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Message from discussion RFC: ULA Replacement for ZX Spectrum SE
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Philip Kendall  
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 More options May 18 2000, 3:00 am
Newsgroups: comp.sys.sinclair
From: Philip Kendall <p...@ast.cam.ac.uk>
Date: 2000/05/18
Subject: Re: RFC: ULA Replacement for ZX Spectrum SE
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Andrew Owen <ao...@brandnewco.org> writes:
> in article ye8ln18jd5c....@cass52.hoyle.star.fleet, Philip Kendall at
> p...@ast.cam.ac.uk wrote on 18/5/00 12:44 pm:

> > I think you need to start thinking in terms of bit fields, rather than
> > simple numbers. The problem here, is that all the low 8 bits are taken
> > by something (ULA, 128Kb/Sinclair joysticks, disk interfaces,
> > Interface 1 x 2, Kempston x 3). To work out what's free, we're going
> > to have to find out the decoding schemes used by all these
> > interfaces...

> Acording to Pera's info bit 4, 2 and 3 are free. If IF1 uses 2 and 3 then
> that leaves bit 4.

It uses 3 and 4, not 2 and 3.

> 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 :-)

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...

> > 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.

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

Phil

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