XEX Loader and U1MB firmware for SIDE3

After my completely rewritten firmware for Ultimate 1MB, Incognito and SIDE/SIDE2 transformed the usability and functionality of those devices and brought tremendous added value to the end user, I am proud to present the SIDE3 Loader for the exciting new SIDE3 cartridge. This fantastic device – designed by Candle O’Sin and sold by Lotharek – has all the functionality of SIDE2, plus cartridge emulation and many other powerful improvements. Features include:

  • Use of inexpensive, easily sourced SD cards instead of CF cards
  • Emulation of a wide range of cartridge types, up to 1MB in size
  • Real time clock with battery-backed NVRAM
  • A massive 8MB of flash ROM and 2MB of fast SRAM
  • Versatile ROM/RAM access with DMA engine
  • Hot-swappable media
  • Dual mode (SDX/Loader) operation, controlled by switch
  • Menu button (doubles as ATR swap button when used with U1MB)
  • Excellent build quality with stylish case design

The SIDE3 loader likewise represents a step forward from the previous SIDE/SIDE2 loader. Features include:

  • Up to 1,000 FAT filenames in memory: no more paging through long folders
  • Mount cartridges, ATRs and partitions simultaneously, all from the same menu
  • Special RAM-based operating system permits booting of some ATRs without U1MB
  • Tight integration with U1MB where present (using special SIDE3 U1MB plugins)
  • Powerful recursive search facility which runs in the background while you browse results
  • Handling of up to fourteen FAT partitions
  • Easy mounting of multi-disk volumes via MAP files
  • Built-in mini DOS allows read-only access to FAT16/FAT32 volumes on ‘D1:’


You can download the latest (development) versions of the SIDE3 loader via the links below.



Improvements in build 0.29:

  • Sixteen line filename/menu display
  • Improved navigation and screen update speed
  • Improved scroll bar rendering
  • Fix for OSS 043M cartridge emulation
  • Fix for MaxFlash 128K (1Mbit) emulation
  • Ability to set on-board real time clock (will be required for FAT writes)
  • Underpinnings of WIP FAT-writes put in place (note: write capability is not yet active)
  • ‘Hard emulation’ setting added (completely disables SIDE3 registers until menu button is pressed)
  • RTC initialisation bugs corrected
  • Fixed regressions regarding FAT volume names and reported device/volume sizes
  • CTRL+H (Root directory) lock-up fixed
  • RTC reset bug fixed
  • Out-of-spec FAT16 volume recognition fixed
  • MAP file processing bug fixed

For proper Ultimate 1MB operation with SIDE3, please install the release candidate main BIOS and PBI BIOS for SIDE3:


Please note: the SIDE3 main BIOS and PBI BIOS are not compatible with the older SIDE and SIDE2 cartridges.


The loader archive contains a self-contained XEX which you can copy to you SD card and launch directly from the loader currently installed on your cartridge. See the README for further details.

The U1MB archive contains an U1MB main BIOS and PBI BIOS which you should flash to Ultimate 1MB.

Note: the SIDE3 main BIOS and PBI BIOS are intended for use with SIDE3 only and are not backwards compatible with SIDE/SIDE2. To use the older SIDE cartridges, please re-flash the proper firmware.

Loader Quick Reference


See the XEX Loader section in the U1MB/Incognito firmware manual for details of how to use the new loader.

Licensing and Donations

While the firmware is free at the point of delivery for personal use, if it is to be included on commercially sold devices, the firmware is subject to a licensing fee which should be arranged with the author in advance. Developers interested in customised versions of the firmware tailored to their particular hardware should contact the author.

In light of the fact that firmware licensing revenue is modest and that updates are offered for free to the end user, donations are welcomed and appreciated. Indeed, it has been my mission to facilitate easy and safe firmware updates, and regular in-the-field upgrades are encouraged to ensure that devices remain up to date and bug fixes are delivered in a timely manner.

Thank you!

SIDE3 Videos