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It's Alive!

There's a working hardware implementation of moxie in the wild!

Intrepid hacker Brad Robinson created this moxie-compatible core as a peripheral controller for his SoC. He had been using a simple 8-bit core, but needed to address more memory than was possible with the 8-bit part. Moxie is a nice alternative because it has a compact instruction encoding, a supported GNU toolchain and a full 32-bit address space. FPGA space was a real concern, so he started with a non-pipelined VHDL implementation, and by all accounts it is running code and flashing LEDs on a Nexys3 board!

The one major "ask" was that there be a little-endian moxie architecture and toolchain in addition to the default big-endian design. I had somewhat arbitrarily selected big-endian for moxie, noting that this is the natural byte order for TCP. In Brad's design, however, the moxie core will handling FAT filesystem duties, which is largely a little-endian task. At low clock speeds every cycle counts, so I agreed to produce a bi-endian toolchain and, for the most part, it's all committed in the upstream FSF repositories (with the exception of gdb and the simulator). moxie-elf-gcc is big-endian by default, but compile with -mel and you'll end up with little-endian binaries.

Brad also suggested several other useful tweaks to the architecture, including changing the PC-relative offsets encodings for branches. They had originally been encoded relative to the start of the branch instruction. Brad noted, however, that changing them to be relative to the end of the branch instruction saved an adder in his design. I made this change throughout the toolchain and (*cough*) documentation.

I'll write more about this as it develops... Have to run now.

Oh. Here's the VHDL on github: Go Brad!


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