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 LED 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 */