rm/rm480z.cpp: Added nmi single step debugging support and corrected alternative character set display logic#15197
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Single step debugging
Basically the same change that I made for the 380Z yesterday, but on the 480Z NMIEN is active high.
The front panel (accessed with <Ctrl>+F) includes a single step debugging command (z) which relies on special hardware to work. When it asserts NMIEN by writing to Port 1 we need to start counting M1 cycles and then trigger a NMI after 8 of those. The NMI service routine will then return to the front panel with PC pointing to the next instruction. With my changes this now works.
Alternative character set display logic
I did not originally appreciate that the alternative character set is actually part of the chargen ROM (starting from offset 0x1000). Hence, we should plot this character data instead of manually inverting the standard set!
The following BASIC program shows that the same characters are produced with the new routine:
NB Changing the escape sequence on line 10 to "=0L" will display the regular character set (teletext characters).