I am not sure those are likely to be free, and if they are they would be 'unofficial', which means they might well not implement all undocumented features correctly.. That's the 'HCF' (Halt and Catch Fire) instruction I mentioned. Let me explain where all this is coming from. I have an HP37201 HPIB Extender on the bench. It's basically a 6800-based microprocessor system with the right I/O chips (FWIW, the HPIB handshake is handled in firmware, there are about 5 6821 PIAs on the board to handle the HPIB lines, the V25 dialer port, status LEDs, etc). Anyway, there's a link on the board labelled 'SA TEST'. I think that's 'Signature Analyser Test', HP having a love of such instruments. From what I've figured out so far, when it's put in the 'SA TEST' position, it disables the data bus buffers (thsoe are the 6881 chips I was asking about the other day) and uses a '241 to force 0000 0010 onto the CPU data lines (at least during CPU read cycles). I would guess this gets the CPU to do something repetitive (incrementing the address bus at full speed would be an obvious choice), so that you get meaningful signatures on all the lines. When I get enough of the instrument back together to power it up, I will flip that link and look at all relevant signals with a logic analyser. -tony

I pulled this off of the web by googling. no way to verify it. this was the website, for credit's sake http://neil.franklin.ch/Usenet/alt.folklore.computers/19971216_HCF ... It was the 6800 that had such an instruction. It was a variant of the 'JMP' (jump) instruction that "jumped to the accumulator". The opcodes are 0x4E and 0x5E (for the A and B registers respectively). While not used in "normal" operations, they are convient for bringing up new designs. One could wire the data buss to either of the codes, and reset the processor. Upon reset, the processor reads out locations 0xFFFE and 0xFFFF for the location of the first instruction to execute (it gets 0x4E 0x4E), then it sets the address buss to that location and reads an instruction from there, getting 0x4E. This causes the address buss to increment and cycle thru, reading every location on the buss. With a scope, properly attached, quite helpful. Shows up decodes and the like for the various I/O, RAM, ROM devices. Might even show up conflicts, or a mal formed decode PAL.