[Simh] RT-11 source

[Johnny Billquist]

On 2016-10-23 14:45, Rhialto wrote:
> On Sat 22 Oct 2016 at 23:03:41 +0200, Johnny Billquist wrote:
>> On 2016-10-22 18:55, Al Kossow wrote:
>>>
>>>
>>> On 10/22/16 7:44 AM, Ray Jewhurst wrote:
>>>> I have an idea for project to teach my self rudimentary OS design and I would like to use RT-11 as an example
>>>>
>>>
>>> Did anyone ever port MINIX to the PDP-11?
>>
>> I don't think so. I suspect it would be non-trivial, as the memory
>> limitations of the PDP-11 require that you sometimes need to design with
>> that in mind from the start, or else face serious problems.
>
> But the original 8086 MINIX (which I used in university in the course on
> operating system design) used "small memory model". That means it sets
> the segment registers for each process to a fixed value and therefore
> its processes (including kernel server processes) were limited to 64KB
> of text and 64KB of data. That would fit with PDP-11s with sepid just
> fine.

Possible. I haven't looked at that code, so I can only guess. However, 
the segmentation registers essentially mean that you have more than 64K, 
as you note (64K+64K). With separate I/D space you accomplish the same 
on the -11, but it works in a different way, so depending on how the 
code is written, it could be easy, or it could turn out to be a real 
headache.
Like I said, you sometimes needs to design things with the hardware in 
mind, as the solutions might be very different, even with similar 
constraints.

And of course, this then requires that you handle the MMU on the -11. 
How did Minix work? I don't think the 8086 had much of a proper MMU. Did 
you have any memory protection, or did the kernel somehow live in your 
address space? If not, how did the remapping changes needed on 
interrupts work? (Yes, I do not know the 8086 that well...)

> At some point during inter-process communication it no doubt uses the
> segmentation registers more actively (I don't remember the details) but
> that can be done in some other ways on the -11.

Everything is always possible. It's just a question of how much work, 
and how slow will the result be.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol