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MAME supports almost every desktop 680x0 Mac now except the Quadra AVs, with the original ROMs and emulating the actual chips. Is there some test I could run on the emulated IIsi for you?

The first-gen PowerMacs supported 2, 4, 8, 16, and thousands of colors on their motherboard video (which is actually borrowed from the LC III).

Bright line in the middle isn't lack of sync, it's lack of vertical drive. Gonna be something on the analog board.

That's likely cold/cracked solder joints on that transformer or a component near it. That was fairly common with the flyback transformer on the Apple IIgs RGB monitors. Touching them up would probably fix it.

Yes, I've been involved with MAME since around 2002.

Fair warning: MAME versions that early got everything wrong with the games. Gameplay speeds, sound, colors, computer AI behavior, even level order and boss hit points in extreme cases. We just fixed a copy protection failure in Kangaroo that made the gameplay wrong dating back to when it was...

In most cases you'll be able to get something that works by rearranging a newer set, but that's not a fun project. It took me a long time to piece together one set that worked on old MacMAME so I could show it running in modern MAME's 68K Mac emulation.

The original ASC has two modes: PCM stereo playback and 4-channel wavetable synthesis. The wavetable mode is what generates the classic original Mac II boot and death chimes. In wavetable mode, it loops the contents of the FIFO and you can freely adjust the pitch and volume of each of 4 voices...

Most instructions you know the length from the first word. There are some where you need to know there's a second word *and* check what its value is to see if there are more words. But it's not as bad as x86.

@dougg3 Interesting that it's different on the '020. That certainly makes it seem more like an actual errata in the chip. Probably a bit late to get Motorola to respond though.

Of course, but TAS is.

I'd think the likely suspect for the contents of memory that affects it would be 4(A4). Also, a thought: CAS and TAS in their legit forms are the only 680x0 instructions that do a special read-modify-write bus cycle so that the operation is not splittable in the middle on a multiprocessor...

Nice work!

Could be the bus transceivers next to the 6502 also. Adrian Black had a video about a year ago where that (among other things) was the problem. On a 40+ year old machine even the "reliable" standard TTL chips can fail.

Video init happens relatively late. After the chime it does the RAM test, and then it hits PrimaryInit where it starts up all the video devices, and then ADB init to scan for keyboards and mice. I'm actually not sure what could hang up the system between the initial chime and video startup...

Because a NuBus video card doesn't work either, I think it's likely the RAMDAC is fine, but the system isn't doing video initialization and so the pixel clock isn't running (it comes from a software-programmable PLL chip). The RAMDAC on board has nothing to do with the operation of NuBus video...

If you have an oscilloscope or logic analyzer that would be the best way to start zeroing in on what the problem is. From the screenshot we can at least tell that the master oscillator is running and the video circuitry is working, but the CPU appears to not be running. Also, that machine...

That single line indicates vertical deflection isn't working at all. If you run it very long like that it can actually burn through the CRT's phosphors and make that line permanently dark when you do fix the problem. What's happening is that a new old stock board probably needs to be recapped...

The IIe Card does *not* have a DeclROM. There's some code scattered around the SuperMario tree cursing that fact, even.

I use IDA for offline disassembly - it automatically inserts Toolbox trap names and things of that nature, but there's far more that could be done. As far as reverse-engineering Valkyrie, I didn't disassemble the driver, I simply watch the reads and writes to the documented register area. MAME...