Re: asm and nasm newbie


wolfgang kern wrote:
> Zhang Huan wrote:
>
> | do you mean that for such code
> | jmp addr32
> | by adding 66h prefix, the operand will be only 16bit and, the high
> | 16bit is ignored
>
> No. I better hadn't mentioned this rare usable options :)
> I once used it for tricky mixed 16/32 code links,
> but I soon replaced it by more flexible ways.

when i knew the 66h prefix, i wondered whether it can help me. actually
i must write an INT13h handler, an implementing a simplied RPC
protocol. it would be much better if i can implement it in c, for time
is so limited. however gcc cannot correctly compile c to 16bit-code in
the case of memory access. but i think i can do something to change the
assemble code gcc compiled.

>
> If you use (assuming 16 bits by default yet):
>
> 66 e9 xx xx xx xx jmp +x ;then the branch offset is also affected.
>
> But as the jumped target cannot be outside the 64KB segment,
> it is somehow useless in 16-bit mode.
>

sorry for my poor english :), do you mean that high 16bit address is
ignored??

and how to add such prefix, for example, in gas

> | and what about the "call" instruction
>
> Same as for jump, but rare recomendable!
> the pushed return address is 32-bits wide then,
> this means it will need a RETD (66 c3) to come back.
>
> The opposite, if in 32-bit mode:
>
> 66 e9 xx xx jmp+x ;will work as jump to Zero-page
> ;because the high word of EIP becomes truncated
>
> | and what about the "call" instruction
>
> Same as for jump, but not recomendable at all!
> the pushed return address is just 16 bits wide then.
>
> I hope I didn't add too many confusion with the uncommon...
>
no, you're justing "unconfusing" me :)

> __
> wolfgang

.

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