There were several operating systems available for the Act Sirius 1. CP/M-80, CP/M-86, MS-DOS 1.25, MS_DOS 2.11 and finally MS-DOS 3.1. With version 3.1 it was necessary to have the Victor Universal Boot ROMS (or the Issue ROMS) fitted in order to run it.

The Sirius was unusual in the fact that the BIOS was included within the operating system (on disk) and not held in a dedicated chip on the motherboard as with modern personal computers.
There were several revisions released for operating system versions:

• CP/M-80 -
• CP/M-86 - Ver 1·0 / 1·0a
• CP/M-86 - Ver 1·0 / 2·2
• CP/M-86 - Ver 1·0 / 2·2a
• CP/M-86 - Ver 1·1 / 2·4 Rev A
• CP/M-86 - Ver 1·1 / 2·4 Rev D

• MS-DOS - Ver 1·25 / bios 2·5x
• MS-DOS - Ver 1·25 / bios 2·6
• MS-DOS - Ver 1·25 / bios 2·61
• MS-DOS - Ver 1·25 / bios 2·61 (HD Version)
• MS-DOS - Ver 1·25 / bios 2·71 (HD Version)

• MS-DOS - Ver 2·11 / bios 2·91 (FD Version)
• MS-DOS - Ver 2·11 / bios 2·91 (HD Version)
  

• MS-DOS - Ver 3·11 / bios 3·02 (See above text)

Should you have the Victor PlusPc retrograde option fitted within your Sirius then you will need the 'duel' operating system that is required to utilise it's functions fully. This is intended to allow switching between the two modes ie 'Victor' mode and 'PC' mode a very basic IBM XT mode that allowed reading and writing to 360kb PC floppy disks and the running of some PC programmes.

There were at least three revisions of this `duel´ system.

• MS-DOS - Ver 2·11 / bios 0·1x (HD Version)
• MS-DOS - Ver 2·11 / bios 3·00 (HD Version)
• MS-DOS - Ver 2·11 / bios 3·04 (HD Version)

Another less well know operating system was the USPD system.
There will be more on this system at a later date.