An interesting use of the 4004 instructions

Paul Koning paulkoning at
Thu Dec 15 18:55:35 CST 2016

> On Dec 15, 2016, at 2:52 PM, dwight <dkelvey at> wrote:
> 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.

It's a bit like a coding convention I've seen used a lot in PDP-11 code, at least in some programs; RSTS/E is full of them.  Consider a function with two entry points, where Carry set or clear is used after entry to distinguish the two cases.  The two entry points look like this:

	fun1:	tst (pc)+
	fun2:	sec
		; common code
		bcs  case2
		; case1...

Or a function that indicates a boolean result (say, success/fail) by Carry clear vs. set:

	good:	tst (pc)+
	fail:	sec
		rts  pc


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