Re: linear vs. logical addresses? how does cpu interpret kernel addrs?
From: Ben Johnson
Date: Wed Sep 17 2003 - 14:43:20 EST
On Wed, Sep 17, 2003 at 07:39:53AM -0400, Richard B. Johnson wrote:
> All stack offsets are accessed relative to SS. No exceptions.
> However a compiler may calculate those offsets based upon
> something else.
> This is why DS must equal SS if 'C' is going to access both
> stack data variables and data segment variables. This is how
> the 'C' code converter is set up. It is not a CPU limitation.
> If you change the SS in the kernel, strange and wonderful
> things will occur.
Let me see if I understand you. If SS and DS point to segments that
have different base addresses then code like this... (I'm an assembly
newbie. hope I get this right.)
# get whatever is at %ss:%esp + 4 and put it in eax
movl 4(%esp), %eax
movl %esp, %edx
# get whatever is at %ds:%edx + 4 and put it in eax
movl 4(%edx), %eax
# eax probably changed twice because while esp and edx have same value,
# if SS->baseaddr != DS->baseaddr, then (%esp) and (%edx) don't point to
# the same memory location.
I'm pretty sure I've seen plenty of code like this, which must mean,
like you just told me, that the C compiler assumes the base address of
DS and SS are the same. So, if I want to change segment base addresses
then I'm up shit creek.
Thanks very much for the info!
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