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

Vol. 2B 4-251
INSTRUCTION SET REFERENCE, N-Z
be determined either by testing the count register with a JECXZ instruction or by
testing the ZF flag (with a JZ, JNZ, or JNE instruction).
When the REPE/REPZ and REPNE/REPNZ prefixes are used, the ZF flag does not
require initialization because both the CMPS and SCAS instructions affect the ZF flag
according to the results of the comparisons they make.
A repeating string operation can be suspended by an exception or interrupt. When
this happens, the state of the registers is preserved to allow the string operation to
be resumed upon a return from the exception or interrupt handler. The source and
destination registers point to the next string elements to be operated on, the EIP
register points to the string instruction, and the ECX register has the value it held
following the last successful iteration of the instruction. This mechanism allows long
string operations to proceed without affecting the interrupt response time of the
system.
When a fault occurs during the execution of a CMPS or SCAS instruction that is
prefixed with REPE or REPNE, the EFLAGS value is restored to the state prior to the
execution of the instruction. Since the SCAS and CMPS instructions do not use
EFLAGS as an input, the processor can resume the instruction after the page fault
handler.
Use the REP INS and REP OUTS instructions with caution. Not all I/O ports can handle
the rate at which these instructions execute. Note that a REP STOS instruction is the
fastest way to initialize a large block of memory.
In 64-bit mode, default operation size is 32 bits. The default count register is RCX for
REP INS and REP OUTS; it is ECX for other instructions. REX.W does not promote
operation to 64-bit for REP INS and REP OUTS. However, using a REX prefix in the
form of REX.W does promote operation to 64-bit operands for other
REP/REPNE/REPZ/REPNZ instructions. See the summary chart at the beginning of
this section for encoding data and limits.
Operation
IF AddressSize = 16
THEN
Use CX for CountReg;
ELSE IF AddressSize = 64 and REX.W used
THEN Use RCX for CountReg; FI;
ELSE
Use ECX for CountReg;
FI;
WHILE CountReg
≠ 0
DO
Service pending interrupts (if any);
Execute associated string instruction;
CountReg ← (CountReg – 1);
IF CountReg
= 0