An interesting use of the 4004 instructions

dwight dkelvey at
Thu Dec 15 13:52:49 CST 2016

I was looking at some disassembled 4004 code when I came across

a SKIP operation.

It isn't normally an instruction but If you do a JCN with all the CCCC = 0,

it will do a NEVER jump.

This is the equivalent of a SKIP instruction.

I would suppose a JCN with CCCC = $8000 would be an always jump,

on page ( not real useful as JUN takes the same cycles and space ).

I thought at first there was some errors in the code because there

were JMS to the middle of JCN instruction but then I noticed that there

were no conditions specified for the JCN. A little thought and I realized

it was a way to skip over a single byte instruction.


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