only there to make the DBcc
series logically complete.
Example:
This piece of code is an efficient implementation of
the strcpy-function of the C-language. A0 contains the
address of the source string and A1 contains the
address of the destination string. In C the end of a
string is marked by a byte containing 0.
MOVE.W #$ffff,D0
LOOP: MOVE.B (A0)+,(A1)+
DBEQ D0,LOOP
This piece of code can easily be transformed into the
strncpy-function by loading D0 with the appropriate
value.


Instructions: Scc (cc stands for Condition Code)
Syntax: Scc <address>
Data sizes: byte.

Condition codes affected:
None
Function: Sets a byte to $ff if the condition codes satisfie. If
the condition is not satisfied the byte is set to 0.
This group of 16 instructions is rarely used. Nearly
all forms are the same as the DBcc group except for the
following two instructions:
SF: the same as a CLR.B instruction
ST: the same as a MOVE.B #$ff, <address>
Example:
Be inventive, invent one yourself!


Instruction: BSR, JSR
Syntax: BSR <address>
JSR <address>
Data sizes: none
Condition codes affected:
none



Addressing modes allowed (only for JSR):
Destination:
(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
Function: The BSR (Branch to SubRoutine) and JSR (Jump to
SubRoutine) instructions are used for calling
subroutines. BSR can branch in a range of 32K. JSR
should be used when a jump out of the 32K range is
needed. Some assemblers optimize JSR into BSR
instructions whenever possible, since BSR is more
efficient than JSR. When executing a BSR/JSR
instruction, the 68000 first pushes the PC (program-
counter) on the stack and then load the PC with the new
address. See below for the RTS (ReTurn from Subroutine)
instruction.


Instruction: RTS
Syntax: RTS
Data sizes: none
Condition codes affected:
none
Function: Counterpart of BSR/JSR instructions. Reloads the PC
with the value on top of the stack. This value will
nearly always have been put on top of the stack by a
BSR/JSR instruction.
Example: * the strcpy function discussed before
STRCPY:
MOVE.W #$FFFF,D0
LOOP: MOVE.W (A0)+,(A1)+
DBEQ D0, LOOP
RTS
* some other code
BEGIN:
MOVE.L #SOURCE,A0
MOVE.L #DEST,A1
JSR STRCPY
RTS

* the strings are put in a data area
.DATA
* 80 bytes for every string
SOURCE .DS.B 80
DEST .DS.B 80
* .DS.B means Define Storage Byte
* so 80 bytes are define as storage for each string


Instruction: JMP
Syntax: JMP <ea>
Data sizes: none
Condition codes affected:
none








Addressing modes allowed:
Destination:
(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
Function: Transfer program control to another address. The PC is
loaded with the specified address. In fact this is a
variant of the MOVE instruction. In this case the
destination register is inherently defined, namely the
PC-register. Therefore we could translate JMP <ea> to
MOVE.L <ea>,PC .

Instruction: RTR
Syntax: RTR
Data sizes: none
Condition codes affected:
none

Function: ReTurn and Restore. Counterpart of BSR/JSR
instructions. Reloads the PC with the value on top of
the stack. This value will nearly always have been put
on top of the stack by a BSR/JSR instruction. The only
difference with the RTS is that with this instruction
also the CCR is reloaded. This instruction is rarely
used but comes in handy when one doesn't want a
subroutine to influence the condition codes. Before the
JSR instruction you should use the instruction:
MOVE.B CCR,-(A7)
which pushes the CCR on the stack

Next time: The last part of the instruction set. These are the
instructions which can only be executed when supervisor-mode is
active.



ASSEMBLY LANGUAGE COURS PART IX by Mark van den Boer

System Control instructions

In this part of the course the last group of instructions will be
explained. This group of instructions deals with the supervisor-
mode and are therefore sometimes referred to as system control
instructions. To remind you: the S-bit in the SR (status
register) of the 68000 determines whether the 68000 is in
supervisor-mode or not. Many of these instructions deal with so
called exceptions. Exception is another word for interrupt and
these are used to force program control immediately to a specific
routint exception handler routine. Exceptions are used to detect
situations that are urgent and need to be handled directly.
Therefore every exception has a vector assigned to it. This
vector is a pointer to a routine which performs some action which
should be taken when such an exception occurs. The exception
vectors are located in the first 256 longwords of memory.




