[HP41] mcode question polling point entries

[Ángel Martin]

(03-22-2023 04:09 PM)MeindertKuipers Wrote:  Hi mcode guru's,

I am struggling with some mcode using the HP41 polling entries, and I am probably messing up some registers. Is there any definition which CPU registers can be freely used and which must be restored during any of these polling entries?

There's a set of "rules" listed in Ken Emery's book, see the Appendix D, page #151 to #153

1. Restore nybbles 10 through 3 of register C to what they were when you took control at the interrupt
2. Have P as the selected pointer
3. Load flags 48 to 55 of the user flags register into CPU register ST. This set of flags is called status set zero (SS0)
4. Have Chip 0 (the status registers) selected
5. The CPU must be in HEX mode
6. You must do a GOTO to 27F3 to end the interrupt and give control back to the calculator so that it may continue polling.