To:	opendos AT delorie DOT com
X-Comment-To:	Robert W Moss <domanspc AT juno DOT com>
Cc:	fd-dev AT topica DOT com
References:	<20010216 DOT 032625 DOT -270235 DOT 0 DOT domanspc AT juno DOT com>
Message-Id:	<2.07b7.RBGP.G8UQW4@belous.munic.msk.su>
From:	"Arkady V.Belousov" <ark AT belous DOT munic DOT msk DOT su>
Date:	Fri, 16 Feb 2001 16:31:16 +0300 (MSK)
Organization:	Locus
X-Mailer:	dMail [Demos Mail for DOS v2.07b7]
Subject:	Re: Max. drive letter, etc. (was Hard Disk 20gb and dos)
Lines:	52
MIME-Version:	1.0
Reply-To:	opendos AT delorie DOT com

X-Comment-To: Robert W Moss

Hi!

16-ζΕΧ-2001 03:26 domanspc AT juno DOT com (Robert W Moss) wrote to
opendos AT delorie DOT com:

RM> ROM BIOS CAPACITY LIMITATIONS
RM> In addition to the capacity limit of 504M, the standard

     This is wrong information. BIOS allows address up to 8Gb per drive (256
heads, 1024 tracks/cylinders, 63 (!) sectors gives 7.9 Gb). 504Mb limit
caused by different limitation _both_ of BIOS and IDE (which allows access
only for 16 heads).

     Initial LBA (Logical Block Addressing) idea based on the fooling IDE
adapters by BIOS: BIOS _translates_ passed for them values into range,
appropriate for IDE. Later IDE standard also allows "linear addressing",
whereas IDE adapter accepts only resulting offset in the sectors.

     Unlike 504Mb limit to break 8Gb limit not sufficient to replace
_internal_ BIOSware, input specification also should be changed and software
should access the new specification. I suggest this is not too complex
addition for _existing_ old OS/DOS, but this was not done.

RM> ROM BIOS is limited to supporting only two hard disk

     This is limit of old IDE also:

RM> drives.  The enhanced BIOS is limited to 128 drives

     BIOS itself accepts drive number in byte-register and even if all
drives number will start from 80h we may address 128 drives.

RM> maximum.  Most SCSI and IDE adapters get around the
RM> standard BIOS two-drive limits by incorporating an
RM> enhanced BIOS on board that  takes over the disk interface.

     This is very long ago when BIOSes adapts all possible changes in this
way.

RM> When DOS boots it automatically assigns C: to the first
RM> primary  partition on the first hard disk and then assigns
RM> D:, E:, etc.. to all the primary partitions it finds and then
RM> assigns the remaining Alpha ID's to the logical partions
RM> on all extended partitions.  After that it assigns all the
RM> removable drives and CD ROM drives.
RM> So what happens when it hits 24?  ANYONE??
RM> If SCSI  can can have 28 drives how are they numbered?

     This hardly depends from OS and don't rack your brains on this until
you meet this limit. :)

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