Docs / Inside the C64 / The Datasette
The Datasette#
Tape stores programs as timed signals on a cassette. Loading involves both transport controls and decoding a sequence of pulses.
The story#
A cassette keeps data in a physical sequence. To reach something later, the transport has to move the tape past the head. Position, winding and recognizing the start of a recording are therefore part of using the medium, rather than background details hidden behind a filename.
Commodore's Datasette connects the cassette mechanism to computer-oriented read, write, sense and motor signals. The 1530 User's Manual introduces the controls and loading workflow. Inside the C64, software still has to interpret the timing of the arriving pulses.
That leaves room for different recording schemes. A custom loader can use a different way to recognize pulses, synchronize with a block or represent data. The same cassette transport can therefore serve several kinds of software loader. The pulse scope makes this underlying timing visible without assuming that every program uses the same encoding.
Follow the timing into the program#
The useful information is the interval between pulse events:
- Movement past the tape head produces a read signal.
- The C64 receives those events through CIA1's FLAG input.
- The loader measures timing and applies its decoding rules.
- Decoded bytes are checked, stored or passed to another loading stage.
A repeated pattern of similar pulse lengths might be a synchronization region. A varied pattern might contain encoded data. The waveform alone is not enough to name a file or assign a bit value, because the interpretation belongs to the loader. The Datasette model follows the signal into the machine's timing and recording paths.
TAP images preserve pulse lengths. For ordinary full-wave TAP entries, the scope draws a representative square-wave shape from that duration. The width is useful evidence, but the drawn duty cycle is not an oscilloscope measurement of a real electrical pin. Recording uses the pulse data being written. See the VICE TAP specification.
Fun fact: PLAY can stay down while the tape stops#
The transport key and the computer-controlled motor are separate. A program can stop the motor while PLAY is still engaged. That is why Monitor shows both states, and why a stationary tape need not mean that someone pressed STOP. The motor, sense and read connections are described in the Datasette signal path.
How it connects#
Tape pulses → read signal → CIA event → loader program
| Signal or control | Meaning |
|---|---|
| Motor | Whether the transport is driven. |
| Sense | Reports the transport key state to the C64. |
| Read pulses | Carry the recorded data's timing. |
| Write signal | Supplies pulses while recording. |
The Datasette model explains pulse playback, recording and the connection to machine timing.
Try it in Monitor#
- Open Storage → Tape while a tape loader is running.
- Choose the play icon to continue loading, then compare the transport key, motor state and tape position with the pulse scope.
- Use Zoom ×2 to look at a shorter timing window. The scope shows pulse counts and timing ranges, without selecting individual pulses.
- Pause to keep the displayed window, then compare the loader's Code and Controls evidence.
What the readings can tell you#
Tape percentage describes position in the available tape stream. It is not a promise that a particular program is that far through loading. A stream can contain pauses, several files or a loader that interprets data differently.
The Monitor observes transport state without moving the tape to another position.
Continue exploring#
Explore CIA timing and the CPU to connect pulse events with the loader that interprets them.
Explore more in Inside the C64, or open the Live Monitor guide for controls and workflows.