Re: Significance of PAGE_OFFSET

["Joseph A. Knapka" <jknapka@earthlink.net>]

Abhishek Nayani wrote:
> 
> Hi,
> 
>         The PAGE_OFFSET is used to give the Kernel
> exclusively the memory range above 3GB, but what is
> its use at bootstrap. In the provisional gdt at bootup
> time the first 4mb of linear addresses are mapped to
> the first 4mb physical addresses and from PAGE_OFFSET
> to PAGE_OFFSET + 4 mb also. why are we doing it ?
> 
>                        Bye,
>                                 Abhi.

Because the code that turns on paging (head.S) logically
relocates the kernel code from its boot-time location
to PAGE_OFFSET. That is, the code that loads
CR0 to turn on paging is executing at whatever
physical address it was loaded at, 9000h or something;
but as soon as that instruction completes, the
next instruction is executing at virtual address
PAGE_OFFSET+9000h (plus the instruction's offset
in the kernel text, naturally). So both of those
mappings have to be valid.


-- Joe


-- Joe Knapka
"You know how many remote castles there are along the gorges? You
 can't MOVE for remote castles!" -- Lu Tze re. Uberwald
// Linux MM Documentation in progress:
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