blastcomp

This project is a minimal, "bare metal" template for programming Mega Drive (Genesis) games and applications in C.

It's in early stages. Watch out!!

Project Goals

• Less External Libraries as possible
• Dockerized Toolchain
• Hardware Direct Access

Project Structure

File	Description
boot.s	Assembly Startup: Sets the Vector Table, writes the SEGA security header, initializes the stack, and jumps to C.
main.c	Game Logic: The C entry point. Initializes the VDP registers and cycles background colors.
sega.ld	Linker Script: Maps code to Cartridge ROM ($000000) and variables to Console RAM ($FF0000).
Dockerfile	Toolchain: Builds a Linux environment with m68k-elf-gcc and binutils pre-installed.
build.sh	Build Helper: Automates the Docker build and compilation process.

Prerequisites

1. Docker Desktop (or Docker Engine on Linux)
2. Bash terminal (Linux, macOS, or WSL on Windows)

How to Build

1. Clone the repository

2. Make the script executable:

   chmod +x build.sh

3. Run the build script:

   ./build.sh

What happens next?

• The script checks for the md-toolchain Docker image. If it doesn't exist, it builds it (this takes a minute the first time).
• It mounts your current directory into the container.
• It compiles boot.s and main.c using m68k-elf-gcc.
• It links them using sega.ld.
• It extracts the raw binary to create game.bin.

Technical Details

The Memory Map

The linker script (sega.ld) enforces the standard Genesis memory map:

• ROM: 0x000000 to 0x3FFFFF (4MB Cartridge space)
• RAM: 0xFF0000 to 0xFFFFFF (64KB Work RAM)

VDP Initialization

Unlike modern PCs, the Mega Drive VDP is not initialized by an OS. The main.c includes a vdp_init() function that manually sends 19 commands to the VDP Control Port to:

1. Enable the Display (Reg 1, Bit 6).
2. Set the resolution to 320x224 (H40 mode).
3. Define memory locations for Scroll Planes and Sprites.

Compilation Flags

We use specific GCC flags to generate ROM-compatible code:

• -m68000: Target the specific M68k CPU found in the console.
• -ffreestanding: Tells GCC there is no Standard C Library (no printf, no stdio.h).
• -O2: Optimization level 2.

License

This project is open source. Do whatever you want.