ASSUME在汇编程序中的含义是什么？

Question

在任何地方它都被解释为将寄存器与段绑定/关联的东西,但我想要理解什么是完全绑定的.

Answer 1

该ASSUME指令告诉汇编器您将使用哪个段寄存器来访问段.

这种"绑定"对于自动化一些常见模式很有用:

1. 它告诉汇编器哪个段寄存器用于访问变量.
   如果在内存访问期间未显式显示段寄存器,则汇编程序将使用ASSUMEd值自动向指令添加段覆盖前缀.
   如果段未被ASSUME任何段寄存器指向,则如果您尝试在该寄存器中加载/存储变量,则汇编器将失败并显示错误.

2. 它告诉汇编器关于哪个段来计算偏移量.
   如果在存储器访问中指定段寄存器,汇编器将使用段ASSUMEd作为该段寄存器来计算存储器访问的偏移量.
   请注意,尽管DSCPU在每次内存访问时都隐式使用,但DS:需要明确覆盖,以明确使用其段作为偏移量的基础.

3. 段ASSUMEd by CS是代码标签所属的段.
   您不能跳转/调用符号,除非它在段ASSUMEd中CS.

考虑下面的程序,不打算运行,只是反汇编.

.MODEL SMALL
.286

;Segment are laid out sequentially, starting from X and aligned on 16 bytes.
;
;_DATI     X
;_DATI2    X + 10h
;_DATI3    X + 20h
;
;All the variables testX are the first variables in a segment so their
;addresses are the same of their segments

_DATI SEGMENT PARA PUBLIC 'DATA' USE16

   test1 dw 0

_DATI ENDS

_DATI2 SEGMENT PARA PUBLIC 'DATA' USE16

   test2 dw 0

_DATI2 ENDS

_DATI3 SEGMENT PARA PUBLIC 'DATA' USE16

   test3 dw 0

_DATI3 ENDS

_CODE SEGMENT PARA PUBLIC 'CODE' USE16

 ;Use CS to access labels defined inside _CODE and use _CODE to compute those offsets
 ;Use DS to access names defined inside _DATI and use _DATI to compute offsets whenever DS is explicitly used as a segment register
 ;... and so on

 ASSUME CS:_CODE, DS:_DATI, ES:_DATI2

 ;NOTE: _DATI3 NOT ASSUMED!

__START__:

  ;No explicit segment override, find the segment of test1 (_DATI) and use
  ;the assumed register (DS).
  ;Assembled into mov ax, WORD PTR [0000] (A1 00 00)     
  mov ax, WORD PTR [test1]  

  ;No explicit segment override, find the segment of test2 (_DATI2) and use
  ;the assumed register (ES).
  ;Assembled into mov bx, WORD PTR es:[0000] (26 8B 1E 00 00)
  mov bx, WORD PTR [test2]

  ;Explicit segment override, use the segment assumed for ES (_DATI2) to
  ;calculate the offset (0000h).
  ;Assembled as the previous mov cx, WORD PTR es:[0000] (26 8B 0E 00 00)
  mov cx, WORD PTR es:[test2]

  ;Explicit segment override, use the segment assumed for DS (_DATI) to
  ;calculate the offset (0010h).
  ;Assembled as the previous mov dx, WORD PTR es:[0010] (8B 16 10 00)
  mov dx, WORD PTR ds:[test2]

  ;OFFSET of X is always relative to the segment X is declared in. 
  ;This is true for MASM mode only, IDEAL mode use the group

  ;Both use an offset of 0, as both test1 and test2 are the first variables
  ;of their segments
  mov ax, OFFSET test1              ;mov ax, 0000  (B8 00 00)
  mov bx, OFFSET test2              ;mov bx, 0000  (BB 00 00)

  ;No explicit segment override, find the segment of test3 (_DATI3) and
  ;use the assumed register (none)
  ;Can't assemly: error -> Can't address with currently ASSUMEd segment registers
  mov ax, WORD PTR [test3]  

  ;Explicit segment override, calculate offset of test3 with respect of the
  ;segment assumed for DS (_DATI)
  ;Offset is 20h
  mov bx, WORD PTR ds:[test3]       ;mov bx, WORD PTR [0020] (8B 1E 20 00)

  ;OFFSET operator don't use assumed register
  mov cx, OFFSET test3
_CODE ENDS

END __START__

如果你没有ASSUME CS,汇编者会抱怨

CS从当前段无法访问

因为你__START__在代码段中定义了一个标签而不是ASSUME任何地方.