Richards M.The BCPL Cintcode and Cintpos user guide.2005

Contents

2.2.1Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.2Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.3Calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2.4Method Calls . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2.5Pre¯xed Expression Operators . . . . . . . . . . . . . . . . 14

2.2.6 In¯xed Expression Operators . . . . . . . . . . . . . . . . 15

2.2.7Boolean Evaluation . . . . . . . . . . . . . . . . . . . . . . 16

2.2.8 VALOF Expressions . . . . . . . . . . . . . . . . . . . . . . 16

2.2.9Expression Precedence . . . . . . . . . . . . . . . . . . . . 16

2.3.1Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.3.2Calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.3.3Conditional Commands . . . . . . . . . . . . . . . . . . . . 18

2.3.4 Repetitive Commands . . . . . . . . . . . . . . . . . . . . 19

2.3.5SWITCHON command . . . . . . . . . . . . . . . . . . . . . . 19

2.3.6Flow of Control . . . . . . . . . . . . . . . . . . . . . . . . 20

2.3.7Compound Commands . . . . . . . . . . . . . . . . . . . . 20

2.3.8Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.4Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.4.1Labels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

i

2.4.2Manifest Declarations . . . . . . . . . . . . . . . . . . . . . 21

2.4.3Global Declarations . . . . . . . . . . . . . . . . . . . . . . 22

2.4.4Static Declarations . . . . . . . . . . . . . . . . . . . . . . 22

2.4.8Procedure Declarations . . . . . . . . . . . . . . . . . . . . 23

2.4.9 Dynamic Free Variables . . . . . . . . . . . . . . . . . . . 25

2.5Separate Compilation . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.1.1Rootnode . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.1.2Other manifest constants . . . . . . . . . . . . . . . . . . . 32

3.1.3Input/Output system manifests . . . . . . . . . . . . . . . 36

3.2Global vector variables . . . . . . . . . . . . . . . . . . . . . . . . 37

3.3Global vector entry points . . . . . . . . . . . . . . . . . . . . . . 38

3.3.1SYSLIB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.3.2sys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.3.3Interpreter Management . . . . . . . . . . . . . . . . . . . 38

3.3.4 Primitive I/O Operations . . . . . . . . . . . . . . . . . . 40

3.4BLIB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.4.1 Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.4.2 Stream Input/Output . . . . . . . . . . . . . . . . . . . . 44

3.4.3Input Functions . . . . . . . . . . . . . . . . . . . . . . . . 46

3.4.4Output Functions . . . . . . . . . . . . . . . . . . . . . . . 46

3.4.5 The Filing System . . . . . . . . . . . . . . . . . . . . . . 48

3.4.6File Deletion and Renaming . . . . . . . . . . . . . . . . . 49

3.4.7Non Local Jumps . . . . . . . . . . . . . . . . . . . . . . . 49

3.4.8 Command Arguments . . . . . . . . . . . . . . . . . . . . 49

3.4.9Program Loading and Control . . . . . . . . . . . . . . . . 52

3.4.10Character Handling . . . . . . . . . . . . . . . . . . . . . . 54

3.4.11Coroutines . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.1Bootstrapping single threaded BCPL . . . . . . . . . . . . . . . . 61

4.2Bootstrapping Cintpos . . . . . . . . . . . . . . . . . . . . . . . . 63

4.2.1The Cintpos BOOT module . . . . . . . . . . . . . . . . . 63

4.2.2klibstart . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Preface

The concept for BCPL originated in 1966 and was ¯rst outlined in my PhD thesis [4]. Its was ¯rst implemented early in 1967 when I was working at M.I.T. Its heyday was perhaps from the mid 70s to the mid 80s, but even now it is still continues to be used at some universities, in industry and by private individuals. It is a useful language for experimenting with algorithms and for research in optimizing compilers. Cintpos is the multi-tasking version of the system based on the Tripos [5]. It is simple and easy to maintain and can be used for real-time applications such as process control. BCPL was designed many years ago but is still useful in areas where small size, simplicity and portability are important.

This document is intended to provide a record of the main features of the BCPL in su±cient depth to allow a serious reader to obtain a proper understanding of philosophy behind the language. An e±cient interpretive implementation is presented, the source of which is freely available via my home page [3]. The implementation is machine independent and should be easy to transfer to almost any architecture both now and in the future.

The main topics covered by this report are:

²A speci¯cation of the BCPL language.

²A description of its runtime library and the extensions used in the Cintpos system.

²The design and implementation of command language interpreters for both the single and multi-threaded versions of the system.

²A description of OCODE, the intermediate code used in the compiler, and Cintcode, the compact byte stream target code used by the interpreter.

²A description of the single and multi-threaded interactive debugger and other debugging aids.

²The e±cient implementation of the Cintcode interpreter for several processors including both RISC and i386/Pentium based machines.

²The pro¯ling and statistics gathering facilities o®ered by the system.

v

Chapter 1

The System Overview

This document contains a full description of an interpretive implementation of BCPL that supports a command language and low level interactive debugger. As an introduction, an example console session is presented to exhibit some of the key features of the single threaded version of the system.

1.1A Console Session

When the system is started (on a machine called meopham) in the directory bcplprogs/demo, its opening message is as follows:

meopham$ cinterp

BCPL Cintcode System (11 June 2004) 0>

The characters 0> are followed by a space character and is the command language prompt string inviting the user to type a command. The integer gives the execution time of the preceeding command. A program to compute factorials can be displayed using the type command as follows:

> type fact.b GET "libhdr"

LET start() = VALOF

{FOR i = 1 TO 5 DO writef("fact(%n) = %i4*n", i, fact(i)) RESULTIS 0

}

AND fact(n) = n=0 -> 1, n*fact(n-1) 0>

The directive GET "libhdr" causes the standard library declarations to be inserted at that position. The text:

1