Greetings:
I apologize in advance if this is a stupid question, or a question
with an obvious answer. The fact is that I have place linux on
and ARM, and an X86 board, however never on a PPC platform. In
fact I have very little experience with the PPC, and RISC processors
in general. I am trying to trace my way through the u-boot code -
since I do not have access to an in circuit emulator, I am somewhat
hindered. However I do have the capability to turn on a LED via a
GPIO register. So I am tracing through the code in a "burn an learn"
fashion.
I have a routine called "led" that turns on an led. The code below is
from start.S from the u-boot distro from the cpu/ppc4xx, my processor is
a ppc 405GPr.
The below code turns the LEN on. Note the branch instruction that branches
to some code to turn the led on. If I put the branch instruction AFTER
the "mtmsr r4" instruction the LED does not turn on. I cannot figure out
what is going on. If anyone has any recommendations on documentation to
read (I have the 600 page "green book") or if there is something I am
obviously missing (i.e. not being familiar enough with how a risc
processor works) any advice is welcome.
. = EXC_OFF_SYS_RESET
.globl _start
_start:
/* Clear and set up some registers. */
addi r4,r0,0x0000
mtspr sgr,r4
mtspr dcwr,r4
mtesr r4 /* clear Exception Syndrome Reg */
mttcr r4 /* clear Timer Control Reg */
mtxer r4 /* clear Fixed-Point Exception Reg */
mtevpr r4 /* clear Exception Vector Prefix Reg */
addi r4,r0,0x1000 /* set ME bit (Machine Exceptions) */
oris r4,r4,0x0002 /* set CE bit (Critical Exceptions) */
/* the LED will turn on if I branch here */
b LED
mtmsr r4 /* change MSR */
/* the LED will NOT turn on if I branch here */
/*b LED */
addi r4,r0,(0xFFFF-0x10000) /* set r4 to 0xFFFFFFFF */
/* dbsr is cleared by setting bits to
1) */
mtdbsr r4 /* clear/reset the dbsr */
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