ALU overflow status bit & COND address

Question

The COND field or the microinstruction word is 2 bits with four possible different codes:

00 for no jump; 01 for jump if N=1; 10 for jump if Z=1; 11 for jump always.

If one of these codes were needed to test a newly implemented ALU overflow status bit, which of the four codes would you recommend I sacrifice? Also, I would love to hear why.

Thanks!

Answer 1

A jump instruction that never jumps is pretty much useless. OTOH, there are many desirable conditions for conditional jumps, more than 4 (think of like 8-16), so supporting more of them directly is a good thing. You could repurpose 00 for the overflow condition.