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A Lisa Inside An FPGA

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AlexTheCat123:
Okay, the floppy controller interfacing problem is fixed now! I ended up adding some delay logic that keeps the 6504's clock paused for a little while after a 68K access to shared RAM ends, which gives some other logic enough time to raise and then lower the FDC RAM CS signal again to re-latch the RAM address that the 6504 was accessing before the cycle got interrupted by the 68K. It was a pretty convoluted fix, but no more Error 57 or hanging!

I also implemented the Lite Adapter, which was super easy. It just compares a constantly-incrementing counter to the value in a shift register (which represents the PWM duty cycle) and sets or clears the PWM output signal depending on whether the counter is greater than or less than the shift register value.

Now that the FDC seems to be working and fully-implemented, I've hooked up a Floppy Emu, and unfortunately we can't quite boot from floppy yet. But we're really close. It can clearly detect whether or not there's a disk inserted, and I've confirmed that seeking and ejecting the disk work perfectly by sending those commands to the FDC manually in Service Mode. The one thing that's not working is reading and (presumably) writing, which is obviously a pretty big problem! Whenever you try to read a sector (either manually in Service Mode or automatically by booting from the disk), the FDC just sits there trying to read the sector forever. This is making me think that something's wrong with the PROM state machine, which is supported by the fact that the only two things I've ever seen it put out onto the 6504's data bus are 0 and 1, which obviously doesn't seem right when reading sectors off a disk. So that's what I'm about to start troubleshooting next; hopefully it's not too hard of a fix!

AlexTheCat123:
After reading some more about the theory of the Apple ][ floppy state machine (which is basically identical to the Lisa's) and tracing through the states, I discovered that I had wired one of the pins on the ROM to the QH pin of the shift register instead of the QA pin, causing it to clear the shift register every time it shifted in a bit instead of only after a full byte was shifted in. And after fixing that, I can boot from floppy! Here's the status of booting various things from floppy, I'm assuming anything that's failing is because of my tiny 256K of RAM:

BLU - Works
Selector - Works
NewWidEx - Works
LisaMandelbrot - Works
MacWorks XL - White screen, floppy activity for a while and then hangs
MacWorks Plus - Gets about halfway through the Loading... screen and then hangs
MacWorks Plus II - Gives error about PFG since we don't have a PFG installed, if we choose to continue it loads about half the sectors from the floppy before hanging
UniPlus - Immediate error 75, I think my UniPlus floppy might just be corrupted
Xenix - Gets to the screen where it prints the Lisa system type, expansion slot contents, and free RAM, then kernel panics
GEM - Can boot to the command line, starting the GUI causes it to hang midway through loading
LisaTest - Error 49 (Line 1010 or 1111 trap)
LOS/Workshop - Error 10727 (Memory exhausted)

I don't have enough room in the FPGA to go to 512K of RAM, and I can't move the RAM to my board's external SPI flash because the write speed is too slow. My board has a DDR3 RAM chip on it too, but I really don't feel like interfacing with that, and I'm worried the latency would be too high anyway. So I think it might be time to design a custom board with an external parallel (or maybe SPI) SRAM before I continue. Unless anyone's got any better ideas (which I would love to hear), I think this will be my next step, and so it'll probably be a little while before another progress update. I guess I can go ahead and try to fix the intermittent ProFile read problems that I was having, but that's about all that I can do with the current version of the hardware.

bmwcyclist:
Fantastic work!

LisaMandelbrot? I will have to look into that.

stepleton:
Amazing progress!

LisaMandelbrot is a set of Mandelbrot set plotters that I wrote a while ago. You can find it here: https://codeberg.org/stepleton/LisaMandelbrot
That page looks sparse because there are three varieties: "Port" which runs on the Office System, "Pro" which runs in the Workshop (no extra features, it's just that the workshop seems like the place for "pros"), and "Solo" which is a standalone program (i.e. boots and runs without an OS). Solo is quite small. Anyway, click through to any of the three to see more detailed information.

My standalone programs (including the Selector) don't really exercise all that much of the Lisa's capabilities; for a start, they all leave the MMU in the boot-up "flat" configuration. So it's not a big surprise that they run. For this reason I wouldn't think there's much point in running my stunt standalone Forth port for Lisa, as it is just as gentle on the machine (the Forth part can't even use RAM above 64k!).

Alex's SRAM idea touches on a project I was thinking of attempting but was in my project pile: the Smallest 2Meg RAM Card On Earth. I had a vision of an SRAM-based RAM card that would be comically small inside of the card cage, and I'd even had my eye on this single, somewhat pricy RAM chip, with its 16-bit data bus and (IIUC) +5V tolerance. But I hadn't started to sweat the details yet, and realistically it is a project that would have sat on a back burner for months at least. All of which is to say: Alex, if you wanted to stretch your SRAM plans slightly to make the Smallest 2Meg RAM Card On Earth, it would be pretty cool, and maybe that IC is worth knowing about  :)

AlexTheCat123:
Ha, it's interesting you mention the 2MB RAM card thing; I was just thinking about doing something like that! All the RAM card logic I've written would easily fit into a small CPLD, so throwing the CPLD, some level shifters, and the SRAM onto a tiny little board would work nicely. And funnily enough, that SRAM chip is the exact one that I've been eyeing for the FPGA!

The COP core I'm using seems to also be small enough to fit into a CPLD, so a CPLD-based COP replacement could be another interesting idea...

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