REP Prefixes

UMBC CMSC 211

REP Prefixes

You can put rep in front of an instruction and cause the instruction to be repeated until cx becomes zero. There are five versions of this:

rep
repe
repz
repne
repnz

rep strop equivalent

	jcxz	stropDone
stropLoop:
	strop
	loop	stropLoop
stropDone:

repE/Z strop equivalent

	jcxz	stropDone
stropLoop:
	strop
	jnz	stropDone	; jnz = jne
	loop	stropLoop
stropDone:

repNE/NZ strop equivalent

	jcxz	stropDone
stropLoop:
	strop
	jz	stropDone	; jz = je
	loop	stropLoop
stropDone:

Example

Let es:di point to an array to 100 bytes. To move a zero into each element:

	mov	al, 0
	cld
	mov	cx, 100
	rep	stosb	; ary[i++] = 0;