386 may be a bit too far, but there was a high resolution shot of an 80286 on the visual6502.org wiki (now sadly offline), were you could almost read the microc

386 may be a bit too far, but there was a high resolution shot of an 80286 on the visual6502.org wiki (now sadly offline), were you could almost read the microcode ROM visually. I spent a lot of time staring at that and got at least a good part of the opcode pattern PLA. Many bits were so blurry that I could only guess what they are, based on what other opcodes I already had. The actual microcode is 53760 bits with a very high density, where the difference between a 0/1 comes down to a fraction of a pixel. I think with a better image, it might be possible to read those bits and even automate it. Not sure if there are any mysteries left to solve though. In the PLA there are patterns with "don't care" bits to match opcodes 64-67 and 0F 07 to 0F FF, so all of these must be invalid (they go to the same address in microcode). Some time ago I found out what the last undocumented opcode 0F 04 - and the F1 prefix - do by experimenting[1]. One slightly odd thing about that instruction is that the CPU seems to grant bus requests for DMA/refresh, but then completely ignore it and write at the same time. On the first machine I tested this on, this caused some crashes and spurious I/O that I didn't notice at first, but on another one it did absolutely not work unless DRAM refresh was disabled. Maybe there is something in the microcode that explains this, but more likely it has to do with other parts of the hardware? [1] https://www.vcfed.org/forum/forum/technical-support/vintage-computer-programming/71519-i-found-the-saveall-opcode https://www.vcfed.org/forum/forum/technical-support/vintage-...