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80386 Programmer's Reference Manual -- Opcode REP

REP/REPE/REPZ/REPNE/REPNZ -- Repeat Following String Operation

OpcodeInstructionClocks DescriptionExample
F3 6Crep insb13+6*(E)CX, Input (E)CX bytes from port DX into ES:[(E)DI]rep insb
F3 6Drep insw13+6*(E)CX, Input (E)CX words from port DX into ES:[(E)DI]rep insw
F3 6Drep insl13+6*(E)CX, Input (E)CX dwords from port DX into ES:[(E)DI]rep insl
F3 A4rep movsb5+4*(E)CX Move (E)CX bytes from [(E)SI] to ES:[(E)DI]rep movsb
F3 A5rep movsw5+4*(E)CX Move (E)CX words from [(E)SI] to ES:[(E)DI]rep movsw
F3 A5rep movsl5+4*(E)CX Move (E)CX dwords from [(E)SI] to ES:[(E)DI]rep movsl
F3 AArep stosb5+5*(E)CX Fill (E)CX bytes at ES:[(E)DI] with ALrep stosb
F3 ABrep stosw5+5*(E)CX Fill (E)CX words at ES:[(E)DI] with AXrep stosw
F3 ABrep stosl5+5*(E)CX Fill (E)CX dwords at ES:[(E)DI] with EAXrep stosl
F3 A6repe cmpsb5+9*N Find nonmatching bytes in ES:[(E)DI] and [(E)SI]repe cmpsb
F3 A7repe cmpsw5+9*N Find nonmatching words in ES:[(E)DI] and [(E)SI]repe cmpsw
F3 A7repe cmpsl5+9*N Find nonmatching dwords in ES:[(E)DI] and [(E)SI]repe cmpsl
F3 AErepe scasb5+8*N Find non-AL byte starting at ES:[(E)DI]repe scasb
F3 AFrepe scasw5+8*N Find non-AX word starting at ES:[(E)DI]repe scasw
F3 AFrepe scasl5+8*N Find non-EAX dword starting at ES:[(E)DI]repe scasl
F2 A6repne cmpsb5+9*N Find matching bytes in ES:[(E)DI] and [(E)SI]repne cmpsb
F2 A7repne cmpsw5+9*N Find matching words in ES:[(E)DI] and [(E)SI]repne cmpsw
F2 A7repne cmpsl5+9*N Find matching dwords in ES:[(E)DI] and [(E)SI]repne cmpsl
F2 AErepne scasb5+8*N Find AL, starting at ES:[(E)DI]repne scasb
F2 AFrepne scasw5+8*N Find AX, starting at ES:[(E)DI]repne scasw
F2 AFrepne scasl5+8*N Find EAX, starting at ES:[(E)DI]repne scasl

Operation




IF AddressSize = 16
THEN use CX for CountReg;
ELSE (* AddressSize = 32 *) use ECX for CountReg;
FI;
WHILE CountReg <> 0
DO
   service pending interrupts (if any);
   perform primitive string instruction;
   CountReg := CountReg - 1;
   IF primitive operation is CMPB, CMPW, SCAB, or SCAW
   THEN
      IF (instruction is REP/REPE/REPZ) AND (ZF=1)
      THEN exit WHILE loop
      ELSE
         IF (instruction is REPNZ or REPNE) AND (ZF=0)
         THEN exit WHILE loop;
         FI;
      FI;
   FI;
OD;

Description

REP, REPE (repeat while equal), and REPNE (repeat while not equal) are prefix that are applied to string operation. Each prefix cause the string instruction that follows to be repeated the number of times indicated in the count register or (for REPE and REPNE) until the indicated condition in the zero flag is no longer met.

Synonymous forms of REPE and REPNE are REPZ and REPNZ, respectively.

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.

The precise action for each iteration is as follows:

  1. If the address-size attribute is 16 bits, use CX for the count register; if the address-size attribute is 32 bits, use ECX for the count register.
  2. Check CX. If it is zero, exit the iteration, and move to the next instruction.
  3. Acknowledge any pending interrupts.
  4. Perform the string operation once.
  5. Decrement CX or ECX by one; no flags are modified.
  6. Check the zero flag if the string operation is SCAS or CMPS. If the repeat condition does not hold, exit the iteration and move to the next instruction. Exit the iteration if the prefix is REPE and ZF is 0 (the last comparison was not equal), or if the prefix is REPNE and ZF is one (the last comparison was equal).
  7. Return to step 1 for the next iteration.
Repeated CMPS and SCAS instructions can be exited if the count is exhausted or if the zero flag fails the repeat condition. These two cases can be distinguished by using either the JCXZ instruction, or by using the conditional jumps that test the zero flag (JZ, JNZ, and JNE).

Flags Affected

ZF by REP CMPS and REP SCAS as described above

Protected Mode Exceptions

#UD if a repeat prefix is used before an instruction that is not in the list above; further exceptions can be generated when the string operation is executed; refer to the descriptions of the string instructions themselves

Real Address Mode Exceptions

Interrupt 6 if a repeat prefix is used before an instruction that is not in the list above; further exceptions can be generated when the string operation is executed; refer to the descriptions of the string instructions themselves

Virtual 8086 Mode Exceptions

#UD if a repeat prefix is used before an instruction that is not in the list above; further exceptions can be generated when the string operation is executed; refer to the descriptions of the string instructions themselves

Notes

Not all input/output ports can handle the rate at which the REP INS and REP OUTS instructions execute.

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