Following on from last month's musings, over the last couple of months I've done a bit more work on the LLVM backend for the MUPS/16. Whilst implementing support for global symbols (which includes function addresses) I realised I'd made a bit of a mistake with the design of the …

Since the first version of the μcode address generator had a few issues, I re-worked it to fix them, and while I was at it I let a bit of feature-creep happen. The 1.1 revision of the board has quite a lot more functionality than the 1.0 one …

Random notes on things I've noticed over the last few weeks that could be improved:

I combined the PC and immediate units into one board since the immediate unit has three outputs (data, B bus, and PC), and combining the imm and PC units onto one board saved 16 lines on the backplane for the imm->PC connection. Techincally, there's a LUI 'unit' here too …

It's been almost 18 months since I had any time to work on this (life got in the way), but I've started assembling and testing the last batch of boards I designed, and working on the next lot.

The μcode address generator was the simplest piece of the control unit …

I finally decided to stop tweaking the design of the pieces of the control unit that have been mostly complete for almost a year now, and just send them to be manufactured. There are four new boards here:

• a new lower backplane
• a new tester board to allow an Arduino …

I've finally had a bit of time around work to get back to this. The last major piece to do is the control unit, and since a) I expect this will take several attempts to get right, and b) I only left myself a 14x20cm space for the whole unit …

Several of the instructions in the current design need a bit of a rethink. In particular, I've been thinking about how this CPU could eventually be pipelined, and as part of that, how to simplify the control unit. I've probably made too much use of the flexibility that microcoding instructions …

My first attempt at implementing interrupt, syscall and exception handling in MUPS/16 currently has 4 types of traps:

• interrupts (currently 4, with equal priority)
• page faults
• syscalls
• privilege errors

Syscalls are handled slightly differently as they are completely syncronous, and so can be treated just as another instruction (R-type …