Mail Archives: opendos/2000/11/02/14:24:14
> I don't understand why any OEM would forget about using the BPB if it is
> part of DOS. They are sent the MS source code for DOS and build on it. Why
> would they want to go to the trouble to delete that from the source code.
I guess they wanted the space to play with for boot sector code, and let DOS
use the default BIOS-based BPBs.
> > Either that one or the one at offset 0Ah in the BPB - I'll guess that
DOS
> > uses the FAT one in preference to the BPB one.
>
> I don't think so. I am not certain what that is used for, it is an
extended
> signature byte. It may be different in some versions, but it seems like it
> was always a 29 hex. (At least for HDD.)
Well, I've found the following table listing some values for the first entry
in the FAT:
F0 Unidentifiable
F8 Fixed disk
F9 Double-sided, 15 sectors/track
F9 Double-sided, 9 sectors/track (720Kb)
FC Single-sided, 9 sectors/track
FD Double-sided, 9 sectors/track (360Kb)
FE Single-sided, 8 sectors/track
FF Double-sided, 8 sectors/track
(The two entries for F9 are verbatim). This coding appears to be the same
used in the BPB. (I just pulled the first sector of a FAT from a Zip disk
and the ID there matches the BPB ID).
I think a fixed disk is F8h - I've checked this both on the actual HDD in
this system and on a random Zip disk (the latter has a BPB as follows:)
16AB:0100 00 02 04 01 00
.....
16AB:0110 02 00 02 00 00 F8 BC 00-20 00 40 00 00 00 00 00 ........
.@.....
16AB:0120 00 F0 02 00 00 00 29 42-2F E6 1A 52 41 4C 31 20 ......)B/..RAL1
16AB:0130 20 20 20 20 20 20 46 41-54 31 36 20 20 20 FAT16
(This is a copy and paste from DEBUG, using the commandsL 100 3 0 1 and D
10B L 33)
I think Win98 (FAT32) uses a different BPB format - the media descriptor is
in the right place, but after that I think the values are laid out
differently to encode the 32-bit locations.
> [Copy protection]
> Some even used tracks beyond 39 and DOS would not copy those. AFAIK there
> were never any HD disk copy protection, it was only used on DD disks.
Hmm... A piece of software I recently disassembled (trying to find out why
it would crash on load on this machine) looks for a special track in the
last cylinder of a disk, and can be installed to a 1.2Mb floppy (the INSTALL
program writes the copy-protected track with a special 'installed' flag).
> This may have changed or may some OEM DOS does it differently, but what
IBM
> DOS DISKCOPY did/does is first it formats the disk then it copies all of
the
> files. It may or may not copy the BR. But as I recall, if I copied a 3.3
> formatted diskette with MSDOS 5.0 installed and used 5.0 DISKCOPY, the BR
> would show the OEM ID as MSDOS5.0.
My Amstrad 3.2 DISKCOPY copies the whole lot as a lump of sectors - the boot
sector and all. I know, since the 'Starting out with the PC' instructions
include 'copy all four system disks supplied', two of which has a DOS Plus
boot-sector while the others are MS-DOS - the boot sectors are copied
correctly. It doesn't even adjust the data in case of a bad sector - so if
you copy a good diskette onto one with known bad sectors, the bad sector
marks in the FAT disappear.
Regards,
Ben A L Jemmett.
(http://web.ukonline.co.uk/ben.jemmett/, http://www.deltasoft.com/)
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