Mark-8 SCELBAL Demo – Loading and Running BASIC via Emulated Paper Tape

In the warm glow of the MicroBasement, SCELBAL (SCELbi BAsic Language) brings classic BASIC to the Mark-8 minicomputer — the first widely published homebrew microcomputer from the July 1974 Radio-Electronics article. Originally loaded from paper tape via the Mark-8's parallel interface, SCELBAL turns the 8008-based Mark-8 into an interactive programming environment. The terminal port (for Tera Term interaction) runs at 110 baud serial, while the original paper-tape reader used a parallel 8-bit interface. This demo shows how to load SCELBAL today using a Raspberry Pi emulating a paper-tape reader, bridging the parallel tape load to the Mark-8's hardware. As time allows, we will be using real paper tape in the future. In the MicroBasement, this emulated paper tape setup keeps the 1974 excitement alive: patient loading, the READY prompt, and the joy of seeing your own code run forever.

Setup and Hardware

The Mark-8's terminal/serial port operates at 110 baud for interaction. The original paper-tape reader was a parallel interface (8-bit data lines). A Raspberry Pi emulates the tape reader by sending SCELBAL data as parallel bytes over a custom adapter (or serial-to-parallel bridge) to the Mark-8's tape input port. Connect the Pi's GPIO or USB-serial adapter accordingly. The Pi runs a script that outputs the tape image with proper timing to mimic the original reader.

Step-by-Step Demo: Loading SCELBAL on the Mark-8 via Emulated Paper Tape

  1. Configure Tera Term (or similar): 110 baud, 7 data bits, no parity, 1 stop bit (for terminal interaction).
  2. On the Raspberry Pi, start the tape-emulation script (outputs the SCELBAL tape image with timing to emulate parallel paper tape reader).
  3. Power on the Mark-8 and set the front-panel switches to the bootstrap address (typically octal 000 000 or the monitor entry point for tape load).
  4. Press RUN (or the appropriate bootstrap sequence to begin tape read on the parallel interface).
  5. Wait ~4–5 minutes as the Pi sends the full SCELBAL tape image. The Mark-8 loads it into RAM via the parallel tape port.
  6. Once loading finishes, the system jumps to SCELBAL's entry point.
  7. You should see the READY prompt in Tera Term (via the 110-baud serial terminal port).
  8. Type SCR and press Enter to initialize SCELBAL’s variable space (required after loading).

Example BASIC Program: Hello World Counter

After SCR, enter this more complex counter loop (all caps, as SCELBAL requires):

10 I=0
20 PRINT "HELLO WORLD";I
30 I=I+1
40 GOTO 20

Press Enter after each line. Then:

The Mark-8 will continuously print “HELLO WORLD” followed by an incrementing number (0, 1, 2, 3...) until you press STOP or reset the machine. Classic 1974 BASIC joy — counting forever on homebuilt hardware.

Legacy

SCELBAL on the Mark-8 — loaded from emulated paper tape straight into RAM — was one of the earliest ways hobbyists could enjoy interactive BASIC programming on their own hardware. Bypassing the need for a separate development system, it made the Mark-8 a true personal computer. In the MicroBasement, this demo — Pi tape emulation, patient loading, READY prompt, and your own counter running forever — keeps that 1974 excitement alive. It reminds us that the personal computing revolution started with simple hardware, clever software, and the patience to wait a few minutes for something wonderful to appear on the terminal. As time allows, we will be using real paper tape in the future to make the experience even more authentic.

Back to Demo's


Copyright 2026 - MicroBasement