Hi

The NMI on the Vic-20 is connected directly to the "RESTORE" key on the keyboard. Not much happens if you press it since the routine just returns to the interrupt break point with a RTI. That is unless the "RUN STOP" key is pressed. In that case, it does not return to the interrupt breaking point. It seems like the Commodore Basic at this point clears the stack by writing $FA into the stack pointer.

During Basic cold start, it clears the stack by writing $FB into the stack pointer.

During reset, it clears the stack by writing $FF into the stack pointer (which kind of makes more sense).

The question is why the "Kernal" writes $FB or $FA into the stack pointer during different cold starts. It retains something there, but I can't understand what.

I don't know how much of this code is Commodore specific since its basically MS Basic v2.0.

Although I am not by any means an 8-bit Commodore expert, I can readily postulate that the behavior you mention is completely Commodore-specific, since there is not a single "RTI" in nearly 7000 lines of the original 6502 source, implying to me that MSBASIC has no stake in any interrupt handling. Why the "Kernal" initializes the stack pointer differently during different cold starts is a question whose answer is beyond my pay grade.

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Got a kilobyte lying fallow in your 65xx's memory map? Sprinkle some VTL02C on it and see how it grows on you!

Mike B. (about me) (learning how to github)

IIRC, the Vicmon cartridge used the NMI to perform single step execution, and it might have been used for the RS-232 implementation using the 6522 VIA. Fortunately, a disassembly of the ROMs are available which you will likely find useful and enlightening.

http://www.zimmers.net/anonftp/pub/cbm/ ... index.html

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Regards, KM
https://github.com/floobydust

In the C64, Commodore calls this a warm start. It restarts BASIC and the shell, but not the kernel (or kernal in Commodore parlance).
The top of the stack me already be in use by the kernel, so it should not be overwritten.

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I trust my somewhat flawed English is comprehensible to all.

Thanks! It looks like the top of page 1 is used for storage, which might help explain why the stack is (sometimes) initialised to a different value:



Here are the three initialisations: