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Message from discussion RFC: ULA Replacement for ZX Spectrum SE
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Andrew Owen  
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 More options May 18 2000, 3:00 am
Newsgroups: comp.sys.sinclair
From: Andrew Owen <ao...@brandnewco.org>
Date: 2000/05/18
Subject: Re: RFC: ULA Replacement for ZX Spectrum SE
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in article ye8k8grkicl....@cass52.hoyle.star.fleet, Philip Kendall at
p...@ast.cam.ac.uk wrote on 18/5/00 4:07 pm:

>> This would make port 88h a possible contender yes?

> No. Remember bit *reset* means active. #88 has 6 bits low! #FF-#88=#77
> would be better :-)

I knew I was missing somehthing. Right, I understand now. Thanks!

> But see below...

>> What stuff uses bit 4 at present?

> If1. Bit 2 is still apparently free though, but that is used by the ZX
> Printer and the Disciple (see eg Ramsoft's Tech Guide, where all the
> ports are listed as being #xB). I believe other disk interfaces use
> the same bit...

> According to Warajevo's docs, the Kempston joystick responds to all
> ports with bit 5 reset. Traditionally, 31 is used for this purpose,
> but this does apparently leave 6 and 7 free. Pera's docs list these as
> `Kempston-soft', but I don't know what this really means -- anybody?

> It does look like 6 and 7 might be available, though...

If I'm following this right, 6 is used by the Kempston mouse, which uses
FADF, FBDF and FFDF. It is also used by the Scorpion's Centronics interface
at FFDD. Bit 2 appears to be used by the Scorpion to switch in RAM banks
8-15.

>>> OTOH, if we're not going to be writing large amounts of data to this
>>> port, we could consider using one of the upper 8 bits of the bus, as
>>> the extra set up time wouldn't then be important.

>> It's not a lot of info but how do we do this without disturbing the lower
>> part?

> Just make the port be of the form #xxFF -- then anything looking at
> the LSB just sees all set and ignores it.

So it's just a case of finding out which 'xx's have been used then.

> Important question time: is there a list out there of how various
> peripherals decode the ports they're on?

I've been looking but I can't find one.

What's the best way of passing 16-bits via an OUT as quickly as possible?

-Andrew


 
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