How to tell...

[Jerome H. Fine]

 >Johnny Billquist wrote:

>Sellam asked if anyone would be able to tell that Magica (a real PDP-11)
>have been replaced by MIM (an emulated PDP-11).
>
>The answer is yes, and no.
>The only telling thing is the speed. It is much faster than a real
>PDP-11/93, which it emulates. But apart from that, I can't see how you
>could tell.
>
Jerome Fine replies:

It also depends on how much of E11 you decide to use.

(a) It is possible to define and implement unused PDP-11
instruction codes that NO real PDP-11 ever uses!  My first
preference would be 32 / 64 Bit Integer Add / Subtract
Instructions.

(b) John Wilson has a DLL which allows a PDP-11 user to
directly access the PC RAM: via a "register" in the I/O
page.  The file is EMEM.DLL (Emulated Memory).  Eventually
I intend to replace the uses of the "hardware MMU" in the
software with the EMEM.DLL so that I don't waste the whole
8,192 bytes of the PAR devoted to the use of the MMU.  It
will also be much faster and can be as large as the user
wants - as much memory (100s of MBytes from PC RAM:) as
the PC has available for E11 to use.  This would be for
VIRTUAL  A(100000000) in FORTRAN, something impossible
with a real PDP-11.

(c) The MTPS instruction could be modified to have a value
that would specify E11 in addition to any other PDP-11.

At present, only (b) is actually available - although this
could be done in hardware for a real PDP-11 so it would not
really say it is not a PDP-11/93.  But (a) and (c) could
also be done very easily.  Does anyone else have any other
requests that they consider important?

Sincerely yours,

Jerome Fine
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