Decoding the NEC V20 Microcode

If anybody's interested in the V20 from a programming perspective, The User's Manual from NEC is here: https://archive.org/details/bitsavers_necV20V30U_11351331/mo....

The V20 instruction set is interesting and has some very 1980s extensions. In addition to the 8080 emulation mode, it has instructions for binary-coded decimal arithmetic and bit fields.

However, NEC uses their own naming scheme which makes picking out the differences from the 8088 unnecessarily difficult. The bits are the same but the mnemonics are different. For example, they renamed all the registers (AX -> AW, SI -> IX, etc) and many opcodes (JMP -> BR, REP MOVSB -> REP MOVBK).

Of course the V20’s microcode was the centre of an interesting legal dispute when NEC was sued by Intel.

https://thechipletter.substack.com/p/intel-vs-nec-the-case-o...

This is an amazing project, I'm always impressed by stuff like this. I'd be very curious to know, though, if there's a market for achievements like that.

My type of article! Thank you

Lovely work, thanks for sharing!

ha I remember that chip so well

instant speed boost for IBM PC

just pop out the 8088 and put in a $20 V20

only problems is some programs ran their clocks too fast

wow that article is amazing, I cannot believe people can reverse engineer at that level, brilliant

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