Mail Archives: opendos/2001/03/20/15:07:25
On 2001-03-20, Rob McGee wrote:
> Suppose you have 4 IDE hard drives, each with 2 primary DOS
> partitions and 2 logical DOS partitions in an extended partition.
> How are drive letters assigned? [...]
> Obviously partition 1 on the primary master is C. Is partition 2 D?
> Then which is E, number 1 on the secondary master or the primary slave?
Not necessarily. Partition C is the first *active* primary partition,
and the other non-active primary partition(s) will be ignored by DOS.
If the first primary partition (1) on 80h is active, then it will be C,
of course.
The rule is, that DOS starts to assign drive letters (starting with C:)
by going through the list of physical non-removable drives (units 80h
and higher), and will for each drive assign a drive letter to the
active primary partition (if itīs FAT), the other primary partitions
of any drive will be ignored. Then DOS will start again with unit
80h, but looks for logical drives in the *first* extended partition
found in each drives partition table - logical drives in optional
other extended partitions will be ignored. (Usually, there is only
one extended partition by definitionem!)
> What happens when you throw some BIOS-controlled SCSI into
> the fray? Say you have 2 SCSI drives on a single bus, partitioned
> just like the IDE gang. Are their primary partitions assigned drive
> letters before the logical partitions on the IDE drives?
There is no exception to the rule. The only question is, if the
INT 13 BIOS controlled SCSI drives will have priority over
the IDE drives or not. On modern systems, you can change this
in the mainboardīs BIOS setup usually under "Boot order".
On systems without such an option, SCSI usually take precedence
over IDE just because the SCSI BIOS is linked in prior to the
(relevant part of the) main BIOS.
So, your two SCSI drives would become units 80h and 81h,
and the IDE drives 82h, 83h, 84h, 85h. This will change the
assigned drive letters, but not the rule: Drive C: will now
by the first active primary partition on the first SCSI drive
(if itīs FAT, as in your case).
> What about a second SCSI bus just like that one? Which SCSI
> bus is first? (Yes, I realize that there are not enough drive
> letters for the second bus, but I am interested in the order
> of assignments primarily.
This depends on the SCSI IDs, your SCSI BIOS, and its
configuration, not on DOS. Usually SCSI units are scanned
from low to high IDs, this will cause SCSI drives with
lower IDs to have lower INT 13 physical unit numbers,
but in most cases, you can change the scan order and
thereby the priority of the busses.
> Since all this is hypothetical we can take out the IDE
> secondary slave and one of the SCSI drives; then we're
> still under 24.)
Again, no difference for DOS. Assuming there would be
no SCSI drives, you would have physical units 80h, 81h,
82h, and 83h for your four IDE drives, removing any of
them (for xample, the one on the secondary slave), you
will have only three left: 80h, 81h, and 82h, thatīs it.
DOS just uses what INT 13 provides, no matter what
type of drive you use.
Matthias
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Matthias Paul, Ubierstrasse 28, D-50321 Bruehl, Germany
<Matthias DOT Paul AT post DOT rwth-aachen DOT de> <mpaul AT drdos DOT org>
http://www.uni-bonn.de/~uzs180/mpdokeng.html
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