Need Help w/ PPC Boot

[Gary Thomas]

On Thu, 2002-12-26 at 12:51, Jerry Walden wrote:
>
> 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) */

This is going to turn on these bits in the MSR: 0x00021000, probably
not what you want.  A better sequence would be:

	mfmsr	r4
	ori	r4,r4,0x1002	/* Set ME, CE */
	mtmsr	r4


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