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From: rcpieter@svin02.info.win.tue.nl (Tiggr)
Newsgroups: comp.sys.acorn
Subject: Re: MODE 21 & MULTISYNC
Message-ID: <1664@svin02.info.win.tue.nl>
Date: 6 Jan 91 15:29:32 GMT
References: <1990Dec21.031201.3334@odin.diku.dk> <1990Dec21.101354.1261@urz.unibas.ch> <1991Jan06.013149.22420@comp.vuw.ac.nz>
Organization: Eindhoven University of Technology, The Netherlands
Lines: 35

banksie@whitu.isor.vuw.ac.nz (Philip Banks) writes:

>  And indeed that is percisely the case. From my experience any screen
>mode with more than about 220k of screen memory will blank out when the
>floppy drive is accessed. You will notice that the hard drive does not
>cause this problem and this is due to timing factors. With a floppy
>drive you have long start rotation times and a slower rotation speed so
>that for fast disk access the computer *must* be ready to accept the
>data whenever it arrives. However with a hard drive the high rotation
>speed (and the disk cacheing on some of the drive controllers...)
>skipping a rotation is not so serious in terms of speed and the Arc can
>accept the data at its own pace. Thus the screen does not blank out!

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