On Fri 2008-02-08 13:02:57, H. Peter Anvin wrote:
> Pavel Machek wrote:
>> On Fri 2008-02-08 17:23:15, Rafael J. Wysocki wrote:
>>> On Friday, 8 of February 2008, Pavel Machek wrote:
>>>> Hi!
>>> Hi,
>>>
>>>>>> I really need the entry point to be at offset 0, so that I can get
>>>>>> pointers to my data. I could not figure out how to do it any other
>>>>>> way. And if 0 is taken, I thought I'd put header at the end.
>>>>>>
>>>>> Why not just put the structure at 0, and put pointers in the structure
>>>>> to everything else you need?
>>>> segments:offsets rear its ugly head here. I need %ds to point to my
>>>> data, and the way to do it is copy it from %cs; that needs start to be
>>>> at 0.
>>> Hm, why exactly is that necessay?
>>
>> It is not _neccessary_. Try to come up with another method that gets
>> relocations right. I could not :-(.
>>
>> (Actually, putting table at the offset 0 and short jump at beggining
>> of the table would probably do the trick. But that still keeps code at
>> offset 0 :-).
>
> Why not just put a pointer to the start of the code in the table, and make
> an indirect call to it?
Indirect call from where?
BIOS jumps to address you provide.
> Where is this code?
arch/x86/kernel/acpi/realmode/wakeup.S
BIOS jumps to wakeup_code. With CS=something, IP=0. If wakeup code is
at other address than zero, I'll not be able to easily address data
below it.
Pavel
.globl wakeup_header
wakeup_header:
...
realmode_flags: .long 0
signature: .long 0x51ee1111
...
.text
.globl _start
.code16
wakeup_code:
_start:
cli
cld
/* Set up segments */
movw %cs,%ax
movw %ax,%ds
movw %ax,%es
movw %ax,%ss
...
/* Check header signature... */
movl signature, %eax
cmpl $0x51ee1111, %eax
jne bogus_real_magic
Pavel
--
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http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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