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From: davidb@brad.inmos.co.uk (David Boreham)
Newsgroups: comp.sys.transputer
Subject: Re: Booting T's from byte-wide ROMs.
Message-ID: <16837@ganymede.inmos.co.uk>
Date: 24 Jun 91 18:40:42 GMT
References: <9106210142.AA05933@multi.ee.usu.edu>
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Reply-To: davidb@inmos.co.uk (David Boreham)
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In article <9106210142.AA05933@multi.ee.usu.edu> stiles@MULTI.EE.USU.EDU (G. S. Stiles) writes:
>Does anybody know if it is possible to boot transputers from byte-wide
>ROMs??
>

Sure, if you have a T225...but I guess that's not your question ?

The T4/T8 devices were not designed with byte-wide ROM in mind.
However rumours persist of various tricks which can be used to 
achieve the desired effect. I should add that I have no evidence
for any of these ideas ever having been properly tested. We always
hang ROMs off T225's or use large amounts where 32-bits is an advantage.

1. Make a demultiplexor which assembles a 32-bit wide word from
   four ROM accesses. This one gets plenty of mentions but almost
   certainly costs more than four ROMs, no matter how you do it !

2. Put the ROM with a state machine, connected to a link adaptor.
   This works. The SM addresses the ROM and counts up, sending 
   byte-by-byte down the link. You connect the link adaptor to the
   transputer and boo from link. (Wastes a link).

3. By some devious cunning with the op-codes, you hang an 8-bit ROM
   on _ONE_ byte of the 32-bit memory interface. Program the ROM 
   with a clever program which only needs one byte to read different
   data. The rest of the bus can be pulled high/low to fake no-ops
   or whatever. Never seen this done but heard rumors. You only need
   a program which copies a bootstrap into RAM and then runs that
   so we're not talking about anything too complex.
   This would be neat if it works except for the bus pulling which 
   is going to screw up the loading and cover the board in resistors.
   (Was that the thing you were mentioning Lyle ?)

4. Order 200,000 parts and we'll put a mask ROM on the chip :)

Cheers, David.

David Boreham, INMOS Limited | mail(uk): davidb@inmos.co.uk or ukc!inmos!davidb
Bristol,  England            |     (us): uunet!inmos.com!davidb
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