Description

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

Synonymous forms of the REPE and REPNE prefixes are the REPZ and REPNZ prefixes, 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:

• If the address-size attribute is 16 bits, use the CX register for the count register; if the address-size attribute is 32 bits, use the ECX register for the count register.

• Check the count register. If it is zero, exit the iteration, and move to the next instruction.

• Acknowledge any pending interrupts.

• Perform the string operation once.

• Decrement the CX or count register by one; no flags are modified.

• Check the ZF flag is the string operation is a SCAS or CMPS instruction. If the repeat condition does not hold, exit the iteration and move to the next instruction. Exit the iteration if the prefix if REPE and the ZF flag is 0 (the last comparison was not equal), or if the prefix is REPNE and the ZF flag is one (the last comparison was equal).

• Return to step 2 for the next iteration.

Repeated CMPS and SCAS instructions can be exited if the count is exhausted or if the ZF 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 ZF flag (the JZ, JNZ, and JNE instructions).