[Simh] Custom ROMs on PDP-11 sim

[Timothe Litt]

On 16-Dec-17 16:38, Johnny Billquist wrote:
>
> However, such a device would be wrong to start with. And I've never
> seen anyone do that, but sure, it could be done.
>
Read the OP.  He has such a device.  It has ROMs with PDP-11 code -- or
at least, that's his assumption.  And it's a Unimate robot controller. 
If that assumption is correct, it fits the architecture that I described.

> But with the limited space in the I/O page, you would not want to have
> the same code replicated all over the space.
It depends.  If I were an OEM, and I anticipated a small number of my
devices per CPU, this would be a good way to avoid having a separate ROM
module.  Robots costing $100Ks fit that bill. 

Further, the typical code isn't very large.  For example, the M9301-YA,
512 words - in I/O space, can bootstrap 8 different devices AND has 7
CPU self-tests and a memory test.  The -YB has a console emulator,
self-tests, and 11 device bootstraps.

So it's quite plausible to have a host-run self-test, bootstrap,
calibration, or other code on- board without consuming unreasonable
amounts of I/O space.

There are other schemes to avoid duplication, but I'll leave those as an
exercise for the reader.

>
> So in the end no, I do not believe that it would ever be the right
> thing to have relative addressing for CSRs. Your example is valid in
> that such a design would make sense to have relative addressing, but
> such a device is already a wrong thing to start with, and should not be.
>
You don't get to decide and enforce what is right/wrong or what should
not be.  As a system architect, I had some power to ordain what should
be.  But one quickly discovers that even that power is limited.  You can
have an opinion, but sometimes reality butts in.  Your idea of
architectural purity and/or design constraints will be tested (and
changed) by Manufacturing, OEMs, customers, and the field - which have
their own challenges.

It is unwise to assume that you know enough to say "never" or "always". 
Unless you like being proven wrong.


>     Johnny
>


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.trailing-edge.com/pipermail/simh/attachments/20171216/c3e878bf/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 4577 bytes
Desc: S/MIME Cryptographic Signature
URL: <http://mailman.trailing-edge.com/pipermail/simh/attachments/20171216/c3e878bf/attachment-0001.bin>