Incognito 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 Incognito. 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 Incognito 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 video standard, CPU speed, etc
  • 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 and recursive searching
  • SIO2BT support (at up to 56kb/s) and high-speed SIO for PCLink and other serial devices
  • Built-in CIO “Z:” RTC device handler

New to Version 3.10

    • Even faster search speeds in loader
    • Keyboard and joystick acceleration in loader allows faster navigation of long directories
    • File extensions on scrolling FAT filenames now remain visible even when highlighted
    • CF card compatibility improved
    • Loader SD/CF adapter issues fixed
    • XF551 now driven at full speed by HSIO driver
    • ATR swap button now works in loader
    • Improved DCB checking in PBI BIOS and support for immediate-mode sector IO
    • Improved boot drive handling
    • ATR button can now be disabled

New to Version 3

Version 3 of the alt firmware brings several improvements, including:

  • Number of BIOS configuration profiles increased from three to four
  • Massively faster and more powerful loader search facility
  • Loader’s built-in FAT DOS now supports subdirectories
  • Loader now remembers the last accessed FAT folder
  • Loader scripts may include paths
  • Loader can now open the location of any already mounted image
  • Usability improvements to the loader and BIOS setup menu
  • Optional synthetic ‘IO sound’ with ATRs and hard disk partitions
  • Improved loader compatibility
  • Several important bug fixes


Incognito Firmware version 3.10 (June 2020)

If you wish your existing configuration profiles to survive the update process, please activate profile 1 before performing the update.

Important: The firmware consists of three separate files: the Main BIOS, SIDE loader and PBI BIOS, which should be updated IN THAT ORDER. The machine should then be power-cycled.

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

Loader Quick Reference

Technical Documentation

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

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


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.

Previous Versions


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