author: Tyson Liddell
tags: 8088 IBM-PC

Writing a boot sector game

I recently completed writing a boot sector game in 8088 assembly language targeting the 5150 IBM PC. It's less than 512 bytes in size, runs without the operating system and explores a significant slice of the hardware-software interface of the IBM PC.

See the detailed technical write up in the git repository for a detailed outline of lessons learned and some neat tricks to make this work. Highlights include:

• Used BIOS interrupts for keyboard, system-timer and video initialisation.
• Worked with memory-mapped IO to drive the display.
• Used VRAM as a storage area by hiding data in character attributes, leaving a tiny memory footprint. This game can run on the most memory constrained systems.
• Iterfaced directly (without using the BIOS) with several hardware systems including the PIT, PPI and PC speaker. Used bit banging to drive the speaker with the CPU (because why not?).
• Navigated BIOS source code to determine further details about the power-on state of the system and hardware.
• Diagnosed and fixed tricky, non-deterministic hardware faults including PC speaker and PIT contention on the PPI, all without access to debugging tools.
• Worked around innaccurate emulation/hardware by implementing a pseudorandom number generator, improving the code and portability in the process.