[TUHS] Redoing "V6on286" or porting V7...?

[Wesley Parish]

On Mon, 14 Nov 2005 09:16, Peter Jeremy wrote:
> I've also looked at this in the past.  I lost interest when my
> intended target box died (and I realised I didn't have the free time).
>
> On 2005-Nov-12 08:18:50 -0500, Brantley Coile <brantley at coraid.com> wrote:
> >i may be wrong, but i don't think you want the segments.  pdp-11
> >segments divided the address space into eight, well, segments.  each
> >could be grow up or grow down and had a physical base and a limit.
> >intel segments don't work that way.  the major difference is that the
> >segment selector, instead of being the upper most three bits of the
> >virtual address, is not even in the address space at all.
>
> Actually the 286 MMU is very primitive compared to the PDP-11.  One
> crucial difference is that Unix has the implicit assumption that the
> stack is in the data space - which is not true on the 286.  This
> difference is fairly critical to Unix and makes it impossible to
> accurately reproduce the traditional Unix memory protection.
>
> >so the eight, for a pdp-11/40 say, or the sixteen for the 11/70 don't
> >really apply.  instead just give each data segment the whole 64k
> >address space and it'll not klobber anybody else.  just itself.
>
> This is fairly wasteful of RAM.  Keep in mind that V6 cannot page -
> a process is either entirely in memory or entirely on disk.  If you
> limit yourself to 640K RAM, you are probably restricting yourself
> to about 6 resident processes.  And swapping means moving 64K of
> data to/from disk.
>
> This approach also makes brk(2) a no-op.  There's nothing to prevent a
> process using as much heap as it wants (or crashing into the stack).
> (No SIGSEGV or SIGBUS errors).
>
> >you're correct to imply that v6 takes up MUCH less space than other
> >systems.  it's amazing how small you can run this.  the root in the
> >6th edition system is only 1.4MB.  and that includes the normal
> >binaries, libraries and the source to the kernel.  it'll run great on
> >a floopy.
>
> You will need to have a swap partition.  And if each process has a
> 64K data segment, you'll need a comparatively large amount of swap.
> The biggest disadvantage of a floppy will be performance - latency
> of about 80msec, I/O of about 40KB/sec.
>
> Have you identified a suitable C compiler?  There aren't many open
> source 16-bit x86 compilers, especially ones that can run in 16-bit
> mode.  None of the ones I found generated particularly good code.
> This may impact you ability to get the larger utilities (and maybe
> the kernel) to fit into 64K I-space.

Would it be possible to use some of the DOS ones?  At least to bootstrap a 
more true-to-Unix one?

I'm thinking of the likes of bcc, available plus source at the FreeDOS web 
archive.  Or OpenWatcom's DOS 16-bit.

Wesley Parish

-- 
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