devkit-mega-everdrive-x7

README

devkit-mega-everdrive-x7

Development utilities for the Sega Genesis/Megadrive with Mega Everdrive X7 (https://krikzz.com/)

The Sega Megadrive (known as Genesis in US) is a video game console released by Sega in 1988. Modern development cartridges have been developed for the system, and one such cartridge is the Mega Everdrive X7, which features a USB Serial TTY and an SD memory card. The Mega Everdrive X7 allows game roms to be loaded from a filesystem on the SD card, but also allows game data to be sent to the game console over the USB serial TTY. After game software loads the USB serial TTY can also be used by the Megadrive to communicate with host programs on a computer, enabling developers to write code for debugging as well as adding new features such as internet connectivity to the game console.

The official software distributed for the Mega Everdrive X7 is windows-native and written in C#, but offers some compatibility with other systems via the mono project. This project is an attempt to increase the accessibilty of development using the Mega Everdrive X7 by creating natively cross-platform utilities.

Running Games from USB

The primary feature of the USB port on the Mega Everdrive X7 cartridge is that it allows the console to run a new game ROM without requiring the SD card or the X7 cartridge to be removed from the system.

I have managed to create a utility called megaedx7-run, written in golang using a cross-platform serial library, which can interact with the Sega Megadrive/Mega Everdrive X7. This program can load arbitrary game roms over USB and execute them on the game console in various modes as supported by the Mega Everdrive. These modes are:

• sms: Sega Master System ROM
  • untested
  • requires supporting SMS BIOS ROMs on SD card of Mega Everdrive X7
• md: Megadrive/Genesis ROM
  • Tested with Sonic 2, Streets of Rage, Toe Jam & Earl
• cd: Mega CD / Sega CD ROM
  • untested
  • may be problematic due to address space collisions between the Mega CD add-on and the Mega Everdrive X7
• m10: Unknown
• os: Unknown; seems to load Mega Everdrive X7 firmware ROMs
• ssf: Allows ROMs > 4MB
  • This mode seems to enable bank switching a la Super Street Fighter 2: The New Challengers. Bank switching allows ROMs larger than the 4MB address space which the Sega Megadrive allocates for the cartridge
  • Some emulators look for the ASCII string 'SSF ' at ROM offset 0x105; this mode might be useful for such ROMs
  • Some technical information about this bank switching technique is available:
    • https://emu-docs.org/Genesis/ssf2.txt
    • https://github.com/ekeeke/Genesis-Plus-GX/issues/67

The Mega Everdrive X7 cartridge seems to expect raw ROM dumps; other formats have not worked in my testing. ROMs with the ASCII string 'SEGA' at offset 0x100 have worked in my testing.

Debugging Over USB

I hope to look into this in the future.