Messages in this thread | | | Date | Wed, 17 Sep 2003 12:42:21 -0700 | From | Ben Johnson <> | Subject | Re: linear vs. logical addresses? how does cpu interpret kernel addrs? |
| |
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!
- Ben
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |