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

Opcode    Instruction         Clocks          Description

F3  6C    REP INS r/m8, DX    13+6*(E)CX,
                              pm=7+6*(E)CX
If CPL <= IOPL/
                              27+6*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Input (E)CX bytes from port
                                               DX into ES:[(E)DI]
F3  6D    REP INS r/m16,DX    13+6*(E)CX,
                              pm=7+6*(E)CX
If CPL <= IOPL/
                              27+6*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Input (E)CX words from port
                                               DX into ES:[(E)DI]
F3  6D    REP INS r/m32,DX    13+6*(E)CX,
                              pm=7+6*(E)CX
If CPL <= IOPL/
                              27+6*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Input (E)CX dwords from port
                                               DX into ES:[(E)DI]
F3  A4    REP MOVS m8,m8      5+4*(E)CX        Move (E)CX bytes from
                                               [(E)SI] to ES:[(E)DI]
F3  A5    REP MOVS m16,m16    5+4*(E)CX        Move (E)CX words from
                                               [(E)SI] to ES:[(E)DI]
F3  A5    REP MOVS m32,m32    5+4*(E)CX        Move (E)CX dwords from
                                               [(E)SI] to ES:[(E)DI]
F3  6E    REP OUTS DX,r/m8    5+12*(E)CX,
                              pm=6+5*(E)CX
If CPL <= IOPL/
                              26+5*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Output (E)CX bytes from
                                               [(E)SI] to port DX
F3  6F    REP OUTS DX,r/m16   5+12*(E)CX,
                              pm=6+5*(E)CX
If CPL <= IOPL/
                              26+5*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Output (E)CX words from
                                               [(E)SI] to port DX
F3  6F    REP OUTS DX,r/m32   5+12*(E)CX,
                              pm=6+5*(E)CX
If CPL <= IOPL/
                              26+5*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Output (E)CX dwords from
                                               [(E)SI] to port DX
F3  AA    REP STOS m8         5+5*(E)CX        Fill (E)CX bytes at
                                               ES:[(E)DI] with AL
F3  AB    REP STOS m16        5+5*(E)CX        Fill (E)CX words at
                                               ES:[(E)DI] with AX
F3  AB    REP STOS m32        5+5*(E)CX        Fill (E)CX dwords at
                                               ES:[(E)DI] with EAX
F3  A6    REPE CMPS m8,m8     5+9*N            Find nonmatching bytes in
                                               ES:[(E)DI] and [(E)SI]
F3  A7    REPE CMPS m16,m16   5+9*N            Find nonmatching words in
                                               ES:[(E)DI] and [(E)SI]
F3  A7    REPE CMPS m32,m32   5+9*N            Find nonmatching dwords in
                                               ES:[(E)DI] and [(E)SI]
F3  AE    REPE SCAS m8        5+8*N            Find non-AL byte starting
                                               at ES:[(E)DI]
F3  AF    REPE SCAS m16       5+8*N            Find non-AX word starting
                                               at ES:[(E)DI]
F3  AF    REPE SCAS m32       5+8*N            Find non-EAX dword starting
                                               at ES:[(E)DI]
F2  A6    REPNE CMPS m8,m8    5+9*N            Find matching bytes in
                                               ES:[(E)DI] and [(E)SI]
F2  A7    REPNE CMPS m16,m16  5+9*N            Find matching words in
                                               ES:[(E)DI] and [(E)SI]
F2  A7    REPNE CMPS m32,m32  5+9*N            Find matching dwords in
                                               ES:[(E)DI] and [(E)SI]
F2  AE    REPNE SCAS m8       5+8*N            Find AL, starting at
                                               ES:[(E)DI]
F2  AF    REPNE SCAS m16      5+8*N            Find AX, starting at
                                               ES:[(E)DI]
F2  AF    REPNE SCAS m32      5+8*N            Find EAX, starting at
                                               ES:[(E)DI]

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.

up: Chapter 17 -- 80386 Instruction Set
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