Re: understanding head.S

["Wang, Baojun" <wangbj@dslab.lzu.edu.cn>]

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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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