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back2root:ibm-pc-ms-dos:instr:rep

REP

REP - Repeat String Operation

Repeats execution of string instructions while CX != 0. After each string operation, CX is decremented and the Zero Flag is tested.

Usage:

	REP MOVSB

The combination of a repeat prefix and a segment override on CPU's before the 386 may result in errors if an interrupt occurs before CX=0. The following code shows code that is susceptible to this and how to avoid it:

again:	rep movs	byte ptr ES:[DI],ES:[SI]	; vulnerable instr.
	jcxz		next				; continue if REP successful
	loop		again				; interrupt goofed count
next:

Modifies flags:

None

Clocks Size
Operands 808x 286 386 486 Bytes
none 2 2 2 1
back2root/ibm-pc-ms-dos/instr/rep.txt · Dernière modification : 2023/02/24 14:16 de frater