学 Win32 汇编[12]: PTR、OFFSET、ADDR、THIS

PTR: 指定要操作的数据尺寸

---

; Test12_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



.data

    val db 11h, 22h, 33h, 44h, 55h, 66h, 77h, 88h



.code

main proc

    xor eax, eax             ;清空 EAX, 同 mov eax, 0

    mov eax, dword ptr val   ;

    PrintHex eax             ;44332211

    

    xor eax, eax             ;

    mov eax, dword ptr val+1 ;

    PrintHex eax             ;55443322

    

    xor eax, eax             ;

    mov ax, word ptr val     ;

    PrintHex eax             ;00002211

    

    xor eax, eax             ;

    mov al, byte ptr val     ;

    PrintHex eax             ;00000011

    ret

main endp

end main


 
   

OFFSET: 获取全局变量或标号的偏移地址

---

; Test12_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



.data

    v1 db 'abcdefg', 0

    v2 dd 11223344h



.code

main proc

    PrintHex offset v1    ;00403000

    PrintHex offset v2    ;00403008

    PrintHex offset main  ;00401000 - 这里的 main 是个标号

    ret

;本例中的 offset 不能用 addr 代替

main endp

end main


 
   

ADDR: 类似 offset 也是获取变量的地址...

---

; Test12_3.asm

.386

.model flat, stdcall



;include    windows.inc

include    kernel32.inc

includelib kernel32.lib

include    user32.inc

includelib user32.lib



.data

    v1 dd 00434241h ;ABC

    v2 dd 00636261h ;abc



.code

main proc

    invoke MessageBox, 0, offset v1, offset v2, 0 ;现在 v1、v2 是全局变量

    invoke MessageBox, 0,   addr v2,   addr v1, 0 ;使用 offset 和 addr 均可

    invoke ExitProcess, 0

main endp

end main


 
   

获取局部变量的地址只能使用 ADDR:

---

; Test12_4.asm

.386

.model flat, stdcall



;include    windows.inc

include    kernel32.inc

includelib kernel32.lib

include    user32.inc

includelib user32.lib



.code

main proc

    LOCAL v1,v2

    mov v1, 00434241h

    mov v2, 00636261h

    ;invoke MessageBox, 0, offset v1, offset v2, 0  ;offset 不能获取局部变量的地址

    invoke MessageBox, 0,   addr v2,   addr v1, 0

    invoke ExitProcess, 0

main endp

end main


 
   

OFFSET 和 ADDR 的异同:
1、offset 不能获取局部变量的地址;
2、addr 只能用于调用函数(invoke)时, 不能用于赋值操作;
3、addr 面对局部变量时会转换为 lea 等指令, addr 面对全局变量时则直接调用 offset;
4、在 invoke 中应尽量使用 addr, 其他只用 offset.

---

THIS:

---

; Test12_5.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



.data

    TextAddr equ this byte   ;伪指令 this 可让当前变量和下一个变量同址 

    szText db 'Asm', 0

.code

main proc

    PrintHex offset szText   ;00403000

    PrintHex offset TextAddr ;00403000

    

    PrintString szText       ;Asm

    mov [TextAddr], 'a'      ;给 TextAddr 赋值

    PrintString szText       ;asm

    ret

main endp

end main