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Moving Block Program

A small interactive program for a 6502 breadboard computer that uses interrupts and LCD output to visually move a block character across the screen. The block shifts one position to the left each time a button is pressed, creating a simple but effective demonstration of interrupt-driven input and direct hardware control.

🎬 Program Behavior

  • Displays the static message: "moving block:" on the first line of a 16x2 character LCD.
  • A full-block character (0xFF) appears on the second line.
  • Each button press moves the block one position to the left.
  • When the block reaches position 0, it wraps back to position 15.

demonstration-video.mp4

🔍 Program Logic Breakdown

  • Initialization:

    • Configures the VIA to trigger an interrupt on button press.
    • Initializes the LCD with a standard 8-bit, 2-line mode.
    • Sets the block's initial position to the rightmost character (15).

  • Main Loop:

    • Waits 100 ms using a software delay.
    • Clears the second line.
    • Sets the LCD cursor to the current block position.
    • Prints the block character (0xFF).

  • Interrupt Routine:

    • On button press, decreases the block position.
    • If it reaches 0, wraps it back to 15.

🧾 File Contents

  • main.asm – 6502 Assembly source code.
  • main.bin – Assembled binary for EEPROM flashing.

🔧 Hardware Overview

  • CPU: WDC 65C02
  • I/O: 65C22 VIA
  • LCD: 16x2 character display
  • Clock: 1 MHz fixed-frequency module
  • Button Input: Connected to VIA CA1 (interrupt line)

📍 Memory Map

  • PORTA = $6001 – LCD Control Lines (RS, RW, E)
  • PORTB = $6000 – LCD Data Lines
  • DDRA = $6003, DDRB = $6002 – Data Direction Registers
  • PCR = $600C – VIA edge trigger setup
  • IFR = $600D, IER = $600E – VIA interrupt flags/control
  • box_index = $0216 – Tracks current block position

💾 Flashing to EEPROM

See the EEPROM Programming Guide for details on how to assemble and flash this program to your EEPROM using a custom or commercial programmer.

🧠 Notes

  • The program runs from address $8000 with vectors defined at $FFFA–$FFFF.
  • Delay loop timing may vary slightly depending on component tolerances.

🙌 Contributions

Improvements and experiments are welcome. Feel free to fork and build on this!