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 almost 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 board …

A big batch of PCBs arrived last week from China (7 different boards!), and I decided to have a go at the reset board first. This is a straightforward conversion of my breadboard prototype, so I thought it would be quick and easy to assemble and test, and I could …

I'm going to just call interrupts, syscalls, page faults, privilege errors etc. exceptions from now on to save typing. Since the current design of exceptions has a few flaws, we need a new design. I'm not keen on redesigning the existing hardware I've made if possible, so that means no …

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 synchronous, and so can be treated just as another instruction (R-type …

I've been toying with the idea of trying to use serial ROM chips in my build for quite a while, but I couldn't find anyone else who had done something similar, and wrote it off as too complicated. A couple of weeks ago, however, I started working on the PCB …

Well, it turned out that there were more problems with the first revision of the ALU than I thought:

• I knew I'd left three pairs of inputs floating across two ICs in the comparison unit, but it turned out I'd also completely neglected to wire up power or ground. Both …

I finally got time to assemble the ALU PCB, after having been away for 6 weeks, waiting for components to arrive, then dealing with the start of the lockdown.

Overall, I'm pretty happy with the result. I had a few problems:

• I ordered the wrong size of component for all …

The ALU PCB design is finished, finally. It's similar in design to the breadboard implementation, but with a few changes:

• a full shifter in place of the fixed right-shift-by-one element on the breadboard
• support for addc and subb instructions, in case I want to add them later
• no dedicated zero …

Currently we don't have add-with-carry or subtract-with-borrow instructions, but they would be useful for doing 32-bit maths, and easy to implement. If nothing else, the ALU should support them in case I want to add them later.

There are four instructions that need to do some kind of equality comparisons of registers:

• BZ (branch if input register is zero)
• BNZ (branch if input register is not zero)
• SEQ (set output register to 1 if both input registers are identical)
• SNE (set output register to 1 if both …

I finished soldering the remaining components onto the register boards last night, and started running a more comprehensive set of tests than I had before, and I've found some of the problems I was worried about.

Firstly, there's a simple mistake in the schematic, in which I swapped pins 14 …