initial stack pointer setup (in head.S)

[h h <harsh512@xxxxxxxxx>]

Hi,

I am trying to understand how does the stack pointer (for kernel itself) get setup in head.S.  I am specifically looking at MIPS CPU.  Here's the code fragment from arch/mips/kernel/head.S :
        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

I looked at PPC implementation and it too seemed to be using init_thread_union and THREAD_SIZE for setting up sp.  

Can somebody here explain what is going on here?  

Thanks!

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