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Date: Sun, 22 Jul 2012 23:03:44 -0500 (CDT)
From: Robert Bonomi <bonomi@mail.r-bonomi.com>
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In-Reply-To: <500d2320.f8bzaFvCSay0b7rK%perryh@pluto.rain.com>
Subject: Re: fsck on FAT32 filesystem?
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> From perryh@pluto.rain.com  Sun Jul 22 22:15:48 2012
> Date: Mon, 23 Jul 2012 03:10:40 -0700
> From: perryh@pluto.rain.com
> To: bonomi@mail.r-bonomi.com
> Cc: freebsd-questions@freebsd.org
> Subject: Re: fsck on FAT32 filesystem?
>
> Robert Bonomi <bonomi@mail.r-bonomi.com> wrote:
>
> > MSDOS/PCDOS had -no- O/S functions to directly access actual disk
> > devices.  The ONLY fuctionality provided to the user, by the "O/S"
> > was filesystem based access.  To get 'raw' device access, one had 
> > to bypass the O/S entirely, and use direct BIOS calls (INT 13h).
>
> FALSE TO FACT.
>
> MSDOS/PCDOS had no _documented_ functions to directly access the
> disks, bypassing the file system, but the functions _did_ exist.

I'm sure you can provide the DOS 'function number' for those calls,
and cites to published data confirming.

> The debugger's "read sector" and "write sector" commands used them,
> and I suspect chkdsk, scandisk, and format probably also used them
> although I never had occasion to verify one way or the other.

My experince in porting MSDOS 3.1 to a non pc-clone architecture was 
that fdisk, format, chkdsk, debug, and sys all invoked INT 13H directly.