16位汇编代码中的OFFSET是什么意思? [英] What does OFFSET in 16 bit assembly code mean?

问题描述

我正在查看一些16位实模式的示例汇编代码.

I am going through some example assembly code for 16-bit real mode.

我遇到了麻烦:

    mov    bx, cs
    mov    ds, bx
    mov    si, OFFSET value1
    pop    es
    mov    di, OFFSET value2

这是做什么的?那里有"OFFSET"有什么作用?

what is this doing? What does having 'OFFSET' there do?

推荐答案

正如其他一些答案所说，offset关键字是指与定义该段的段的偏移量.但是请注意，段可能会重叠，一个段中的偏移可能会与另一段中的偏移不同.例如，假设您在实模式下具有以下细分

As some of the other answers say, the offset keyword refers to the offset from the segment in which it is defined. Note, however, that segments may overlap and the offset in one segment may be different in another segment. For instance, suppose you have the following segment in real mode

data SEGMENT USE16 ;# at segment 0200h, linear address 2000h

    org 0100h
    foo db 0

    org 01100h
    bar db 0

data ENDS

汇编器会看到foo与data SEGMENT的底距偏移0100h，因此无论在什么位置看到offset foo，它都会将值0100h放入，而不管DS的值是多少.时间.

The assembler sees that foo is at offset 0100h from the base of data SEGMENT, so wherever it sees offset foo it will put the value 0100h, regardless of the value of DS at the time.

例如，如果我们将DS更改为data段的基数以外的其他值，则汇编程序将假定:

For example, if we change DS to something other than the base of the data segment the assembler is assuming:

mov ax, 200h            ; in some assemblers you can use @data for the seg base
mov ds, ax

mov bx, offset foo          ; bx = 0100h
mov byte ptr [bx], 10       ; foo = 10


mov ax, 300h
mov ds, ax

mov bx, offset foo          ; bx = 0100h
mov byte ptr [bx], 10       ; bar = 10, not foo, because DS doesn't match what we told the assembler

在第二个示例中，DS是0300h，因此DS指向的段的基数是03000h.这意味着ds:[offset foo]指向与02000h + 01100h相同的地址03000h + 0100h，该地址指向bar.

In the second example DS is 0300h, so the base of the segment pointed to by DS is 03000h. This means that ds:[offset foo] points to the address 03000h + 0100h which is the same as 02000h + 01100h, which points to bar.

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