Hidden codes in NEC 8080A

From: Dwight K. Elvey <dwightk.elvey_at_amd.com>
Date: Wed Jul 24 13:59:01 2002

Hi
 I started to look and see what hidden
codes might be in this processor. My memory
said that there was something useful but I
couldn't find anything. Here is what I saw:

08 nop
10 nop
18 nop
20 nop
28 nop
30 nop
38 nop
CB JMP
D9 CALL
DD CALL
ED CALL
FD CALL

 I did see something unusual. Looking at
the PSW, bit# 5 would change, depending
on if an add or subtract was done on an
earlier instruction. If there was a subtract,
it would be set to a '1'. It would set to '0'
on anything that did an add, ADD, ADI, ACI, ADC,
DAD, INR and INX. It would go to '1' on DCR,
DCX, SUB, SUI and SBB.
 I looked for anything related to overflow
conditions but didn't see anything. None
of the NOP codes effected any of the registers
or flags. The PSW bits 1 and 3 were always
set to '1'.
 How dull!
Dwight
Received on Wed Jul 24 2002 - 13:59:01 BST