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 …

I had thought the MMU was pretty solid, since I gave it a good test after its v1.1 rework, and didn't find any issues. In testing the control unit, though, I've noticed some weirdness. Mapping addresses didn't work consistently. More often than not, about half the bits were incorrect …

Notes:

• low-temperature solder paste does not flow nearly as well as original version. Maybe because it's not thermally stable, and I'm not storing it at a constant temperature?

• short between VCC and GND after assembly and soldering. Removed 48 of 60 capacitors before finding the culprit on the 49th.

• bug …

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 …

Before testing the new control boards, all of which are driven off a single SPI EEPROM chip containing the microcode, I needed to add code to the Arduino server to read and write SPI ROMs, again - this was in the old version of the server, but not ported over.

Adding …

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 …

Requirements for a new clock:

• the output of the clock is a pair of square waves. clk is a 1/4 duty cycle (i.e. high for one quarter of the cycle, low for the remaining three quarters). clk2 is a normal 1/2 duty cycle:

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 …

The control lines are designed with a couple of common features:

• they all have a safe or no-op state when the lines are high. For simple lines, this means that they are active-low. For encoded lines, this means that all ones is a no-op command
• they all have pullup resistors …

Whilst trying to write up some work I've been doing on the control unit I ran into a problem: when building the web pages I wasn't getting overbars on control signals (e.g. THIS). I had a vague memory of having solved this a couple of years ago when I …

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 …

I finally got a bit of time to fix the bugs in the memory and MMU boards, and to solder up the memory board (waiting for some chips for the MMU still).

The rev 1.1 design of the memory board is alomst the same as the 1.0, except …

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 …

I finally put together the MMU, the fourth of the seven PCBs that arrived in my August batch. A couple of things were obvious right off the bat: 1. I'd somehow screwed up the silkscreening, and all the text labels for the current MMU mode on the left of the …