Path: utzoo!utgpu!cunews!cognos!alzabo!andras
From: andras@alzabo.uucp (Andras Kovacs)
Newsgroups: comp.sys.acorn
Subject: Re: MODE 21 & MULTISYNC
Message-ID: <1991Jan7.040708.29118@alzabo.uucp>
Date: 7 Jan 91 04:07:08 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>
Reply-To: andras@alzabo.UUCP (Andras Kovacs)
Organization: Brian's XENIXlings, Ottawa, Canada
Lines: 69

In article <1664@svin02.info.win.tue.nl> rcpieter@svin02.info.win.tue.nl (Tiggr) writes:
>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

    Observation #1: On my A310 which has a ComputerWare ST506 Hard Disk
podule connected to a MiniScribe 3053 40MB (24 msec), in Mode 21 (which
chews up 320K video memory) the hard disk reads are noticeably slower.
The screen doesn't blank out, but the read (with an already positioned head)
takes approx. 1/4 of a second, which is much-much slower than other reads.
    Observation #2: when you do a read from the floppy while in high-res mode,
the blankout is similar to the effect when you shut down the video refresh
to drive the processor as fast as it goes.
    Quotation #1: "The processor will not be stopped if it is about to perform
a sequential memory access; instead, the DMA operation is postponed until the
processor requests an internal cycle or a non-sequential memory access.
Excessive DMA hold times are avoided by limiting the maximum number of
consecutive processor S-cycles to three." (VLSI VL86C010 book)
    Quotation #2: "Maximum video/cursor DMA latency: ... 1070 ns." (Same book)
    Speculation: IMO the DMA accesses take priority over anything; this is
why the processor itself slows down on large video buffers. This effect works
with the hard drive also. In the case of the floppy, I suspect that they(*)
intentionally shut down the video refresh 'cause it would take away to much
of the memory bandwidth and/or processing power; without it the floppy reads
would be atrociously slow.
    Footnote on (*): When I say "they", I mean the Acorn dudes. When the vote
was on for comp.sys.acorn, I counted approx. 30 ("yea") votes from acorn.uk
sites. It would be really nice if someone would come on line from Acorn and
explain to us mortals the nitty-gritty details of this problem.

    Please excuse me for the long posting...

  Andras
-- 
Andras Kovacs       "Could somebody tell me what kind of a world we live in
andras@alzabo.UUCP   where a man dressed as a BAT gets all of my press..."
Nepean, Ont.                                               The Joker