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SubjectRe: paging question
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On Wed, 2002-02-20 at 21:26, Jason Yan wrote:
> Hi,
>
> I have a question about the code to enable to initialize page
> tables in linux/arch/i386/head.S
>
> I search the internet again and again but fail to find any answer
> so far, maybe you gurus can help me out, here it goes:
>
> 48 cld
> 49 movl $(__KERNEL_DS),%eax
> 50 movl %eax,%ds
> 51 movl %eax,%es
> 52 movl %eax,%fs
> 53 movl %eax,%gs
> 81 /*
> 82 * Initialize page tables
> 83 */
> 84 movl $pg0-__PAGE_OFFSET,%edi /* initialize page tables */
> 85 movl $007,%eax /* "007" doesn't mean with right to kill, but
> 86 PRESENT+RW+USER */
> 87 2: stosl
> 88 add $0x1000,%eax
> 89 cmp $empty_zero_page-__PAGE_OFFSET,%edi
> 90 jne 2b
>
> I remove the SMP code. According the setup.S, gdt_table is setup as
> gdt_table:
> #.quad 0x0000000000000000; // null
> #.quad 0x0000000000000000; // not used
> #.quad 0x00cf9a000000ffff; // 0x10 kernel 4GB code at 0x00000000
> #.quad 0x00cf92000000ffff; // 0x18 kernel 4GB data at 0x00000000
>
> 1) So, what's in %eax after line 49 ? 0x0 ?

0x18, its the index of the 4th item in GDT. Lines 49-53 set the segment
registers to use the settings in gdt[3];

> 2) Isn't __PAGE_OFFSET 0xC0000000 ? what's the result of $pg0-__PAGE_OFFSET ?

the physical address of $pg0, the kernel is linked to PAGE_OFFSET so it
thinks the address of $pg0 is PAGE_OFFSET+something. Suptracting
PAGE_OFFSET is the simple way to obtain the physical address.

--
// Gianni Tedesco <gianni@ecsc.co.uk>
80% of all email is a figment of procmails imagination.

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