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Re: understanding head.S

To: Chetan Nanda <chetannanda@gmail.com>, linux-mips@linux-mips.org
Subject: Re: understanding head.S
From: "Wang, Baojun" <wangbj@dslab.lzu.edu.cn>
Date: Thu, 05 Jun 2008 19:15:24 +0800
In-reply-to: <7f245da80806050302m6b449e6m1f84dc7ef7efff46@mail.gmail.com>
Organization: DSLab, Lanzhou University
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <7f245da80806050302m6b449e6m1f84dc7ef7efff46@mail.gmail.com>
Reply-to: wangbj@dslab.lzu.edu.cn
Sender: linux-mips-bounce@linux-mips.org
在 2008-06-05四的 15:32 +0530,Chetan Nanda写道:
> 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
make sp point to the stack top so that we can allocate stack space
later, -32 is a reserved area for local variable, saved GPR, arguments
(function stack frame).. due to the abi
>       set_saved_sp    sp, t0, t1
set_saved_sp is defined (as a assembler macro) in
include/asm-mips/stackframe.h (if you're using vim, I think it will be
easy to use vim with cscope..)
>       PTR_SUBU        sp, 4 * SZREG           # init stack pointer
SZREG is defined to the general purpose register size, it is 32bit in
MIPS32 and 64bit in MIPS64, so the last line allocates 16/32 bytes to
build a stack frame.. You could refer to the MIPS ABI if you need
further information (the kernel don't follow the standard ABI all the
time though..)
> 
> 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
> 
-- 
Wang, Baojun                                                Lanzhou University
Distributed & Embedded System Lab                      http://dslab.lzu.edu.cn
School of Information Science and Engeneering          wangbj@dslab.lzu.edu.cn
Tianshui South Road 222. Lanzhou 730000                             .P.R.China
Tel: +86-931-8912025                                      Fax: +86-931-8912022

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