Hackaday · Creativity & design

Decoding NEC V20 Microcode ROM

To create a cycle-accurate emulator of the NEC V20 processor, its microcode ROM needs to be decoded. This involves extracting approximately 30,000 bits from a CPU die shot.

Manually decoding the ROM is time-consuming. Automation using machine vision is an alternative. While a tool like MaskRomTool exists, it struggled with contrast in the die shot.

The tool did provide bit locations, generating 42x42 pixel PNGs for each bit. A convolutional neural network (CNN) was then trained to distinguish between 0s and 1s, requiring manual classification of 1,000 images.

Ambiguous bits were classified manually, which was deemed easier than refining the CNN. With the microcode decoded, it can be matched against the V20's internal architecture to understand its functions.

Progress is available on GitHub. The V20's microcode was previously a subject of legal disputes between NEC and Intel regarding CPU compatibility.

AI-samenvatting op basis van de bron.