(ARM).Using the ARM assembler

Using the Barrel Shifter

Value resulting from rotating right with extend

The value of the operand is taken by rotating the register and the carry bit one bit right. Because the carry is included in the shift, 33 bits are affected. This is known asrotate right extended.

Syntax: register-name, RRX

Example: r0, RRX

Example use:

MOV r1, r0, RRX ; move rotate right extended contents of r0 into r1

Register shifted by n bits

The register operand is shifted by n bits, where the value n is taken from the least significant byte of a register.

Syntax: register-name, shift-type register-name

Example: r0, LSR r2

Example use:

MOV r1, r0. LSR r2 ; move contents of r0 divided by 2^r2 into r1

Application Note 50

ARM DAI 0050A

19

Open Access

Access to C Global Variables from Assembly Code

5 Access to C Global Variables from Assembly Code

The simplest way to access C global variables from assembly language source is to

IMPORT your global variables into your assembler source and access them directly.

For example:

;File sub1.s:

AREA globals,CODE

EXPORT asmsubroutine

IMPORT globvar

asmsubroutine
LDR	r1,	=globvar
LDR	r0,	[r1]
ADD	r0,	r0, #2
STR	r0,	[r1]
MOV	pc,	lr
END

This loads the address of globvar into r1, loads the value contained by r1 into r0, manipulates it, then stores the new value back into globvar.

A problem occurs if the variable’s memory address is greater than 4Kbytes from the LDR instruction, which may be caused when using scatter loading or large source files. The way to avoid this is to give globvar a label and then access it through the label:

;File sub2.s
AREA	globals,CODE
EXPORT asmsubroutine
IMPORT globvar
asmsubroutine
LDR	r1,	globaddr
LDR	r0,	[r1]
ADD	r0,	r0, #2
STR	r0,	[r1]
MOV	pc,	lr
globaddr		DCD	globvar

;More code can follow here without putting "globaddr" out of

;range of the LDR.

END

	Application Note 50
20	ARM DAI 0050A

Open Access

Access to C Global Variables from Assembly Code

The above are effectively equivalent pieces of code: using the LDR Rn,=expression syntax causes the Assembler to create a “label DCD expression” for itself and then try to access it via LDR Rn,label (choosing label so as not to conflict with any other labels in the program). If you write globaddr DCD globvar at the end of the program, you encounter LDR offset problems at precisely the same point with the second piece of code above as with the first.

The difference is that you can write globaddr DCD globvar where you like, and you can usually find a safe place to put it that lies within range of the LDR, or create a safe place by inserting an unconditional branch around the DCD.

However, rather than explicitly placing labels in your code at a place you know can be reached from the current PC, you can use the LTORG directive to tell the Assembler that data items generated by LDR Rn,=expression instructions can safely be placed where the directive appears in your source. If this directive is not used, the Assembler assumes that the only such place is at the END directive of the outermost file being assembled.

So the first sample piece of code above can be made to work, even if the assembler source file becomes very long, by changing it to:

; File sub1.s

AREA globals, CODE

EXPORT asmsubroutine

IMPORT globvar

asmsubroutine
LDR	r1,	=globvar
LDR	r0,	[r1]
ADD	r0,	r0, #2
STR	r0,	[r1]
MOV	pc,	lr

LTORG

;More code can follow here without putting the location

;containing"globvar" out of range of the LDR.

END

Application Note 50

ARM DAI 0050A

21

Open Access