Instruction: CHK
Syntax: CHK <ea>,Dn
Data sizes: word
Condition codes affected:
X not affected
N Set if Dn is less than zero, cleared if <ea>
less than Dn, in all other cases undefined
Z
V
C always undefined












Addressing modes allowed:
Source:
Dn
(An)
(An)+
-(An)
w(An)
b(An,Rn)
w
l
w(PC)
b(PC,Rn)
#
Destination:
Dn







Function: Compares the contents of the effective address operand
with the data-register. If the data register is less
than zero (the data register is always considered to be
a signed word) or greater than the contents of <ea>,
then an exception occurs. The pointer to this
exception-routine is located at address $18. This
instruction is used to check if a data register is
within a range. It is often used by high-level
languages such as PASCAL to perform array-bound
checking.
Examples:
Instruction Before After
CHK #50,D0 D0=45 D0=45
No exception occured, if D0 had been 51
or greater then an exception would have
occured.


Instruction: MOVE USP (privileged instruction)
Syntax: MOVE USP,An or MOVE An,USP
Data sizes: long

Condition codes affected:
X
N
Z
V
C not affected
Addressing modes allowed:
See syntax
Function: As you all should know, the 68000 has in fact two A7
registers. One A7 register is used when in supervisor-
mode, the other when in usermode (this is: not in
supervisor-mode. It is sometimes desirable for a
program which is executing in supervisor mode to know
the value of the usermode A7-register. This instruction
provides a way to obtain and change the value of A7
usermode-register.
Example:
Instruction Before After
MOVE USP,A6 A7user=$12345678 A7user=$12345678
A6 =$00000000 A6 =$12345678
A7sup =$87654321 A7sup =$87654321

Instruction: RESET
Syntax: RESET (privileged instruction)
Data sizes: none
Condition codes affected:
X
N
Z
V
C not affected
Function: Reset all external devices. A device can be a chip like
the 6850.


Instruction: RTE (privileged instruction)
Syntax: RTE
Data sizes: none
Condition codes affected:
none




Function: Every exception is terminated by this instruction. It
can be compared to RTS. The only difference is that RTE
will restore the SR in addition. Note that an
exception-routine has the responsibility to save
registers if this is important.


Instruction: STOP (privileged instruction)
Syntax: STOP #
Data sizes: word
Condition codes affected:
All set as a direct result of the operand
Addressing modes allowed:
Source:
#
Function: Stop execution of a program until an exception occurs.
The operand stored in the SR. Note that with the
operand a minimum interrupt level can be determined.
With this instruction it is possible to wait for a
videochip interrupt to occur.


Example:
STOP #%0010011000011111
Wait for an exception with a priority of 6 or 7 to
occur and set the XNZVC-bits.


Instruction: TRAP
Syntax: TRAP #
Data sizes: # must be >=0 and <=15
Condition codes affected:
X
N
Z
V
C not affected







Function: This instruction generates an exception. The operand
indicates an exception number. The vectors for the
exceptions are located at addresses $80 to $BC. This
instruction is mainly used to allow programs that
execute in user-mode to call supervisor-mode routines.
This way a user can be given a number of specific
functions. In the ST trap vectors 2, 14 and 15 are used
for GEM, BIOS and XBIOS functions. In the case of the
ST the TRAP-instructions is preceded by instructions
that put function numbers and parameters for these
functions on the stack. This way it is possible to
assign groups of functions to one trap-vector.
Note that when calling a TRAP in user-mode the
stackpointers change (supervisor-mode and user-mode,
remember??). Thus, the MOVE USP instruction can be used
to retrieve parameters that had been put on the stack.


Instruction: TRAPV
Syntax: TRAPV
Data sizes: none

Condition codes affected:
none
Function: When the V-bit is set an exception occurs. The
exception vector is located at address $1C. When the V-
bit is clear nothing happens. This instruction can be
used by high-level languages to inform the user that an
overflow error has occured.



