From: Ed Beroset 
Subject: Re: Directly writing to m
Brian McCloud wrote:
> 
> EB>> SM> (assuming, of course, that mov ax,ss:[sp] worked ;)
> EB>>
> EB>>DG> Why wouldn't it?
[...]
> EB>there is no way to specify [sp] as a destination address.
> 
> However, in 32-bit addressing modes, the SIB byte allows ss:[esp] as a
> destination.  So,
>         mov     ss:[esp], eax
> encodes as (hexidecimal, assuming a Use32 segment)
>   8B 04 24
That's certainly true, but [esp] and [sp] are not the same thing. 
There's a way to use [esp] but not [sp] as a destination address.  Sorry
if that wasn't clear.
Ed
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