User guide

168 CHAPTER 9. THE DESIGN OF CINTCODE

These instructions are used move val ue s between register A, B and C.

NOP

This instruction has no eﬀect.

SYS

This instruction is used in body of the han d written library routine sys. If A is zero

then the inter pr e te r returns with exception code P!4.

If A is -1 it set register count to P!4, setting A to the previous value of count.

Changing the value of count may change which of the two interpreters is used. For

more details see Section 4.3.

Otherwise, it performs a system operation returning the result in A. In the C im-

plementation of the interpreter this is done by the following code:

c = dosys(p, g);

MDIV

This instruction is used as the one and only instruction in the body of the hand wri t t en

library rout i ne muldiv, see Section 3.3. It divides P!5 into the double length product of

P!3 and P!4 placing the resul t in A and the remainder in the global variable result2.

It then performs a function return (RTN). Its eﬀect is as follows:

A := <the result>

G!Gn_result2 := <the remainder>

PC := P!1 // PC := P!1

P := P!0 // P := P!0

CHGCO

This inst r uc ti on is used in the implementation of coroutines. It is the one an d only

instruction in the bo dy of the hand wri t te n library routine changeco(val,cptr) where

val is passed in Cintcode register A and cptr is in P!4. Its eﬀect, which is rather subtle,

is shown below. For more information see page 46.

G!Gn_currco!0 := P!0 // !currco := !P -- changeco’s old P pointer

PC := P!1 // PC := P!1 -- changeco’s return address

G!Gn_currco := P!4 // currco := cptr

P := P!4!0 // P := !cptr

BRK

This instruction is used by the debugger to implement break points. It causes the

interpreter to return with exception code 2.