Q: Register usage in mpc860 ports

Franz Sirl Franz.Sirl-kernel at lauterbach.com
Sat May 22 02:36:16 EST 1999

At 17:35 21.05.99 , Jim Chapman wrote:

>I am porting LinuxPPC to a custom embedded mpc860 board. I am starting
>with the mbx port and using home-built egcs-1.1.2 tools on  a Linux
>host, using the instructions in
>There seem to be register usage conflicts in the MMU TLB handlers. The
>code tries to use location 0 (according to the comment) to store a
>couple of register values, e.g. in arch/ppc/kernel/head.S
>  mtspr M_TW, r20 /* Save a couple of working registers */
>  mfcr r20
>  stw r20, 0(r0)
>  stw r21, 4(r0)
>However, I am finding that r0 isn't zero when the MMU exceptions are
>taken. Instead, it points to somewhere in the text or data segment. I
>hacked the code to save r20 & r21 to a specific memory location and that
>fixed that particular problem.

r0 represents 0 in a number of PPC instructions, lookup stw in your PPC 
assembler book.


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