CBW、CWDE、CDQ、CWD: 扩展指令

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, stdcallinclude    windows.incinclude    kernel32.incinclude    masm32.incinclude    debug.incincludelib kernel32.libincludelib masm32.libincludelib debug.lib.codemain 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    retmain endpend 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, stdcallinclude    windows.incinclude    kernel32.incinclude    masm32.incinclude    debug.incincludelib kernel32.libincludelib masm32.libincludelib debug.lib.codemain proc    mov al, 68    cbw    PrintHex ax ;0044    PrintDec ax ;68        mov al, -68    cbw    PrintHex ax ;FFBC    PrintDec ax ;-68    retmain endpend main

CBW、CWDE 和 MOVSX 的关系

---

; Test25_3.asm.386.model flat, stdcallinclude    windows.incinclude    kernel32.incinclude    masm32.incinclude    debug.incincludelib kernel32.libincludelib masm32.libincludelib debug.lib.codemain 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    retmain endpend main