STOSB 使用额外段有什么特殊原因吗? [英] Is there any special reason for STOSB to use extra segment?

问题描述

我读过 STOSB 的功能如下:

I have read that STOSB functions like this:

ES:[DI] <-- AL

如果 DF = 0 递增 DI 否则递减 DI.

If DF = 0 increment DI else decrement DI.

• 那么为什么STOSB 不会改变DS:[DI]?
• 使用额外段有什么特殊目的吗?
• 在大多数字符串指令中，我们使用额外的段.为什么?

推荐答案

那么为什么 STOSB 不改变 DS:[DI]

So why STOSB doesn't change DS:[DI]

因为这个定义会与已经使用 DS:[SI] 的 LODSB 的使用发生冲突.使用单独的段寄存器可为您提供更大的灵活性.

Because this definition would collide with the use of LODSB which already uses DS:[SI]. Using a separate segment register gives you more flexibility.

使用额外段有什么特殊目的吗?

Is there a special purpose for using extra segment?

是的.您可以在处理段时轻松地在段之间传输字节.例如，您可以使用 LODSB 从 DS:[SI] 加载 AL，修改 AL 然后存储它使用 ES:[DI] 到一个不同的段，Extra Segment，STOSB.在具有 16 位段大小和 20 位地址空间的 8086 中，这非常有用.

Yes. You can transfer bytes between segments easily while processing them. For example, you can use a LODSB loading AL from DS:[SI], modify AL and then store it to a different segment, the Extra Segment, with STOSB using ES:[DI]. In 8086 with its 16-bit segment size and a 20-bit address space this is really useful.

另一个说明用法的指令是REP MOVSB指令，它从DS:[SI]<复制一个字节序列(长度在CX中)/code> 到 ES:[DI].

Another instruction illustrating the use is the REP MOVSB instruction which copies a sequence of bytes (with its length in CX) from DS:[SI] to ES:[DI].

(如果您在复制时不需要检查每个字节，您只需使用 rep movsb 或 rep movsw 以获得比 更好的性能lods/stos 循环.)

(If you don't need to examine each byte as you copy it, you'd simply use rep movsb or rep movsw for better performance than lods/stos in a loop.)

在大多数字符串指令中，我们使用额外的段.为什么?

In most string instructions we use extra segment. why?

嗯，不是大多数，但可能大约有一半.使用另一个段寄存器为您提供了快速访问不同段的优势——您不仅限于处理一个 64KB 段中的数据，而且不必在每次访问不同段之前更改 DS 寄存器.

Well, not in most, but maybe roughly in half of them. Using another segment register gives you the advantage of quick access to different segments - you are not limited to processing data only in one 64KB segment and do not have to change the DS register before each access to a different segment.

stos 和 movs 写 es:[di]，这是有道理的，因为 DI 是目标索引"寄存器.

stos and movs write es:[di], which makes sense because DI is the "destination index" register.

cmps 和 scas 从 es:[di] 读取，这对于 scas 来说可能令人惊讶，因为它只有一个内存操作数，因此您可能希望它像 lods 一样从 ds:[si] 读取.特别是因为 SCASB 设置了来自 AL - [mem] 的标志，而不是相反，所以它就像一个 cmp，其中内存是右操作数(源)，而不是左(目标).像 cmp al, es:[di].

cmps and scas read from es:[di], which is maybe surprising for scas because it only has one memory operand so you might expect it to read from ds:[si] like lods. Especially because SCASB sets flags from AL - [mem], not the other way around, so it's like a cmp where memory is the right operand (source), not left (destination). Like cmp al, es:[di].

也许 8086 指令集的架构师想象了一个循环的用例，它执行 lods 和 scas 以在段之间实现 strcmp.

Perhaps the architect of the 8086's instruction set imagined a use-case of a loop that does lods and scas to implement strcmp between segments.

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