学 Win32 汇编[25] - 符号扩展指令: CBW、CWDE、CDQ、CWD

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