User guide

72 CHAPTER 3. THE LIBRARY

oldcount := sys(Sys

setcount, newcount) CIN:y, POS:y, NAT:n

One of the Cintcode registers is called count which is inspected just before the

interpreter processes the next instruction. If count>0 it is decremented and the in-

struction processed. If count=0 the interpreter returns to the calling (C) program with

error code 3.

The Cintcode System nor mal l y has two resident interpreters. One is calle d cinterp

implemented in C and the other is called fasterp which is sometimes implemented in

assembly l anguage . fasterp is faster than cinterp since it provides fewer debugging

aids, does not count instruction executions and does not implement t he proﬁling feature.

Setting count to a negati ve value causes this faster interpreter to be invoked and setting

count to a positive value causes the slower interpreter to b e used. Normally the CLI

command interpreter is used to make this switch, see Section 4.3.

With some debugging versions of fasterp, setting count to -2 causes it to exe-

cute just one instruction b ef ore returning with error code 10. This feature assists the

debugging of a new versions of fasterp and is particularly useful when fasterp is

implemented in assembly language.

regs!0 A register – work register

regs!1 B register – work register

regs!2 C register – work register

regs!3 P register – the stack frame pointer

regs!4 G register – the base of the global vector

regs!5 ST register – the status regi st er (unused)

regs!6 PC register – the program counter

regs!7 Count register – see below

regs!8 MW register – Used onl y on 64-bit systems, see below

The count register is normally decremented every time a Cintcode instruction is

interpreted. When the count reaches zero the interpreter saves the registers and returns

with a result (=3) to indicate that this has happened. If the count register is positive,

it indicates how many Cintcode instructions should be executed before the interpreter

returns. A count of -1 is treated as inﬁnity and causes the fast interpreter fasterp to

be used.

Either interpreter returns when a fault, such as div i si on by zero, occurs or when

a call of sys(Sys_quit,...) or sys(Sys_setcount,...) is made. When returning,

the current state of the Cintcode registers is saved. The r et ur ne d result is either

the second argument of sys(Sys_quit,...) or on e of the builtin return c odes in the

following table: