==================================Outline====================================
Repeat Prefixes and More String Instructions
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Repeat Prefixes and More String Instructions
p/239
'rep' 명령어는 ‘ecx’ 레지스터를 카운터로 사용한다.
'rep'를 풀어서 쓰면 다음과 같다.
rep movsb
jecxz endCopy ;skip loop if count is zero
copy: movsb ;move 1 character
loop copy ;decrement count and continue
endCopy:
'ecx' 값은 'rep' 명령어 operand가 수행되기 전에 0인지 확인되고 'ZF'플래그는 operand가 수행되고 나서 검사가 된다.
첫 번째 문자가 같지 않으면 'ecx'값과 ‘ZF' 값이 동시에 0이 아님으로 ’repz'명령어는 멈추게 된다.
//‘repz’/‘repe’는 같은 명령어이다.
교재 p/244 String search program
[sca.asm]
.386
.MODEL FLAT
ExitProcess PROTO NEAR32 stdcall, dwExitCode:DWORD
INCLUDE io.h
cr equ 0dh
Lf equ 0ah
.STACK 4096
.DATA
prompt1 BYTE "String to search? ", 0
prompt2 BYTE cr, Lf, "Key to search for? ", 0
target BYTE 80 DUP (?)
key BYTE 80 DUP (?)
trgtLength DWORD ?
KeyLength DWORD ?
lastPosn DWORD ?
failure BYTE cr, Lf, Lf, "The key does not appear in the string.", cr, Lf, 0
success BYTE cr, Lf, Lf, "The key appear in the string."
position BYTE 11 DUP (?)
BYTE " in the string.", cr, Lf, 0
PUBLIC _start
.CODE
_start: output prompt1 ;
input target, 80 ;
lea eax, target ;
push eax ;
call strlen ;
mov trgtLength, eax;
output prompt2 ;
input key, 80 ;
lea eax, key ;
push eax ;
call strlen ;
mov keyLength, eax ;
mov eax, trgtLength ;
sub eax, keyLength ;
inc eax ;
mov lastPosn, eax ;
cld
mov eax, 1 ;
whilePosn: cmp eax, lastPosn ;
jnle endWhilePosn ;
lea esi, target ;
add esi, eax ;
dec esi ;
lea edi, key ;
mov ecx, keyLength ;
repe cmpsb ;
jz found ;
inc eax ;
jmp whilePosn ;
endWhilePosn:
output failure ;
jmp quit ;
found: dtoa position, eax ;
output success ;
quit:
INVOKE ExitProcess, 0 ;
strlen PROC NEAR32 ;
push ebp ;
mov ebp, esp ;
pushf ;
push ebx ;
sub eax, eax ;
mov ebx, [ebp+8] ;
whileChar: cmp BYTE PTR [ebx], 0 ;
je endWhileChar ;
inc eax ;
inc ebx ;
jmp whileChar ;
endWhileChar:
pop ebx ;
popf ;
pop ebp ;
ret 4
strlen ENDP
END
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