? about kernel size..

[John Clark]

Not to barge in to your discussion… but yes there were Unix variants that worked on the 286. Xenix was one such OS. Xenix being Microsoft’s idea of avoiding the Unix name…

Originally Xenix ran on PDP-11, and was ported to other machines that have long passed into the oblivion of technological Hell.

As for ‘preemetive multitasking… of course one can run a preemptive scheduler on almost any CPU that has a clock interrupt.

I wrote one for the 286 based on the kernel described in Douglas Comer’s “Xinu” book.

Products with that hack where produced for about 8 years… in the early-mid-80s. I wrote it such that it would run in a DOS box and allow for machine control of various robotic systems for industrial inspection machines.

In any case one can develop a multitasking kernel to run in a non-MMU based system… just lends itself to crapping out on the least provocation…

What an MMU provides is hardware ‘address translation’ such that the application can run in an ‘virtual absolute’ addressing space,
have memory protection such that errant code can’t clobber other tasks or the kernel, and also given appropriate devices,
have ‘swapping’ for larger than real memory applications.

The 286 ‘segment’ registers gave a bit of ‘translation’ capability in that one could address relative to the segment registers, and so, code and data could be place in available member and context switched would update the segment registers.

The segment registers were a crappy way of accessing memory if one had long linear arrays of data to process… such as image processing… which happened to be the application of my work at the time…

And one could always implement an external MMU which was popular with the Motorola M68K which was sort of the contemporary alternative to the Intel x86 line.

John Clark.

On Mar 8, 2016, at 2:25 PM, Markus Hitter <mah at jump-ing.de> wrote:

> Am 08.03.2016 um 22:56 schrieb Brad Walker:
>> But, are you saying that no engineering has been done on this yet OR no
>> amount of engineering could make it work?
> 
> If I recall correctly from some 25 years ago, memory address mapping
> (which is what a MMU does) is mandatory for preemtive multitasking. An
> i286 can't run a Unix-like OS either.
> 
> 
> In 2008 I tried to get FreeBSD down to its minimum, too. The success
> post is about all what's left today:
> 
> https://lists.freebsd.org/pipermail/freebsd-embedded/2008-October/000604.html
> 
> The task to get there is simple and straightforward, but time consuming:
> go step by step through the kernel configuration to disable whatever you
> can spare. Configure, build, try, repeat. If you need a small entire
> system, do the same for packages and every single file you copy into
> your system image.
> 
> 
> Markus
> 
> -- 
> - - - - - - - - - - - - - - - - - - -
> Dipl. Ing. (FH) Markus Hitter
> http://www.jump-ing.de/
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