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understanding head.S

To: linux-mips@linux-mips.org
Subject: understanding head.S
From: "Chetan Nanda" <chetannanda@gmail.com>
Date: Thu, 5 Jun 2008 15:32:42 +0530
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Hi All,
I was reading the boot code for MIPS.
in head.S file before jumping to 'start_kernel' it calculate the stack
pointer address as follow:

"
        PTR_LA          $28, init_thread_union
        PTR_ADDIU       sp, $28, _THREAD_SIZE - 32
        set_saved_sp    sp, t0, t1
        PTR_SUBU        sp, 4 * SZREG           # init stack pointer
"

Can anyone please explains me this 4 lines of code?
Why ' _THREAD_SIZE - 32' is added in 'sp' ?
What 'set_saved_sp' will do ?
and then why we subtract '4 * SZREG' from 'sp' ?

Please hep me to understand this code better.

Thanks,
Chetan Nanda

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