Progress report time....
I need RAM in order to implement/test most instructions. To that end, I've implemented a fake data cache that is always accessed within a single cycle during the WRITE pipeline stage. Eventually this will have to be replaced with a real data cache that reads/writes to real memory over the wishbone bus while the processor pipeline stalls.
The push instruction was easy enough to implement. It's the first one
that writes to both memory and a register (to update the stack pointer).
This meant reworking the interface between the EXECUTE and WRITE stages.
pop is a little more tricky because we need to update two registers:
the stack pointer and the register we're loading memory into. I'm going
to work this out tomorrow night, but I can see now how making it work in
a single cycle will require a little more logic than splitting it up
into two cycles. It will be interesting to experiment with changes like
that once everything is working.
Also, I reorganized the HDL source to cleanly separate the moxie core from the muskoka SoC and related firmwares and cores. As usual, everything is in github.