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From: Nir Mor<nir.mor@g...>
Date: Thu Feb 10 15:05:57 CET 2005
Subject: [openrisc] Exception General Purpuse Register for first-level exception.
First level first-level exception handlers can only use relative to r0
addressing, It is a limit on one processor and a problem on SMP
environment since each CPU need to have it own information.

I suggest to add two System SPR registers, with R/W access in SM mode
to help the software.

EGPR0 = Exception General Purpuse Register 0
EGPR1 = Exception General Purpuse Register 1

First level exception would look like this:

set the first-level exception Stack Pointer (after reset):

l.movhi r1,hi(first_level_exception_stack)
l.ori r1,lo(first_level_exception_stack)
l.mtspr r0,r1,OR1200_SPR_EGPR1

Put CPU information structure on the stack:
l.sw r3,(CI_INFO)r1 /* Store CPU Infomation pointer to stack*/

Dispatch first-level exception:

l.mtspr r0, r1,OR1200_SPR_EGPR0 /* Save Stack Pointer to EGPR0 */
l.mfspr r1, r0,OR1200_SPR_EGPR1 /* Load Exception Stack Pointer from EGPR1*/
l.sw (GPR2)r1, r2 /* Save Frame Pointer*/
l.mfspr r2,r0,OR1200_SPR_EGPR0 /* Load previous Stack Pointer from EGPR0 */
l.sw (GPR1)r1,r2 /* Save previous Stack Pointer */
l.add r2,r1,new_frame_value_reteleve_to_stack_pointer /* Calculate
new frame */
l.sw (GPR3)r1,r3 .
l.sw (GPR4)r1,r4 .
.
.
l.sw (GPR31)r1,r31

Get CPU information structure:

l.lwz (CI_INFO)r1,r3 /* Load CPU Infomation pointer from stack*/

This suggestion is inspired from powerpc :

"SPRG0-SPRG3 - Only the 600 family has these general-purpose SPR
registers, which the program can use as needed. Conventionally, for
the 600 family processors SPRG0 is reserved as register containing the
value for a stack pointer to be used by the first-level exception
handler. SPRG1 is then used in first-level exception handlers to save
a GPR, which is then used as a stack pointer for saving other
registers by copying the content of SPRG0 to the saved GPR. The EABI
specifies that GPR1 is used as the stack pointer and is always valid,
so saving it using by means of an SPRG, or any other method, is not
necessary when running EABI compliant programs. "

http://www.xtrj.org/ppc/ppc2.htm

Best Regards.

Nir Mor.