Ultimate 1MB BIOS and SIDE Loader

In 2012, Sebastian Bartkowicz (Candle) asked me if I could write a BIOS to realize the PBI hard disk functionality he was developing for the 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, we had something approaching a final version.

In 2015, a desire to significantly broaden the functionality of Ultimate 1MB and correct some problems with the PBI BIOS led me to completely re-write the the firmware for the device, including the main BIOS, PBI BIOS and XEX loader. Among new features provided by the firmware are the following:

  • 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 2

Version 2 of the alt firmware brings several improvements, including:

    • Number of configuration profiles increased from two to three
    • Greatly increased XEX loading speed
    • Better VBXE integration
    • Numerous usability improvements to the loader and BIOS setup menu
    • Better control of internal BASIC state
    • Improved compatibility with SD/CF adapters
    • Several critical bug fixes

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.

Ultimate 1MB Firmware Update (July 2018)

Note: Owners of Ultimate 1MB with the original v.1 CPLD code from Candle’s first run of boards 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, it is guaranteed to be compatible with the new firmware.


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 and 1088XEL-U1MB Firmware Manual

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


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.


While it has been my privilege to write software for Ultimate 1MB, Incognito and SIDE, I enjoy no licensing agreements with hardware vendors. Since the hardware itself cannot function without firmware, please consider making a small donation (via the “Donate” button) to help ensure the continuation of software development and technical support. Donations also assist with website hosting and equipment costs.

Thank you!