MC 68000 ASSEMBLY LANGUAGE COURSE PART X by Mark van den Boer

Now all instructions of the 68000 have been explained it's time
to bring this knowledge into practice. This last part of the
course will deal with the subject of translating high-level
constructs to the equivalent assembler constructs. The C program-
ming language will be used as the high-level language which has
to be translated to assembler. A note to those of you who are
more familiar with Pascal or BASIC: litte imagination is needed
to deduct the similar constructs in Pascal and BASIC.
What now follows is a C-program which containing several commonly
used data- and control structures. The examples show you how to
translate these structures into assembler.
There should also be a file called M68000.DOC on your ST NEWS
disk. This file contains a quick-reference card containing all
instructions and allowed addressing modes. This reference card
has been made by Trustware, Inc. (an unregistered trademark of
Victor Langeveld). I am very grateful to Victor for allowing me
to include this card, since it's a rather tedious job to put
together such a card. One thing's for sure: this card is one of
the better of its kind and it's the most compact reference card
for the 68000 I've ever seen.
/*
A function. (Called a procedure or function in Pascal
and a subroutine in BASIC)
Note how parameters are passed in the assembly language
translation.
Als pay attention to how local variables are stored.
*/
int function (number, pointer)
int number;
char *pointer;
{
register int i, j;
char *c;

i = number - 1;
c = pointer;
c = "new Queensryche album: Operation Mindcrime";
/* Note how a string is stored */
return i; /* Note how a value is returned */
}


.text
function:
* offset of number = 8
* offset of pointer = 10
LINK A6,#-4
* save registers
MOVEM.L D6-D7,-(sp) * sp = A7
* i in D7
* j in D6
* offset of c = -4
* i = number - 1
MOVE.W 8(A6),D7
SUB.W #1,D7
* c = pointer
MOVE.L 10(A6),-4(A6)
* c = "new Queensryche album: Operation Mindcrime"
.data





L2:
.dc.b 'new Queensryche album: Operation Mindcrime',0
MOVE.L #L2,-4(A6)
* D0 is used for resulting values from functions
* return i; D0 is always used for the result of a function
MOVE.W D7,D0
* restore registers
MOVEM.L (sp)+,D6-D7
UNLK A6
* }
RTS

* global variables
.bss
* int i;
i: .ds.w 1
* char character
character: .ds.b 1
* int *i_pointer
i_pointer: .ds.l 1


* int i_array[10][5]
i_array: .ds.w 10*5
* one struct is in fact 3 bytes long, but for every structure
* 4 bytes are reserved. This is because the 68000 can only
* address words at even addresses.
* struct { /* this is the equivalent of a record in PASCAL */
* int i;
* char c;
* } structure[5];
structure: .ds.b 4*5


main()
{
/* assignment of a constant to a variable */
i = 9;

/* assignment of a constant to a variable */
character = 'c';

/* assignment of a constant to a variable */
i_pointer = &i;
/* watch how indexing of array is done */
/* integer is 2 bytes, so the address of
array-element [3][4] is:
(3 * 5 (the length of i_array[3]) + 4)
* 2 (size of an integer) = 38.
So the integer should be stored at i_array + 38.
*/
i_array[3][4] = *i_pointer;

/* Now the distance in bytes from the beginning of the
array must be computed during program execution,
in contrary to the previous example.
*/
i_array[i][i - 1] = 2;

/* Assignments to arrays of structures */
structure[1].i = 3;
structure[i].c = character;

/* expression evaluation and assignment */
i = i_array[0][0] * i_array[0][1] +
i_array[0][2] / i_array[0][3];
/* conditional statement */
if (i < i_array[i][i]) i = 1;
else i = 2;

/* while loop */
while (i <= 10) i++;
/* continue and break statements */
while (i++ <= 10) {
if (i != 4) continue;
else break;
}

/* for loop */
for (i = 4; i >= 0; i--) i_array[i][i] = i;

/* do loop */
do i++; while (i < 10 && i != 5);

/* switch statement; watch the application of a
jump-table. Pay special attention to how 'case 4'
which must 'default' is solved.

*/
switch (i) {
case 0:
i = 0;
break;
case 1:
i = 5;
break;
case 2:
case 3:
i = 7;
break;
case 5:
i = 1;
break;
default:
i = 2;
break;
}



/* switch statement;
watch how 'case 999' has destroyed the
jumptable-optimization.
*/
switch (i) {
case 0:
i = 0;
break;
case 1:
i = 5;
break;
case 2:
case 3:
i = 7;
break;
case 5:
i = 1;
break;
case 999:
/* This case should be t