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
