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Reverse Engineering the Commodore 64 Freezer Cartridge: A Deep Dive

2025-06-14

This article delves into the reverse engineering of Commodore 64 freezer cartridges, such as the Final Cartridge III. These cartridges leverage the C64's Ultimax mode and NMI interrupts to achieve functionalities like freezing programs, applying cheat codes, and saving game states. The article meticulously explains the technical challenges of the freezing process, such as coordinating 6502 CPU instruction cycles with Ultimax mode activation, and how limited memory resources are utilized for displaying menus and managing state backups. The author also analyzes the cartridge's backup mechanisms and game trainer functionality, praising the developers' deep understanding of the C64 hardware and their masterful coding skills.

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Hardware Freezer Cartridge

Siemens PC 100 Assembly Manual Reveals Undocumented 6502 Opcodes

2025-05-09

A 1980 Siemens PC 100 assembly manual surprisingly documents "illegal" opcodes for the 6502 processor. The PC 100, based on the Rockwell AIM-65, features extra instructions labeled "Sonderbefehle" (special instructions), including AAX, DCM, LAX, and ISB, offering potential performance gains for programmers. While not officially supported and subject to change, these undocumented opcodes reveal a deeper understanding of the 6502 by Siemens engineers, adding a fascinating layer to retrocomputing history and highlighting the creative exploration of hardware and software capabilities at the time.

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Hardware Siemens

Microsoft's Hidden Easter Egg: A Retro Tech War Story

2025-04-26

This article unravels the story behind a hidden "MICROSOFT!" Easter egg embedded in early versions of Microsoft's BASIC interpreters. From the 1975 Altair BASIC to Commodore PET's Commodore BASIC V2, Microsoft cleverly concealed this message to assert code ownership. The egg resurfaced across various platforms, even leading to a second, more overt Easter egg in the TRS-80 Color Computer. The article delves into authorship, removal reasons, and its intriguing place in computer history, showcasing the competition and technical details of early tech companies.

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Tech Easter Egg

The Mystery of the 6502's Illegal Opcodes

2025-04-23

The MOS 6502, powering classics like the Commodore 64, Apple II, and NES, is famous for its 'illegal' opcodes: 105 undefined instructions out of 256. While many articles document their effects, this one delves into their origins. By analyzing the 6502's internal Programmable Logic Array (PLA), the author reveals how these undocumented instructions arise from the chip's design. Examples like the 'LAX' instruction (a combined LDA and LDX) and the 'KIL' opcodes (which halt the CPU) are explained, showcasing how the 6502's architecture unintentionally created functional, albeit undefined, instructions.

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Hardware