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How did the C64 interface tape drives?
I'm aware that the external interface was basically a audio input - but how was this audio converted into digital data, and how was it loaded into RAM and executed?
Basically what I am asking is how the computer handled the audio, as it entered the machine. How was it digitized, and how was this transferred to RAM?
The analogue audio is turned into a 1-bit signal — either high or low.
The machine then detects positive transitions, counting the amount of time between each. That allows them to be bucketed into one of three types:
Each byte is preceded by a byte marker, which is a mark wave and a long wave.
From there onwards, 0s are stored as a short wave followed by a long wave, and 1s are stored as a long wave followed by a short wave, and each byte is completed by an odd parity bit.
So reading a byte is as simple as watching for a mark wave, then tracking the sequence of short and long waves, using the parity bit as confirmation.
All files are also preceded by periods of lead-in tone, which is just a prolonged period of short waves. The computer can use that section to calibrate itself to different tape speeds.
A complete program file then looks like:
Commodore used repeated data as a basic means of checking integrity; it's an outlier in this regard — other micros tend to do more intelligent things but the Commodore ROM just stores bytes directly to their intended destination on first run through, then checks them on the second.
