(SOLVED) 41CL - DOUBLE HEPAX ACCESS and MMU CONFIG

[Monte Dalrymple]

(08-24-2019 05:06 PM)rprosperi Wrote:  If the HEPAX, Advantage and IR modules handled these special OpCodes in h/w, how are CL, V41, etc. able to reproduce that behavior, meaning how do you know exactly what to have the emulation layer do to emulate the module's h/w behavior?

Is it "simply" educated guessing (e.g. looking at disassembled code, then taking a stab), followed by lots of testing, then rinse and repeat until it works?

For the Advantage and IR modules the special opcode is an ENROM instruction. This is also one of the cases for Hepax. The operation of this opcode is understood at a macro level, but there has always been the question of whether the following instruction is fetched from the current bank or the next bank. It seems that even HP wasn't sure about this, always placing the same instruction in that location in both banks. I don't remember which way I did it in the NEWT, but it works because the code tolerates either way. Unfortunately I don't know of an easy way to determine which way the legacy hardware actually works, because it would require modifying the contents of the physical ROM to test.

The Hepax uses a different opcode to "move" the code to a different page. Since this is unique to the Hepax, and wouldn't work anyway given the MMU, the Hepax image used in the 41CL actually has that piece of the code removed from the image.

The IR printer also uses the special Peripheral instructions, which are also NOPs as far as the CPU is concerned. This operation is also well understood as far as the CPU is concerned, but I have never been able to find any documentation on how the IR module interprets them. I started looking at the code some time ago, but there are at least a dozen different "instructions" used and it's probably going to be impossible to figure out what they do from just looking at the code.