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CBW、CWDE、CDQ、CWD: 扩展指令

;CBW(Convert Byte to Word):             将 AL 扩展为 AX
;CWDE(Convert Word to Extended Double): 将 AX 扩展为 EAX
;CDQ(Convert Doubleword to Quadword):   将 EAX 扩展为 64 位数 EDX:EAX
;CWD(Convert Word to Doubleword):       将 AX 扩展为 DX:AX

;它们都是用符号位填充多出的空间

;它们对 EFLAGS 无影响, 也无参数

; Test25_1.asm .386 .model flat, stdcall include windows.inc include kernel32.inc include masm32.inc include debug.inc includelib kernel32.lib includelib masm32.lib includelib debug.lib .code main proc ;CBW mov al, 7Fh cbw PrintHex ax ;007F mov al, 80h cbw PrintHex ax ;FF80 ;CWDE mov ax, 7FFFh cwde PrintHex eax ;00007FFF mov ax, 8000h cwde PrintHex eax ;FFFF8000 ;CDQ mov eax, 7FFFFFFFh cdq PrintHex edx ;00000000 PrintHex eax ;7FFFFFFF mov eax, 80000000h cdq PrintHex edx ;FFFFFFFF PrintHex eax ;80000000 ;CWD mov ax, 7FFFh cwd PrintHex dx ;0000 PrintHex ax ;7FFF mov ax, 8000h cwd PrintHex dx ;FFFF PrintHex ax ;8000 ret main endp end main

符号扩展指令的本质

;一个正数(无符号)或负数(有符号)在扩展储存空间时, 使用这些指令可保证原值不变

; Test25_2.asm
.386
.model flat, stdcall

include    windows.inc
include    kernel32.inc
include    masm32.inc
include    debug.inc
includelib kernel32.lib
includelib masm32.lib
includelib debug.lib

.code
main proc
    mov al, 68
    cbw
    PrintHex ax ;0044
    PrintDec ax ;68
    
    mov al, -68
    cbw
    PrintHex ax ;FFBC
    PrintDec ax ;-68
    ret
main endp
end main


CBW、CWDE 和 MOVSX 的关系

; Test25_3.asm
.386
.model flat, stdcall

include    windows.inc
include    kernel32.inc
include    masm32.inc
include    debug.inc
includelib kernel32.lib
includelib masm32.lib
includelib debug.lib

.code
main proc
    ;CBW 等价于 movsx ax, al
    mov al, 80h
    cbw
    PrintHex ax ;FF80
    
    mov al, 80h
    movsx ax, al
    PrintHex ax ;FF80
    
    ;CWDE 等价于 movsx eax, ax
    mov ax, 8000h
    cwde
    PrintHex eax ;FFFF8000
    
    mov ax, 8000h
    movsx eax, ax
    PrintHex eax ;FFFF8000
    ret
main endp
end main


posted on 2010-04-15 15:13  万一  阅读(...)  评论(...编辑  收藏