Some registers are protected, some are free game.  For details, see the
ABI (Application Binary Interface) page 3-14 and EABI (Embedded
Application Binary Interface) specifications:
   http://www.esofta.com/softspecs.html

gvb


At 04:33 PM 9/25/00 -0400, Zhaobin Zhu wrote:

Hi,

This is a newbie question regarding (PowerPC) calling a C routine
within a .S
file:

Let me take an example:

in arch/ppc/kernel/head.S,
...
        mr      r3,r31
        mr      r4,r30
        mr      r5,r29
        mr      r6,r28
        mr      r7,r27
        bl      identify_machine
...
The code calls rountine identify_machine(..) in setup.c.

Questions:

   The identify_machine(..) in setup.c will certainly use some of the
registers
(r0-r31).
   Will the compiler protect the registers used in the routine (for
example
   push them into stack before entering the routine and pop them out
   after exiting the routine ) ?

   If yes, how the compiler does the protection ? Will it simply save
all the
regs
   into stack before entering routine and restore them after
returning from the

   routine (except R3 wiich may be used as a pointer to some returned
value).

   If compiler does no reg protection, then there is no gurantee that the
register set
   will be the same before and after calling a C routine. It's the
calling
code's
   responsibility to make sure some important regs will not be
altered after
return
   from a C routine.

Thanks,

--
 Zhaobin Zhu
 zzhu@emc.com

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