User guide

Chapter 4

The Command Language

The Command Language Interpreter (CLI) is a simple interactive interface between the

user an d the system. It loads and exe cut e s previously compiled programs that are held

either i n the current directory or one of the directories speciﬁed by the shell environ-

ment var i abl e (typically BCPLPATH or POSPATH) whose name is in rootnode!rtn

path.

These commands are describ ed in Section 4.3 and their source code can be found in

the com directory. The command language is a combination of the features provided

by the CLI and the collection of commands that can be invoked. Under Cintpos, a

similar CLI program provides command language interpreters in several contexts such

as those created by the commands: run, newcli, tcpcli and mbxcli. Details of the

implementation of both CLIs are given at the end of this chapter from page 128.

Commands can set a return code in the global returncode with zero meaning

successful ter mi n ati on and other values indicating the severity of the fault. Commands

that set a non zero retur n code are expected to leave a reason co de in result2. The

CLI copies the return code and reason code of the previous command into the CLI

variables cli

returncode and cli result2, respectively. These can be inspected by

commands such as if and why and also used by the CLI to terminate a command-

command if the failure was severe enough. For details, see the command failat on

page 115 below.

4.1 Bootstrapping Cintsys

When Cintsys is started, control is passed to the int er p re t er which, after a few initial

checks, allocates vectors for the memory of the Cintcode abstract machine and the tally

vector available for statistics gathering. The Cintcode memory is initialised suitably

for sub-allocation by getvec, which is then used to allocate space for the root node,

the initial stack and the initial global vector. The initial state shown in ﬁgure 4.1 is

completed by loading the object modules SYSLIB, BLIB and BOOT, and initialising the

root node, the stack and global vector. Interpretation of Cintcode instructions now

begins with the Cintcode register PC, P and G set as sh own in the ﬁgure, and Count set

to -1. The other registers are cleared. The ﬁrst Cintcode instructi on to be executed is

the ﬁrst instructi on of the body of the function start deﬁned in sysb/boot.b. Since