0580-CLEAR.md

0580 Clear input/output file records and interrupt vector array

SYS0580: MOV DI,Input

Start at Input (in system data segment).

SYS0583: PUSH DS
SYS0584: POP ES

Copy DS into ES. STOSW stores word (in AX) into ES:[DI].

SYS0585: MOV CX,SizeOf(DATA)
SYS0588: SUB CX,DI
SYS058A: SHR CX,1

Clear memory from 0050h (Input) to SizeOf(DATA) = 029Ch (SaveInt75) or around ((029Ch - 0050h) / 2) = 294 words. 029Ch is the size of the DATA segment (see: MAIN.MAP).

SYS058C: XOR AX,AX

Clear memory with 0000.

SYS058E: CLD

Proceed in forward direction, i.e. DI increases each iteration.

SYS058F: REPZ
SYS0590: STOSW

Clear until CX = 0. CX is decremented by 1 each time STOSW is executed

SYS0591: RET

Return to caller. Because it returns with a NEAR RET, it can only be called from within the system library.

See also: DATA or go back