While the NM-186 was based on a standard x86-compatible 80186, its architecture was very different compared to the IBM PC’s. The major difference is the memory map, which enabled the NM-186 to make more memory available to applications than was possible in real mode on the IBM PC: as far as I can determine, up to 708KiB for applications, and an additional 512KiB in a RAM drive (the system officially supported up to 1.5MiB of RAM, and I’ve seen claims that up to 925KiB could be made available for applications). The NM-186 also had a different floppy disk controller and graphics controller, providing higher-resolution graphics (up to 640×350 with four colours), and it included a sound chip, the [AY-8910](https://en.wikipedia.org/wiki/General_Instrument_AY-3-8910). Software written specifically for the Nimbus could take advantage of all that; as you’d imagine, it was mostly educational software.

As standard the NM-186 ran an OEM version of MS-DOS, specifically adapted to its architecture; this was DOS 3.10 (3.05 on early models). It also had various custom versions of Windows, starting with 1.0 and going up to 3.0 (which was adapted for use in schools: icons and groups couldn’t be moved around in Program Manager). Other titles were ported as well. The base system sold in education environments included an “IBM mode” with an “IBMulator” which was supposed to handle well-behaved PC software, at least software which could work on MDA or CGA.

There was an expansion board which provided better PC compatibility, the [IBM Mode Utility Board](https://www.thenimbus.co.uk/nimbus-parts-list/IBM-Mode). This added a PC speaker, RTC and various other components necessary to provide hardware PC compatibility.