Intel 210200-002 manual Repeat While Equal Repeat While Zero

REP REPEATREP

REPE/REPZ REPE/REPZ

REPEAT WHILE EQUAL/

REPEAT WHILE ZERO

REPNE/REPNZREPNE/REPNZ

REPEAT WHILE NOT EQUAL/

REPEAT WHILE NOT ZERO

Description:

REP/REPE/REPZ/REPNE/REPNZ

Repeat, Repeat While Equal, Repeat While Zero, Repeat While Not Equal and Repeat While Not Zero are mnemonics for two forms of the prefix byte that controls subsequent string instruction repetition. The different mnemonics are provided to improve program clarity. The repeat prefixes do not affect the flags.

REP is used in conjunction with the MOVS (Move String) and STOS (Store String) instructions and is interpreted as "repeat while not end-of-string" (CX not 0). REPE and REPZ operate identically and are physically the same prefix byte as REP. These instruc- tions are used with the CMPS (Compare String) and SCAS (Scan String) instructions and require ZF (posted by these instructions) to be set before initiating the next repetition. REPNE and REPNZ are mnemonics for the same prefix byte. These instructions function the same as REPE and REPZ except that the zero flag must be cleared or the repetition is terminated. ZF does not need to be initial- ized before executing the repeated string instruction.

Repeated string sequences are interruptable; the processor will recognize the interrupt before processing the next string element. System interrupt processing is not affected in any way. Upon return from the interrupt, the repeated operation is resumed from the point of interruption. However, execution does not resume properly if a second or third prefix (i.e., segment override or LOCK) has been specified in addition to any of the repeat prefixes. At interrupt time, the processor "remembers" only the prefix that immediately precedes the string instruction. After returning from the interrupt, processing resumes, but any additional prefixes specified are not in effect. If more than one prefix must be used with a string instruction, interrupts may be disabled for the duration of the repeated exe- cution. However, this will not prevent a non- maskable interrupt from being recognized. Also, the time that the system is unable to respond to interrupts may be unacceptable if long strings are being processed.

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