CBW、CWDE、CDQ、CWD: 扩展指令

CBW、CWDE、CDQ、CWD: 扩展指令

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;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

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; 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

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符号扩展指令的本质

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;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

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CBW、CWDE 和 MOVSX 的关系

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; 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

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