Dec TU-56 Question

From: Ben Franchuk <>
Date: Sun Aug 11 15:44:00 2002

Tony Duell wrote:

> Tobe honest, it would probably be easier to make a new processor from
> scratch if you wanted to do something like this to the hardware. Just as,
> I susepct, if you wanted to design an emulator for a ficticious
> processor, then you wouls wnt to start by designing a clean, orthogonal,
> instruction set. But it would be possible to start from an existing
> 'machine' in either case.

Not in all cases. Working on a homebrew FPGA 12/24 bit cpu
I don't really have a base machine to work from. The current
machine I am working on will not have a emulator but be rather
debugged on-line. I do have a earlier similar CPU design but
it was not clean as I liked but since I have a emulator
written and mostly debugged hardware I can use this for software
testing and debuging for the new machine.

> But no real PDP11 has those instructions. So if you add them to a PDP11
> emulator, then the result is no longer a PDP11 emulator, in that there is
> a sequence of instructions that behaves differently on the PDP11 and the
> emulator. Just as if you wire up a 32 bit ALU and registers to a PDP11
> processor board, the resulting machine is no longer a true PDP11.

No it would be just a PDP-11 with strange I/O board. While I have not
looked at the PDP-11 instruction set in depth there does not seem to
be room for more instructions.

> There have been all sorts of interefaces proposed and made over the years
> to let one computer have access to the memory of another. People have
> designed cards that appear as I/O ports to the 'master' system and act
> as proper bus master on the other side (so they can request the bus from
> another genuine processor on, say, a VME bus and then access the memory
> and peripherals over there). Simpler ones just insist on being the only
> master on the 'slave' side -- there have been many attempts to add ISA
> slots to other machines in this way to allow the use of cheap ISA I/O
> cards, for example.

But really other than video or network stuff what is there?
You are better off building your own card as ISA I/O sucks!

> In the second case you'll find things like a 2N3904 transistor that's
> open-circuit base-emitter, and which is therefore not a transistor at
> all. Or a 10uF capacitor that behaves as though it has a 100 ohm resistor
> in series with it, inside the can (this is what's mean by ESR --
> effective series resistance). The components are not what they claim to
> be any more. If you rebuilt this cirucit with known-good components then
> it would behave as originally intended. I really can't think of a
> software equivalent of this.

True, but data formats can change over time requiring software tweeks now and then.
Received on Sun Aug 11 2002 - 15:44:00 BST

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