[m6526] implement TOD and alarm

[nippur72]

this PR implements TOD (time of the day) and ALARM in the chip MOS6526 (Commodore 64's CIA).

To make it work with c64.h the _c64_tick() needs to set M6526_PIN_TOD into the pins at the rate of 50 or 60 Hz. This can be easily done with a counter. I use something like this in my c64.h:

    // create a 50 Hz signal for TOD (CIA1)
    if(++sys->tod_counter > (C64_FREQUENCY / 50)) sys->tod_counter = 0; 

    // ...
   
        // feed the TOD signal to the CIA1
        if(sys->tod_counter == 0) cia1_pins |= M6526_TOD;  

I haven't made a PR for c64.h yet because I still use an old version of it, but will do once I upgrade.

I did not test the alarm functionality, as my program only makes use of the time of day, and I don't have any test programs.

[floooh]

Just a heads up, I'll most likely won't be able to look at this PR for a long time (too much other stuff on my table atm), and also without being able to properly test its behaviour it's unlikely to be merged.

[nippur72]

Hi, just to let you know that I fixed the code in order to pass all the VICE tests for TOD. I hope that can help you for a possible merge in the future.

And if you are curious to see why I needed the TOD feature, here we are emulating an italian Videotel modem cartridge for the C64:

• click here
• hit F5 to go to Videotel menu
• hit F1 to dial a number
• enter a telephone number (whatever)
• wait for connection to the BBS
• in the status bar, the connection time is counted using the TOD 😄