AlphaCatch: x86 Real-Time Assembly Game

Project Overview

AlphaCatch is a 16-bit arcade application developed in x86 Assembly Language (NASM). Built as a final project for the Computer Organization and Assembly Language (COAL) curriculum, this software demonstrates proficiency in low-level system architecture, direct hardware manipulation, and custom interrupt handling within a DOS environment.

The application bypasses standard BIOS routines to interact directly with video memory and hardware interrupts, resulting in a high-performance, flicker-free real-time rendering engine.

Technical Architecture

This project implements several advanced low-level programming concepts:

1. Direct Video Memory Access (DMA)

Instead of relying on slow DOS interrupts (INT 21h) for output, the engine writes directly to the Video Graphics Array (VGA) memory segment at 0xB800.

• Performance: Achieves zero-latency updates and eliminates screen tearing.
• Double Buffering: Implements a screen save/restore mechanism to preserve the user’s original command prompt state upon application exit.

2. Custom Interrupt Service Routines (ISRs)

Standard hardware interrupts were hooked and replaced with custom handlers to manage concurrency.

• Keyboard ISR (INT 0x09): A non-blocking input handler was developed to process scancodes directly from the keyboard controller ports. This allows for N-key rollover simulation, enabling simultaneous movement for multiplayer modes and "Shift" key speed modifiers without input buffer delays.
• Timer ISR (INT 0x08): Manages the game loop, physics calculations, and the countdown clock independently of the rendering cycle.

3. Algorithmic Logic

• Linear Congruential Generator (LCG): A custom implementation of the LCG algorithm utilizes the system timer to seed pseudo-random number generation for dynamic character spawning.
• Collision Detection: Real-time logic compares memory offsets of the player sprites against falling objects to determine valid "catches" or "misses."

Game Specifications

Modes of Operation

• Single Player: The user controls a primary collection basket. Difficulty scales dynamically; as the score increases, the drop velocity of characters accelerates.
• Multiplayer (Co-operative): Two users share the same input interface. The custom ISR allows independent control of two separate baskets simultaneously on the same screen.

Controls Interface

Action	Player 1 Keybind	Player 2 Keybind
Lateral Movement	Left / Right Arrow	A / D
Velocity Boost	Shift (Hold)	Shift (Hold)
Menu Navigation	Up / Down Arrow	N/A
Confirm Selection	Enter	N/A
Terminate Program	Esc	N/A

Setup and Execution

Prerequisites

To compile and execute this 16-bit application on modern architecture, the following tools are required:

• NASM: The Netwide Assembler for building the source.
• DOSBox: An x86 emulator for the runtime environment.

Build Instructions

1. Clone the Repository

   git clone https://github.com/rauf-babar/Alphabet-Catcher.git
   cd Alphabet-Catcher

2. Assemble the Source Compile the assembly file into a .COM executable using NASM:

   nasm main.asm -f bin -o main.com

Runtime Instructions

1. Launch DOSBox.
2. Mount the project directory as a virtual drive (ensure the path matches your local directory):

   mount c C:\path\to\AlphaCatch-Assembly
   c:
   

3. Execute the binary:

   main.com
   

Gameplay Demonstration

Coal.Project.mp4