>You may be running code in kseg0, but you're apparently trying to load from
>or store to one of the TLB-mapped segments. Be careful about what
>assumptions you make about where that exception is coming from. You could
>be triggering some other exception, then that exception could be causing a
>TLB miss, possibly as a result of going haywire (or possibly normal). If
>you don't have the exception handlers going into an infinite loop after
>turn on the LED blink, they're going to go on their merry way, handle the
>exception as best they can, then return (or panic and die if they discover
>something really bad).
Good thinking. I put the exceptions into inifinite loop after the blink
code. That showed me that the first exception I'm getting is an "other"
Interrupts are disabled, so it's not one of those. But if I can't even jump
to a function (!), I'm miles away from dumping the exception info registers
to the screen or serial port to figure out what the "other" is.
So then I decided to set up a "heartbeat" blink (distinguishable from all
other blinks - dit, pause, dit) right off.
Aside: What would I do without the LED?
Then I put an infinte loop right *before* jal prom_init.
Heartbeat came on and continued as expected.
Then I removed the infinite loop right before prom_init and put one first
thing *inside* promp_init. Guess what? I get an "other" exception.
What? Check this out. It's not even touching the stack now, and it still
// disable interrupts
80000784: 40086000 mfc0 $t0,$12
80000788: 35080001 ori $t0,$t0,0x1
8000078c: 39080001 xori $t0,$t0,0x1
80000790: 40886000 mtc0 $t0,$12
// setup heartbeat
80000794: 24020001 li $v0,1
80000798: 3c01ab00 lui $at,0xab00
8000079c: a4220240 sh $v0,576($at)
800007a0: 24020010 li $v0,16
800007a4: 3c01ab00 lui $at,0xab00
800007a8: a4220242 sh $v0,578($at)
800007ac: 24020001 li $v0,1
800007b0: 3c01ab00 lui $at,0xab00
800007b4: a4220248 sh $v0,584($at)
// probe the cpu
800007b8: 1000001e b 80000834 <cpu_probe>
800007bc: 00000000 nop
// setup the stack
800007c0: 3c1c8000 lui $gp,0x8000
800007c4: 279c6000 addiu $gp,$gp,24576
800007c8: 27881fe0 addiu $t0,$gp,8160
800007cc: 3c018008 lui $at,0x8008
800007d0: ac2880cc sw $t0,-32564($at)
800007d4: 251dfff0 addiu $sp,$t0,-16
// clear bss
800007d8: 3c088008 lui $t0,0x8008
800007dc: 25086a30 addiu $t0,$t0,27184
800007e0: ad000000 sw $zero,0($t0)
800007e4: 3c09800a lui $t1,0x800a
800007e8: 2529a40c addiu $t1,$t1,-23540
800007ec: 25080004 addiu $t0,$t0,4
800007f0: 1509fffe bne $t0,$t1,800007ec <probe_done+2c>
800007f4: ad000000 sw $zero,0($t0)
// when an infinite loop is placed here, the heartbeat continues
// when one is not here, I get an "other" exception
// jump and link to prom_init
800007f8: 0c01db5b jal 80076d6c <prom_init>
800007fc: 00000000 nop
// adjust the stack pointer for local vars
80076d6c: 27bdfff0 addiu $sp,$sp,-16
// endless loop
80076d70: 0801db5c j 80076d70 <prom_init+4>
80076d74: 00000000 nop
How can that possibly except? Any ideas?