Mackerel-30 board bringup is going well for the most part. I've got the IDE interface and DRAM controller hooked up and functional. I did make a mistake in IDE wiring though. One of the buffer chips between the IDE interface and the CPU data bus has the bits wired in reverse order, i.e. bits 0-7 are mapped to 7-0. Fortunately, the rest of the control circuitry is wired correctly and the bits can be reversed in software for now. This adds a bit of overhead, but Mackerel-30 is not winning any speed contests right now anyway. I'll add it to the list of fixes for the next PCB revision.

I also ported over my DRAM controller from Mackerel-10. Going from four 30-pin SIMMs to a single 72-pin SIMM required a bit of adjustment to the CAS and RAS logic, but the state machine and refresh timing is almost unchanged. Unfortunately when I bought DRAM sticks for this project, I didn't notice that the 64 MB modules I ordered were 3.3v only. I've ordered some more 32 MB and 64 MB modules that will work at 5v, but for now I'm using a backup 8 MB stick.

There seems to be some inconsistencies with how 72-pin SIMMs have their RAS and CAS lines wired up to the individual DRAM ICs. The 8 MB sticks I have seem to map one RAS line to each of the 4 8-bit DRAM ICs on the stick, but based on other datasheets and information I've seen online, this may not be 100% standardized. My interpretation is that normally the four RAS pins operate in two pairs of two, effectively acting as a bank switch for double-sided modules. I'll have to experiment when I receive the new batch of SIMMs, but in the worst case, I can make some minor adjustments to the DRAM controller logic to compensate. I'm feeling pretty confident in the core functionality of the controller at this point.

My intention was to finish out this update with a proof-of-concept FPU test, but I realized the libc library I'm using doesn't support floating point at all. I could still add the new decoding logic to the CPLD and do some tests in assembly, but I think I will wait on the FPU for now. I've got the hardware far enough along to start porting Linux to Mackerel-30 and I'm much more excited to get started on that than to worry about the FPU at the moment.