blinker01

Here is a little sample GPU program that flashes the OK or status light.

Installation:

It runs in place of bootcode.bin.

Download the blinker01.bin here and replace the usual bootcode.bin file on the SD card you use to boot your pi. Don't forget to keep a backup of your old bootcode.bin so you can set it back to normal operation!

Now when the GPU starts up, it looks for a program called bootcode.bin - so you'll have to rename blinker01.bin to bootcode.bin.

How it works:

The boot routine in the GPU doesn't use the first 0x200 bytes as code, so we fill it with padding:

00000000: 0000 0000 0000 0000: ..     :0000           ; zero bit instruction
00000002: 0000 0000 0000 0000: ..     :0000           ; zero bit instruction
...
000001fe: 0000 0000 0000 0000: ..     :0000           ; zero bit instruction

Now we configure the GPIO so that the pin driving the status led is configured as an output signal by writing to GPFSEL1. (see page 90 of http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf). Notice how the GPU uses bus addresses starting at 0x7e000000.

00000200: 1110 1000 0000 0001: .... ~ :e801 0004 7e20 ; mov r1, #0x7e200004
00000206: 0010 0000 0001 0000: .      :2010           ; ld  r0, 0x00(r1)
00000208: 1110 1000 1110 0000: ...... :e8e0 ffff ffe3 ; and r0, #0xffe3ffff
0000020e: 1110 1001 1010 0000: ...... :e9a0 0000 0004 ; or r0, r0, #0x00040000
00000214: 0011 0000 0001 0000: .0     :3010           ; st  r0, 0x00(r1)

Set up some registers ready for the main loop:

00000216: 1110 1000 0000 0001: .... ~ :e801 001c 7e20 ; mov r1, #0x7e20001c
0000021c: 1110 1000 0000 0010: ..(. ~ :e802 0028 7e20 ; mov r2, r0, #0x7e200028
00000222: 1110 1000 0000 0011: ...... :e803 0000 0001 ; mov r3, r0, #0x00010000

Now lets begin the main loop. First we turn the led off (or on depending on active high/low - I forget :) ) by writing to GPSET0 (0x7E20001C) to set the output high.

00000228: 0011 0000 0001 0011: .0     :3013           ; st  r3, 0x00(r1)

Loop to delay for a while:

0000022a: 1110 1000 0000 0000: ...... :e800 0000 0000 ; mov r0, #0x00000000
00000230: 1110 1000 0100 0000: @..... :e840 0001 0000 ; add r0, r0, #0x00000001
00000236: 1110 1001 0100 0000: @..... :e940 0000 0010 ; cmp r0, r0, #0x00100000
0000023c: 0001 1000 1111 1010: ..     :18fa           ; bne 0x00000230

Turn the led on (or off depending on active high/low - I forget :) ) by writing to GPCLR0 (0x7e200028) to clear the output low.

0000023e: 0011 0000 0010 0011: #0     :3023           ; st  r3, 0x00(r2)

Loop to delay for a while:

00000240: 1110 1000 0000 0000: ...... :e800 0000 0000 ; mov r0, #0x00000000
00000246: 1110 1000 0100 0000: @..... :e840 0001 0000 ; add r0, r0, #0x00000001
0000024c: 1110 1001 0100 0000: @..... :e940 0000 0010 ; cmp r0, r0, #0x00100000
00000252: 0001 1000 1111 1010: ..     :18fa           ; bne 0x00000246

Loop back to do it all again:

00000254: 0001 1111 0110 1010: j.     :1f6a           ; b 0x00000228