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

Brantley Coile brantley at coraid.com
Tue Nov 15 01:45:13 AEST 2005

this is a nice conversation.  you're observations are all correct.
see my additional comments below.

> 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.

absolutely correct.  in fact, i've never really figured out
what intel was thinking with the stack segment in protect mode.
you can't use it in C, you can't even use it in a Pascal like language,
or any other language that uses call-by-reference procedure
>>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.

true, but your numbers are a bit off.  640k / 64k = 10 not 6.  the kernel
will take only 2, so you should have 8.  the shared segment for the shell
is about 32k, so maybe you're not that far of.

if the objective is to have a system that is real and has the feel of what
it was like to use a 6th Edition Unix system, this isn't a big deal.  you
and edit, compile and run programs on it.
> 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).

ture enough.  but that's the case if you use the 286 segmens as well.
either that or you make the data==stack segment a little short,
then set the stack to a fixed size near the bottom of the data segment.
if you run off the end or grow the stack too deep, you wrap around
and hit the limit of the segment.
>>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.

i wouldn't bother with a swap device on the floopy version.  you'd just
just use this version to get the disk driver working so you could
have real swap.  swaping to modern disks is pretty fast.  i can swap
an entire 64k segment in less than a millisecond, so it would seem
much faster than the original.
> 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.

the mit x86 stuff would be where i'd start.  i haven't looked to see
if you need to tweak the assignment operators to avoid having
to s/=+/+=/, but it might already be done.

it's all 16 bit stuff: port of PCC, assembler and loader.

an enjoyable discussion.  wish i had time to work on it.


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