The iconic, standalone font used by the Commodore 64 "Dead Test" diagnostic cartridge has been documented for the first time, revealing its unique design and a hidden Easter egg.

Key Points

• The "Dead Test" diagnostic cartridge (Rev. 718220) uses a custom 8K ROM font that functions independently of the C64’s built-in character ROM.
• The font design is heavily inspired by the MICR E-13B character set, commonly used for magnetic ink recognition on bank checks.
• Researchers identified a "mystery" character at screen code $21, which is a "transit" symbol from the MICR set, serving as a deliberate Easter egg.
• The cartridge operates in "Ultimax mode," a compatibility feature originally designed for the short-lived Commodore Max Machine, which allows the system to bypass standard ROMs.
• The font includes 58 specific characters, including digits, uppercase letters, and box-drawing symbols, while using a $AA fill pattern for undefined codes.

Why it Matters

This documentation preserves a piece of computing history by detailing the technical implementation of a specialized diagnostic tool used by professional repair technicians. By reverse-engineering the font and its Ultimax mode integration, enthusiasts can now better understand and emulate the low-level hardware operations of the Commodore 64.