Creating a 256-Byte DOS Demo: The Endbot Experience

Endbot is a remarkable 256-byte demo that showcases the power of compact coding. This DOS-based program, designed to run via DosBox-X, features a dynamic robot sprite, a growing explosion, a scrolling checkerboard landscape, and a MIDI soundtrack, all packed into a single .com file. To build this demo, you'll need FASM (Flat Assembler) to compile the code into a raw binary. Running it requires specific DosBox-X configurations to ensure proper MIDI emulation.

## Building and Running Endbot

To create the Endbot demo, use FASM to compile the assembly code into a .com file. This process is straightforward, involving a single command without a linker step. Running the demo requires DosBox-X, configured to emulate MPU-401 for MIDI output. The configuration file specifies settings for VGA display, CPU type, and MIDI device, ensuring the demo runs smoothly.

## Code Walkthrough

The code begins by setting up the VGA mode and initializing the timer. The main loop processes each pixel, syncing with the timer to maintain a consistent frame rate. MIDI notes are output at specific intervals, creating a rhythmic soundtrack. The explosion effect is calculated using a clever trick to derive X and Y coordinates from the pixel index, allowing for a smooth, expanding circle.

The robot sprite is animated through a series of bit manipulations, creating the illusion of damage over time. The sprite's movement is controlled by toggling bits of the timer, resulting in a zigzag pattern. The background features a scrolling landscape, achieved through perspective calculations and a checker pattern.

## Data and Size Optimization

The sprite and MIDI data are cleverly combined into a single memory block, with some bytes serving dual purposes. The use of efficient assembly instructions, such as `xlat` for color lookup and `btc` for sprite damage, minimizes the code size. The demo's exit routine is a simple `ret` instruction, leveraging DOS's program termination mechanism to save space.

Endbot exemplifies the art of size optimization, using every byte to its fullest potential. The demo's compact size is achieved through a combination of clever coding techniques and efficient use of DOS and assembly language features.