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The "35-cent" Commodore 64 softmodem
Rockwell famously used 6502-based cores in modems for many years, but that doesn't mean other 6502s couldn't be used. If only there were a w...
An important warning before we continue: from the telephone company side the line pair carries voltage used to power the phone and ringer, so never plug this cable into a wall jack — doing so could potentially send up to 48 volts to the computer, with likely undesirable and even fiery results. After initializing the screen editor, the code waits for the next Timer A interval to fire and scans the keyboard manually (since the IRQ isn't running anymore), then fetches the next key, if there is one. Vanilla Xmodem, written by Ward Christensen, uses a fixed 128 data bytes per block and a simple checksum with known deficiencies, so John opted for the more complex version with a cyclic redundancy check to ensure errors could be promptly detected.
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