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An On-Chip Bootloader

Good news: we can access external memory! The logic for my pseudo-static RAM controller is working, and big programs can finally run on hardware.

You may recall that I had previously only been accessing fake memory that was configured directly out of limited FPGA resources. I could squeeze a tiny C program in there, but not use anything like newlib, the embedded C runtime library. This new memory controller lets the moxie-based Marin SoC access 16MB of external RAM on the Nexys3 board.

When we were limited to on-chip resources, the C binary would be coupled with the synthesized logic and loaded directly into the FPGA. This means any changes to the code meant resynthesizing the logic to rebuild the FPGA bitstream (I think there are ways around this, but I never got there with my work-flow). Now that I have access to external RAM, I can separate my code from my logic.

The trick is to use an on-chip bootloader - code that is loaded with the FPGA bitstream as described above. It does some basic hardware initialization, and sends this message to the serial port:

MOXIE On-Chip Bootloader v1.0 Copyright (c) 2013 Anthony Green

Waiting for S-Record Download...

At which point I can send any program I like over my laptop's serial port in the form of an S-Record ASCII file. This generally looks like...

$ moxie-elf-gcc -Os hello.c marin.S -T../../moxie-marin.ld -o hello.elf -lnosys $ moxie-elf-objcopy -O srec --srec-forceS3 hello.elf hello.srec $ cat hello.srec > /dev/ttyUSB1

And then, back on the Nexys3 serial port I see:
Jumping to code at 0x30000000. Hello World!

A couple of things can happen now:

  • with a little bit of dejagnu hacking, we can get the GCC testsuite to run directly on hardware. The simple thing here is to just have libgloss' _exit() jump back to the on-chip bootloader @ 0x1000.
  • test the "stage-2" bootloader, u-boot. U-Boot was one of the first programs I ever ported to moxie. I've run it on the simulator, but never on hardware.

As usual, everything is in the moxie-cores github repo here:

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