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