WIP: 16C firmware hack for more memory

[brouhaha]

(11-11-2023 01:56 AM)blackjetrock Wrote:  Could you not have the following:?

When address x is executed,

1. Execute the instruction from the ROM
2. Execute extra instructions

Yes, I could do that. I prefer to stick to code that can execute on the HP Nut processors. When I first hacked HP-15C firmware in 2004-2005 (leading to the expanded memory of the SwissMicros DM15[L] and the HP 15C CE), I cut the ISA line between the Nut and R2D2 chips of an old 12C, and inserted an FPGA and two parallel NOR flash chips between, to replace the ROM of the R2D2. I'd hoped to later do the same with a low power microcontroller, that might actually fit inside the calculator and run from its batter without completely devastating the battery life. I didn't complete that, but I still prefer code that could, in principle, run on the original hardware.

If I was going to patch it in native code, I'd do the same thing HP has done on some ARM-based Voyagers, which is to usurp some undefined opcode(s), to avoid having any address comparisons added to the interpreter loop.