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