Ultimate 1MB BIOS and SIDE Loader

In 2012, Sebastian Bartkowicz (Candle O’Sin) asked me to write a PBI BIOS to for Ultimate 1MB. During the months that followed, Candle thought of more and more things to add (such as the ability to read and write disk images mounted in the FAT partition of the CF card), and after weeks of coding and debugging, I had produced something approaching a final version.

In 2015, a desire to significantly broaden the functionality of Ultimate 1MB and overcome some of the limitations of the PBI BIOS led me to completely re-write every firmware component and produce a new main BIOS, PBI BIOS and XEX loader.


  • A PBI implementation of Matthias Reichl’s High-Speed SIO code which does not require OS patching and which works with external cartridges
  • The ability to set the system date and time from inside the BIOS setup utility
  • Expanded PBI BIOS settings on a dedicated menu page
  • System information page showing CPU type, audio and video hardware, and more
  • User-selectable BIOS hotkeys
  • Configuration profiles
  • BIOS plugin API for user-defined external device control
  • New, faster XEX loader supporting multiple FAT partitions, APT partition mounting, fast recursive searching and built-in FAT DOS
  • Dedicated High-Speed SIO menu
  • SIO2BT support (at up to 56kb/s) and high-speed SIO for PCLink and other serial devices
  • Built-in CIO “Z:” RTC device handler
  • Rapidus plug-in and support for PBI BIOS extensions in fast linear 65C816 RAM

New to Version 4.2

  • SIDE3 Loader-style menu design
  • Fixed Sophia 2 plugin
  • Fixed JED version reporting

Firmware Download

The Ultimate 1MB firmware update can be applied by running UFLASH.XEX and flashing the 64KB FIRMWARE.ROM file to the ‘Firmware’ slot. One should also update SpartaDOS X using the customised version provided, which includes the latest hard disk tools on the CAR: volume. Alternatively, one may re-flash the entire ROM with one of the 512KB images provided, which include a useful selection of BASIC and OS ROMs (highly recommended when upgrading from the original firmware).

Ultimate 1MB Firmware version 4.2 (24 January 2023)

Important: Owners of Ultimate 1MB with the original v.1 CPLD code from Candle’s first run of boards (with glossy black PCB and requiring current limiting resisitors on the MMU cable) must ensure that they update to the current JED prior to installing the new BIOS, otherwise they risk bricking the device. Boards with the PBI feature (including all manufactured by Lotharek and those from Candle’s second run) will safely run the new BIOS without modification. If your U1MB has a white PCB or was purchased new from a commercial vendor since 2013, it is guaranteed to be compatible with the new firmware.


The most recent JED file for owners of older U1MB boards:


Note: this JED still reports as version 1 in the BIOS, and it’s HIGHLY UNLIKEY that you need to update any but the very oldest U1MB boards.


Be sure to download the toolkit ATR which includes several utilities and drivers not included in the SDX ROMs:


User Manual

The User Manual contains installation instructions and usage notes for the Main BIOS setup utility and SIDE/XEX Loader:

Ultimate 1MB, Incognito, 1088XEL/XLD, SIDE Firmware Manual

Loader Quick Reference

Technical Documentation

Technical documentation describing the firmware memory map, configuration data layout and Plugin architecture is provided below:

Ultimate 1MB, Incognito and 1088XEL-U1MB BIOS Technical Documentation

Plugin Source Code

To complement the Technical Documentation, you can also download the source code for the U1MB BIOS plugins:

Ultimate 1MB BIOS Plugin Source Code

Previous Versions


If wish to report any issues with the new firmware of have any suggestions on how it might be improved, you may email me directly via the address on the contact page, or post in the discussion thread on the AtariAge Forum.

Ultimate 1MB Links

Please lend your support to Lotharek at lotharek.pl. Thanks to him, Ultimate 1MB and other excellent Atari 8-bit upgrades are always available for convenient online purchase.

Purchase Ultimate 1MB from lotharek.pl

Purchase Ultimate 1MB with my firmware pre-installed from lotharek.pl

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!

Tutorial Videos