On Fri, 30 Dec 2022, Paul Ruizendaal wrote:

> One could argue that one of the drivers of the success of CP/M in the 
> 1970’s was due to its clear separation between the boot rom, BIOS and 
> BDOS components. As far as I am aware, Unix prior to 1985 did never 
> attempt to separate the device drivers from the other kernel code. I am 
> not very familiar with early Xenix, it could be that Microsoft had both 
> the skill and the interest to separate Xenix in a standard binary (i.e. 
> BDOS part) and a device driver binary (i.e. BIOS part). Maybe the 
> differences in MMU for the machines of the early 80’s were such that a 
> standard binary could not be done anyway and separating out the device 
> drivers would serve no purpose. Once the PC became dominant, maybe the 
> point became moot for MS.

Certainly Microsoft *did* have an operating system, as early as 1981, that 
had the concept of separated BIOS and BDOS, but they didn't write it, they 
bought it 🤪

That said, given that it existed in MS-DOS, I can't imagine it wouldn't 
have been impossible to also implement in Xenix...

-uso.