[vbox-dev] Patch to PC BIOS file: \src\VBox\Devices\PC\BIOS\floppy.c

Dave Meagher stope19 at gmail.com
Wed Feb 18 07:00:22 UTC 2015

> From: Klaus Espenlaub <klaus.espenlaub at oracle.com>
> Subject: Re: [vbox-dev] Patch to PC BIOS file:
>         \src\VBox\Devices\PC\BIOS\floppy.c
> To: vbox-dev at virtualbox.org
> Message-ID: <54E1F3D1.1060501 at oracle.com>
> Content-Type: text/plain; charset=windows-1252; format=flowed
> On 16.02.2015 12:18, Michal Necasek wrote:
> >
> >     Hi Dave,
> >
> >   Thanks for the patch! I'll take a look at it in detail soon (at first
> > glance it looks very reasonable). However, I'm not going to commit
> > anything unless I can run at least the most basic verification that the
> > code does something useful. So what can I use as a testcase so check
> > that the modified code does what it's supposed to?
> It seems to do some additional 0x40:0x90 and 0x40:0x91 updating only
> (the 0x18 one doesn't update the DPT which I assume a real BIOS would),
> and for that it's surprisingly much code. Proper handling of these calls
> should also include telling the floppy controller about the density etc.
> - of course I know that it doesn't have an influence on a software
> floppy controller implementation behavior, as it doesn't care about
> precompensation and so on. Some even smarter software out there might
> check the floppy controller registers and get confused.
> In any case there's significant functionality overlap with function
> floppy_media_sense, and as the BIOS code size has hard limits any code
> redundancy should be eliminated.
> Also I wonder if 0x40:0x92 and 0x40:0x93 should be updated by the BIOS,
> too. They aren't as well documented as the others.
> Klaus

Hi Klaus,

My understanding is that 17h/18h are most often used when a format is
required of a diskette at something other than the maximum capacity of the
drive. For example a format of a 720k disk in a 1.44MB drive (eg: dos
command FORMAT a: /F:720). In this scenario, one of these entry
points would be called prior to the first int13 5h format call. My
reading  indicates
that the 17h/18h calls only 'setup' the media status for the specified
parameters (although 18h also points to a DPT entry). I suspect normally
the DPT would contain an entry for each possible combination of
media/drive, so there would be an entry for a 1.44MB dive, and also one for
720k media in a 1.44 drive. VirtualBox ‘sort of’ does this (there seems to
be an entry for a 360k disk in a 1.2mb drive – but not for a 720k disk in a
1.44mb drive).

This really means that the 18h call could possibly be somewhat smaller if
it could just iterate over the DPT looking for the matching entry to
return. While I considered this, I thought that - particularly as a first
time contributor - changes of this magnitude (splitting out the DPT
structures into a header file, adding new entries to the DPT ..etc) would
not help my cause (larger changes to the BIOS code I suspect would be
resisted). So, I tried to introduce these new functions using code with the
same ‘look & feel’ (I hope) as the existing code.

As for these entry points needing to tell the floppy controller 'what has
changed', I'm not sure this is required, as I suspect this this is one of
the first tasks done by the format (5h) call – I suspect via a call to
'floppy_prepare_controller'. However I suspect your knowledge of this area
exceeds mine.



> >     Regards,
> >        Michal
> >
> > ----- Original Message -----
> > From: stope19 at gmail.com
> > To: vbox-dev at virtualbox.org
> > Sent: Sunday, February 15, 2015 10:07:41 AM GMT +01:00 Amsterdam /
> > Berlin / Bern / Rome / Stockholm / Vienna
> > Subject: [vbox-dev] Patch to PC BIOS file:
> > \src\VBox\Devices\PC\BIOS\floppy.c
> >
> > Hi,
> > I'd like to contribute a patch to the file:
> > '\src\VBox\Devices\PC\BIOS\floppy.c'. This patch is submitted under the
> > MIT License.
> > The patch provides an implementation for 2 BIOS entry points that are -
> > in the current source tree - stubbed out as 'not available'. The 2 entry
> > points are:
> > 3x9,XT2,XT286,PS)
> > As mentioned, these are not currently implemented - so I see little risk
> > to existing software. The patch was developed on a Win32 XP platform,
> > the usual dev tools and Open Watcom 1.9
> > The motivation for this patch is a requirement to get an old (early
> > 90's) database system to load in VirtualBox. It uses these entry points
> > not to perform format operations, but to setup the floppy for subsequent
> > IO operations. Why it does this I cannot say...
> > The patch is in the attached text file, and was created against the
> > current file image from the source repository.
> > Regards,
> > Dave
> >
> ------------------------------
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