Stack Frame Calc Problem in head_4xx.S

[Gary D. Thomas]

On Sat, 2003-05-03 at 15:20, Jerry Walden wrote:
> Greetings:
>
> I am having trouble understanding what is happening to my stack pointer.
>
> At line 1 r1 = 0x03f9_ebe8
>
> After line 15 executes r1 = 0xc00f4ff0
> which seems fine so far (according to the map file it is pointing to the
> proper location)
>
> After line 16 executes r1=0xc00f6ff0
> which is still within the bounds of init_task_union
>
> After line 17 execute r1 = 0xc00f6fe0 which seems like a problem to me,
> because it is not with the
> bounds of init_task_union - (see map file below)
>

Why do you think this?  It seems that init_task_union comprises the
space from 0xc00f4ff0..0xc00f6ff0.  That value is certainly within
that range.

> I would expect r1 to be within the bounds of init_task_union after this code
> is executed -
> is my guess correct?  If so how is it possible that line 17 comes up with
> the result
> that it did?

Read how 'stwu' works.  After the store takes place, r1 is updated
with the new value.  In other words
  stwu r0,-STACK_FRAME_OVERHEAD(r1)
means
  [r1] = r0
  r1 = r1 - STACK_FRAME_OVERHEAD(r1)
which is exactly the results you are getting.

>
> TASK_UNION_SIZE = 8192
> STACK_FRAME_OVERHEAD = 16
>
> Thanks for any help
>
> Jerry
>
> 1 start_here:
> 2
> 3	/* ptr to current */
> 4	lis	r2,init_task_union at h
> 5	ori	r2,r2,init_task_union at l
> 6
> 7	/* ptr to phys current thread */
> 8	tophys(r4,r2)
> 9	addi	r4,r4,THREAD	/* init task's THREAD */
> 10	mtspr	SPRG3,r4
> 11	li	r3,0
> 12	mtspr	SPRG2,r3	/* 0 => r1 has kernel sp */
> 13
> 14	/* stack */
> 15	addi	r1,r2,TASK_UNION_SIZE
> 16	li	r0,0
> 17	stwu	r0,-STACK_FRAME_OVERHEAD(r1)
>
>
> c00f4ff0 D init_task_union
> c00f6ff0 d aligninfo
> c00f70f0 D cpuinfo_op
> c00f7100 D cpu_specs
> c00f7280 D ppc_htab_operations
>
>
>
> Jerry Walden
> Program Manager
> Digital Atlantic Inc
> http://www.digitalatlantic.com
> jwalden at digitalatlantic.com
> 1-877-494-6073 x407
> cell - 703-431-2413
>
>
--
Gary D. Thomas <gary.thomas at mind.be>


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