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6

As you stated, the results for the C64 make sense, given that the interrupt source is based on a 50 Hz timer which is a bit off from a real 50 Hz signal, like for example the PAL screen refresh. For the Plus/4, both models, the PAL and the NTSC version use a rasterline interrupt as an interrupt source. This can be checked by looking at address $ff0a which ...


2

The video timing is generated by the onboard crystal oscillator. On the C64, the 50/60 Hz clock comes from the stepped down mains frequency of the 9VAC from the transformer, so it ticks at whatever the current mains frequency happens to be. So the C64 clocks are asynchronous. It is not tied to screen refresh. The Plus/4 works differently, it does not use the ...


4

The problem is that you're trying to access the function parameter data, but the compiler does not expose that as a symbol to the internal assembler, so it's looking in the public, global namespace for it -- and can't find it. According to SDCC - Interfacing with Z80 assembler code, you do indeed need to access the parameters from the stack. I don't know if ...


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