dwelch67 / Rpigpu

Start here to see a possible instruction set for the Raspberry Pi GPU

https://github.com/hermanhermitage/videocoreiv/

disclaimer: I have no other information other than what is publicly available, the information in the above repo and the Broadcom documents at the Raspberry Pi site. I have done my own experiments as needed to see that the instructions on the above site execute as described on the above site.

Based on the above information I did some experiments, the assembly was tedious, adding instructions (to the tinyasm) as I needed them, etc. Historically I have been a failure at making compiler backends. Some day I wont be. I have had success with static binary translation though, and that is the path I took to give myself some sort of compiler.

So what you will see here at least initially are some examples written in C, compiled to thumb (ARM's 16 bit instruction set, original thumb, not thumb2 extensions), the thumb binary is disassembled, as it is disassembled raspberry pi gpu machine code is created in its place.

This is not beginner stuff in any way, this is not a generic C compiler for the Raspberry Pi gpu. This is for people are capable of writing their programs in both thumb MACHINE CODE and gpu MACHINE CODE but want to save some time. So far every example has required more work on the translator. It took three clean slate starts to get this far. I just realized that I have been treating immediates as unsigned when they are signed, so I have to go back through and fix a bunch of stuff. For the examples I have so far it does appear to work.

I have a bootloader for the gpu that I wrote using tinyasm but am not publishing it, it greatly helps with development though. When I get a C based, translated, bootloader I will publish that. To be honest the only reason I started on this translator was to make a bootloader using C in some form not all asm.

Understand: The Raspberry Pi boots like this, there is what I assume is a flash, OTP or not, in the part that the gpu boots off of, it could be hardware and not the gpu, I have no idea, doesnt matter. The first file loaded is bootcode.bin, you can replace bootcode.bin with your own, but you wont have any sdram. I dont know how much memory is on chip that bootcode.bin is able to boot and use, have not figured that out yet. The Raspberry Pi project bootcode.bin appears to initialize sdram. The next thing that happens is loader.bin is loaded, this is also gpu code. If you replace loader.bin with your program then you will have sdram. At least on entry to both bootcode.bin and loader.bin the gpu is running at the crystal rate which from the schematic is 19MHz, and experiments confirm that (dividing that clock down for the uart, blinking leds, etc).

From the published documents the ARM's view of peripherals is from address 0x20000000, and it shows that to be 0x7E000000 for the GPU and that appears to work. I have no idea how to load the ARM with the GPU nor do I know how to change the system clock to the 250Mhz (or whatever) that we are used to with the stock gpu firmware.

So starting with the blinkers then the uarts and eventually a bootloader these programs are progressively helping me develop the translator. Bootloader01 is working both with gcc and llvm as compilers. I dont know if it is sad or funny that when things dont work you assume it is the translator, put time in to debugging to find that the C code being compiled then translated has the bug (forgot to setup the rx gpio pin). It didnt help that I was using a bootloader written in asm which had already setup the uart and pins correctly so the fact that the test program didnt, wasnt standing out as a problem.

If you do nothing else check out the tinyasm/asm.c file in the above repo. It a mixture of pure genius and mad scientist. I love it, I may model all of my assemblers after that in the future. I have taken it and ripped out everything but the emits to create the doemit.c program I use to aid in the translation.