15 Dec
2016
15 Dec
'16
4:55 p.m.
On Dec 15, 2016, at 2:52 PM, dwight <dkelvey at
hotmail.com> 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
paul