Decoding the NEC V20 Microcode

Posted by mariuz 4 days ago

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Comments

Comment by ataylor284_ 1 day ago

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).

Comment by klelatti 1 day ago

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...

Comment by rep_lodsb 1 day ago

Mentioned in the article, same link too. The court documents included a diagram of the microcode format, and also a comparison of the reset sequence between Intel and NEC.

Comment by jeanmichelselli 1 day ago

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.

Comment by msephton 1 day ago

My type of article! Thank you

Comment by eigenform 1 day ago

Lovely work, thanks for sharing!

Comment by ck2 1 day ago

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

Comment by tverbeure 1 day ago

What I remember of the V20 was that there was some benchmark (Norton?) that was way faster, but I never saw a difference for any real application that I tested.

Comment by EvanAnderson 1 day ago

I have a soft spot for the V20. The laptop I carried in high school was V20-based (a NEC Ultralite PC1701). I remember a CGA 3D billiards game I played on both the laptop and an XT clone desktop. The V20 was definitely faster for that game.

For most of what I actually used it for (typing notes and playing with QBASIC) I didn't notice a difference.

Comment by 486sx33 1 day ago

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