Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

4-250 Vol. 2B
INSTRUCTION SET REFERENCE, N-Z
Description
Repeats a string instruction the number of times specified in the count register or
until the indicated condition of the ZF flag is no longer met. The REP (repeat), REPE
(repeat while equal), REPNE (repeat while not equal), REPZ (repeat while zero), and
REPNZ (repeat while not zero) mnemonics are prefixes that can be added to one of
the string instructions. The REP prefix can be added to the INS, OUTS, MOVS, LODS,
and STOS instructions, and the REPE, REPNE, REPZ, and REPNZ prefixes can be
added to the CMPS and SCAS instructions. (The REPZ and REPNZ prefixes are synon-
ymous forms of the REPE and REPNE prefixes, respectively.) The behavior of the REP
prefix is undefined when used with non-string instructions.
The REP prefixes apply only to one string instruction at a time. To repeat a block of
instructions, use the LOOP instruction or another looping construct. All of these
repeat prefixes cause the associated instruction to be repeated until the count in
register is decremented to 0. See Table 4-3.
The REPE, REPNE, REPZ, and REPNZ prefixes also check the state of the ZF flag after
each iteration and terminate the repeat loop if the ZF flag is not in the specified state.
When both termination conditions are tested, the cause of a repeat termination can
F2 AE REPNE SCAS m8 Valid Valid Find AL, starting at ES:[(E)DI].
F2 REX.W AE REPNE SCAS m8 Valid N.E. Find AL, starting at [RDI].
F2 AF REPNE SCAS m16 Valid Valid Find AX, starting at ES:[(E)DI].
F2 AF REPNE SCAS m32 Valid Valid Find EAX, starting at ES:[(E)DI].
F2 REX.W AF REPNE SCAS m64 Valid N.E. Find RAX, starting at [RDI].
NOTES:
* In 64-bit mode, r/m8 can not be encoded to access the following byte registers if a REX prefix
is used: AH, BH, CH, DH.
Table 4-3. Repeat Prefixes
Repeat Prefix Termination Condition 1* Termination Condition 2
REP RCX or (E)CX = 0 None
REPE/REPZ RCX or (E)CX = 0ZF = 0
REPNE/REPNZ RCX or (E)CX = 0 ZF = 1
NOTES:
* Count register is CX, ECX or RCX by default, depending on attributes of the operating modes. In
64-bit mode, if default operation size is 32 bits, the count register becomes RCX when a REX.W
prefix is used.
Opcode Instruction
64-Bit
Mode
Compat/
Leg Mode Description