|
| |
Repeat String Instruction
|
86/88 |
Y |
186 |
Y |
286 |
Y |
386 |
Y |
486 |
Y |
REP string_instruction |
|
Ovfl |
N |
Dir |
N |
Int |
N |
Trap |
N |
Sign |
N |
Zero |
N |
Aux |
N |
Prty |
N |
Carry |
N |
REP is a prefix that may be specified before any string instruction
(CMPS, LODS, MOVS, SCAS, and STOS). REP causes the string instruction
following it to be repeated, as long as CX does not equal 0; CX is
decremented after each execution of the string instruction. (For CMPS
and SCAS, REP will also terminate the loop if the Zero Flag is clear
after the string instruction executes.)
Notes: If CX is initially 0, the REPeated instruction is
skipped entirely.
The test for CX equal to 0 is performed before the
instruction is executed. The test for the Zero Flag
clear (done only for CMPS and SCAS) is performed
after the instruction is executed.
REP, REPE (Repeat While Equal), and REPZ (Repeat
While Zero) are all synonyms for the same
instruction.
REPNZ (Repeat Not Zero) is similar to REP, but when
used with CMPS and SCAS, will terminate with the
Zero Flag set, rather than cleared.
REP is generally used with the MOVS (Move String)
and STOS (Store String) instructions; it can be
thought of as "repeat while not end of string."
You do not need to initialize ZF before using
repeated string instructions.
A REPeated instruction that is interrupted between
repeats will correctly resume processing upon return
from the interrupt. However, if other prefixes are
used on a single instruction (for example, segment
override or LOCK) in addition to the REP, all
prefixes except the one that immediately preceded
the string instruction will be lost. Therefore, if
you must use more than one prefix on a single
instruction, you should disable interrupts before
the instruction, and enable them afterwards. Note
that even this precaution will not prevent a non-
maskable interrupt, and that lengthy string
operations may cause large delays in interrupt
processing.
------------------------------------ Timing ----------------------------------
OpCode Instruction 386 286 86
F3 6C REP INS r/m8, DX 13+6*CX 5+4*CX
F3 7D REP INS r/m16, DX 13+6*CX 5+4*CX
F3 7D REP INS r/m32, DX 13+6*CX
F3 A4 REP MOVS m8, m8 5+4*CX 5+4*CX 9+17*CX
F3 A5 REP MOVS m16, m16 5+4*CX 5+4*CX 9+17*CX
F3 A5 REP MOVS m32, m32 5+4*CX
F3 6E REP OUTS DX, r/m8 5+12*CX 5+4*CX
F3 6F REP OUTS DX, r/m16 5+12*CX 5+4*CX
F3 6F REP OUTS DX, r/m32 5+12*CX
F3 AA REP STOS m8 5+5*CX 4+3*CX 9+10*CX
F3 AB REP STOS m16 5+5*CX 4+3*CX 9+10*CX
F3 AB REP STOS m32 5+5*CX
F3 A6 REPE CMPS m8, m8 5+9*N 5+9*N 9+22*N
F3 A7 REPE CMPS m16, m16 5+9*N 5+9*N 9+22*N
F3 A7 REPE CMPS m32, m32 5+9*N
F3 AE REPE SCAS m8 5+8*N 5+8*N 9+15*N
F3 AF REPE SCAS m16 5+8*N 5+8*N 9+15*N
F3 AF REPE SCAS m32 5+8*N
------------------------------------ Logic -----------------------------------
while CX <> 0 ; for MOVS, LODS, STOS
execute string instruction
CX = CX - 1
while CX <> 0 ; for CMPS, SCAS
execute string instruction
CX = CX - 1
if ZF = 0 terminate
See Also REPNE MOVS STOS CMPS SCAS LODS REPE CLD STD |