X-Authentication-Warning:	delorie.com: mailnull set sender to djgpp-bounces using -f
From:	"Tim Nicholson" <djgpp AT gizzy DOT co DOT uk>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Protected Mode disk access
Date:	Mon, 21 Jan 2002 19:48:11 +0000 (UTC)
Organization:	BT Openworld
Lines:	69
Message-ID:	<a2hr9q$b8g$1@helle.btinternet.com>
References:	<1011635483 DOT 28296 DOT 0 DOT nnrp-08 DOT c1ed6b28 AT news DOT demon DOT co DOT uk> <a2hlp7$g58$1 AT nets3 DOT rz DOT RWTH-Aachen DOT DE>
NNTP-Posting-Host:	host217-35-53-59.in-addr.btopenworld.com
X-Trace:	helle.btinternet.com 1011642491 11536 217.35.53.59 (21 Jan 2002 19:48:11 GMT)
X-Complaints-To:	news-complaints AT lists DOT btinternet DOT com
NNTP-Posting-Date:	Mon, 21 Jan 2002 19:48:11 +0000 (UTC)
X-Newsreader:	Microsoft Outlook Express 6.00.2600.0000
X-MSMail-Priority:	Normal
X-Priority:	3
X-MimeOLE:	Produced By Microsoft MimeOLE V6.00.2600.0000
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp
Reply-To:	djgpp AT delorie DOT com

Thanks, I can see the problems - Ho Hum :-)

I thought that was the probably the case but it never hurts to ask.

I take your point on the cache, but multi tasking and virtual machines are
the real problems in our field.


Tim



<broeker AT physik DOT rwth-aachen DOT de> wrote in message
news:a2hlp7$g58$1 AT nets3 DOT rz DOT RWTH-Aachen DOT DE...
> Tim Nicholson <djgpp AT gizzy DOT co DOT uk> wrote:
> > Am I correct in thinking the DJGPP switches from protected mode to real
mode
> > to access the HDD and transfers data in 64K blocks (variable) via the
DPMI
> > to and from DOS?
>
> Yes.  Practically all I/O is done this by, in DJGPP.  It has to be.
>
> > Is it not possible to access the HDD directly in protected mode via a
FAT32
> > Library?
>
> A library wouldn't do it. To make sure you don't blow up the
> underlying DOS, it would at least need a 16bit DOS driver counterpart
> that informs DOS and its programs (smartdrv most notably), of what is
> happening.  Otherwise, you'ld almost certainly corrupt data.
>
> > - Does such a thing exist? and would it speed up disk I/O where
> > reasonable quantities (several MB) of data was been transferred to
> > and from the drive?
>
> Not by very much.  That's what you have that several 10's of KB of
> transfer buffer for, after all: to make sure the mode switch overhead
> is as small as possible compared to the time the realmode call itself
> (i.e. actual disk writing) takes.  For linear reads from protected
> mode, I don't think you can be much faster than DJGPP already is.
>
> Of course, it would still be crazy to implement a large database
> management system in DJGPP... The tininess of the written blocks used
> by such systems would drown your performance in modeswitch overhead.
>
> > The reason for asking is that disk I/O seems much faster under
> > Windows 98/ME etc. and I am guessing this is partly because the
> > whole thing is happening in protected mode.
>
> To some extent.  There's also the fact that Windows talks directly to
> the hardware, whereas DOS always goes via the BIOS, and thus can't use
> some of the modern features, plus it may have to execute lots of code
> that was and continues to be written with a 1980's MFM disk in mind.
>
> > going much faster! I can't use Windows or Linux because the code must be
> > deterministic to meet certification requirments, besides, I like DOS!
>
> "Deterministic behaviour" and "caching" contradict each other to quite
> some extent.
>
>
>
>
> --
> Hans-Bernhard Broeker (broeker AT physik DOT rwth-aachen DOT de)
> Even if all the snow were burnt, ashes would remain.

- Raw text -