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From: banksie@ono.isor.vuw.ac.nz (Philip Banks)
Newsgroups: comp.sys.acorn
Subject: Re: MODE 21 & MULTISYNC
Message-ID: <1991Jan13.012153.10118@comp.vuw.ac.nz>
Date: 13 Jan 91 01:21:53 GMT
References: <1990Dec21.031201.3334@odin.diku.dk> <1990Dec21.101354.1261@urz.unibas.ch> <1991Jan06.013149.22420@comp.vuw.ac.nz> <1664@svin02.info.win.tue.nl>
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Reply-To: banksie@ono.isor.vuw.ac.nz (Philip Banks)
Organization: Dept. of Comp. Sci., Victoria Uni. of Wellington, New Zealand.
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In article <1664@svin02.info.win.tue.nl>,
rcpieter@svin02.info.win.tue.nl (Tiggr) writes:
|>What a nonsense.
|>
|>I'm not sure about what I'm going to put below this line, but I expect
|>it to be slightly nearer the truth than what is quoted above.
|>
|>The major difference between reading from floppy and reading from
|>harddisk is that reading from floppy involves the ARM needing to
|>service an FIQ for every byte that is read.  When reading from
|>harddisk, the ARM is only involved every sector, servicing an IRQ.  The
|>next observation to make is that the FIQ routine resides at an optimal
|>spot in memory for it to take advantage of the next-read-is-sequential
|>feature of ARM/MEMC.  When doing sequential reads, the ARM has priority
|>on the bus over VIDC.  I expect that in hires modes this might cause that
|>VIDC sometimes can't get the next words for the screen in time.
|>
|>Now, all this sounds very fuzzy and doesn't explain why the screen
|>becomes a nice shade of grey, but I guess it has something to do with it.
|>
|>Then pooh drops by and suggests this is done on purpose by the floppy
|>reading routine.  Hohum, reminds me of the good ol' Electron :-)
|>
|>Tiggr

	 Hey! Hey! Hey! Before you completely rubbish my explanation (while it is
	wrong) could you please consider a few mitigating factors instead of being
	somewhat rude! My geographic location places me almost exactly on the 
	oppposite side of the world from Britain and this leads to two big
	problems :- The nearest Branch of VLSI is in Australia some distance
	away from me and consequently unlike most of you Britishers I have not
	yet got a copy of the VLSI data manual on the ARM hardware. Also due
	distance I find it *very* hard to just drop into Acorn and ask their staff
	about the hardware of the Arc. Accordingly I am a little in the dark 
	about the hardware of the Arc.
	  Now when I noticed this screen blanking I noticed two things:- 1)
	it only occurs when the screen mode uses more than about 220K of memory.
	2) It only occurs with floppy drives not hard drives! So I thinks to
        myself what is the difference between hard drives and floppies? The 
	bigest is their rotation speed. So I feel I was quite reasonable in making
	the conclusions I did. I suggest next time you want to correct some one
	you do it a little more tactfully!

                                   
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|   BANKSie! (aka Philip Banks) banksie@isor.vuw.ac.nz            |   @@@@@/#| 
|      An Arc owner stuck in a non Arc spot.                      |   @@@@/##|
|  Well you try drawing the Arc Symbol in Ascii 'Graphics'!       |   @@@/---|
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