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NEC V20 microcode ROM extracted from die photos using CNN bit classification
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A developer commissioned high-resolution die photography of the NEC V20 CPU and used a convolutional neural network to extract the microcode ROM contents, enabling cycle-accurate V20 emulation in MartyPC.
The previous V20 emulation core in MartyPC was not cycle-accurate because it repurposed the 8088 core with V20 instructions grafted on. With the microcode decoded, emulator authors can now match the V20's actual timings rather than approximating them, the same way reenigne's 8088 microcode decoding enabled accurate 8088 emulation in 2020.
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The NEC V20 microcode ROM is 258x116 bits (29,928 total), with a word length of 29 implying 1032 microcode words, eight more than the expected 1024.
Standard thresholding in MaskRomTool failed due to poor contrast from the metal layer, so the author exported bit positions to JSON and extracted 42x42 pixel sub-images for each of the 29,928 bits.
A CNN trained in PyTorch on roughly 1,000 manually classified bits successfully sorted the remaining ROM bits as 0 or 1, replacing what would have been hours of tedious manual classification.
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