September 3rd, 2024

Oya, I've heard of mega-ROMs

The article outlines the process of creating a bootleg cartridge for the MSX to play "Knightmare II," emphasizing the importance of understanding the Konami mapper and memory management techniques.

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Oya, I've heard of mega-ROMs

The article discusses the challenges and processes involved in creating a homemade cartridge for the MSX computer platform, specifically to play Konami's game "Knightmare II: The Maze of Galious." The author reflects on the high prices of original cartridges, which have become collectibles, prompting the desire to build a bootleg version. The project required understanding the unique mapper used by Konami, which involves complex page-switching techniques to access larger game data than the MSX's standard memory could accommodate. The author details the learning curve associated with the MSX's memory mapping system, which divides the 64kB address space into pages and slots, allowing for more efficient memory management. The project gained momentum after receiving support from a fellow MSX enthusiast, leading to the acquisition of necessary components. The article also explains the technical aspects of the Konami MegaROM mapper, which allows for the paging of larger ROM sizes, and the specific hardware components required to implement this system. The author emphasizes the importance of understanding the mapping scheme to successfully create a functional cartridge.

- The project aims to create a bootleg cartridge for the MSX to play "Knightmare II: The Maze of Galious."

- Understanding the unique Konami mapper and its page-switching techniques was crucial for the project.

- The MSX's memory management system divides its address space into pages and slots for efficient data access.

- Support from fellow enthusiasts helped advance the project, highlighting the collaborative nature of retro gaming communities.

- The article details the technical requirements and challenges of implementing the MegaROM mapper for larger game cartridges.

Link Icon 4 comments
By @dietrichepp - 7 months
> Normally, I would expect floating address pins to be completely random, but this was very consistent.

I think “random” assumes some kind of perfect conditions. There will be leakage current to ground, leakage current from Vcc, and the actual threshold between high and low for the input will be some particular value.

Just a quick look at the data sheets for the SN54LS670 from TI, and I see that the output pin leakage current is rated at no more than 20 µA. That’s worst-case, but if you apply Ohm’s law and plug in 3.3 V, you get something on the order of 160 kΩ output impedance. I have a limited understanding of electronics and I’m sure I’m missing something, but I’d expect a lot of the output pins to settle on a reasonably stable voltage when they are turned off. Maybe natural variations in geometry or chemistry will push some pins higher and other pins lower.

You can run an entire CPU core with less than 20 µA current these days, just to give it a sense of scale.

By @ibizaman - 7 months
> [About buying a used cartridge] In retrospect, after all this adventure, that doesn’t seem like much, but “spending fifty bucks instead of learning something” is not the name of this blog.

Absolutely love this quote.

By @jbverschoor - 7 months
The Maze of Galious is such an underrated game in terms of IP.

I had the pleasure of attending MSXGOTO40 last december.

Great explanation what megarom actually means, I never knew

By @wileydragonfly - 7 months
Very interesting post. I played some MSX recently and there’s some good games there. I’m sure I’m going to end up with an MSX2 some day.

I recently started making some SEGA master system cartridges. Your only option there is to destroy a Monopoly or After Burner. :-|