Mail Archives: opendos/2001/02/24/17:54:08
----- Original Message -----
From: "Robert W Moss" <domanspc AT juno DOT com>
To: <opendos AT delorie DOT com>
Sent: Friday, February 16, 2001 4:26 AM
Subject: Re: Max. drive letter, etc. (was Hard Disk 20gb and dos)
> From 'Upgrading and Repairing PC's', by Scott Mueller,
> sixth edition, 1997, QUE Books. ISBN: 0-7897-0825-6.....
>
> PAGE 762:
>
> SCSI
>
> Most systems support up to four SCSI host adapters,
> each with with up to 7 hard disk drives for a total of 28
> physically installed drives.
This is outdated. There are now controllers that can handle 31
drives each. In fact if you look at the SCSI specs, a single
controller can handle up to 256 devices
> (Since this book was published I recall reading some
> place about a new super wide SCSI setup where you
> can install 16 devices per adapter.) (?)
Currently 31.
> ROM BIOS CAPACITY LIMITATIONS
Actually, the BIOS limitations do not really apply to SCSI. The
motherboard BIOS will find the SCSI controller BIOS. The SCSI
controller BIOS and components does the rest. Note: When setting
up CMOS, you do not put any entries in it for SCSI drives. The
system BIOS does not really limit SCSI. There may be limits of
drive sizes and other drive characteristics by the system BIOS if
the SCSI BIOS cannot lie to it.
> In addition to the capacity limit of 504M, the standard
> ROM BIOS is limited to supporting only two hard disk
This is just a limitation of the system BIOS. For example, if you
had a drive larger than 504MB and it came with Disk Manager, the
DM drivers could overcome this. As for the two drive limit, there
has been controllers made for many years that overcame this. I
believe the company's name that produced one such controller
is/was PSI (PSR? or something similar.) They had a controller in
which you could plug in up to four modules in each controller. You
could have any combination of four plug in modules for the
following types: 2 IDE drives each, 4 IDE drives each, 2 MFM/RLL
drives each, 2 ESDI drives each, and SCSI. You could mix and match
these modules anyway you wanted. These were primarly used on UNIX
servers, but also had DOS drivers as I recall. They cost something
$600 each plus an amount for each module installed. As I recall
these were being sold back about the early and mid 90's. I sent in
for some information on these several years ago and still probably
have their brochures boxed up some place. At the time I recalled
the possibility of having 4 SCSI modules, each supporting 7 drives
for a total of 28 drives. Since you could have up to four of these
controllers, that would mean a total of 96 drives per
computer/server.
> So what happens when it hits 24? ANYONE??
> If SCSI can can have 28 drives how are they numbered?
Keep something in mind about SCSI. When talking about SCSI, we are
talking devices. These devices do not have to be drives. There are
SCSI devices such as: printers, plotters, and numerous other
devices I have seen over the years (believe or not, even SCSI
floppy drives.)
Let me give you an example on my own system. I have an ST-150
SCSI-II FAST tape drive. DOS/WINDOZE never assigns it a drive
letter, yet they know it is there and can use it with the
appropriate software. When the SCSI drivers are loaded in DOS, it
shows all the available SCSI devices.
As for you question about what happens after 24 drives have been
assigned a letter, in DOS nothing happens, it will not see the
drive. I played with this some time ago when putting more than 24
total partitions on my drives and making them all FAT 16 so that
DOS could see them. Those above 25 could not be accessed. Linux,
however, saw all of them.
> If the Enhanced BIOS allows up to 128 drives How Are They
> Numbered, or is that just for UNIX and LINUX, where they list
There are many operating systems that will work on the IBM PC
platform that do not have DOS's limitations that could use that
many and probably more. In Linux, for example, you can make all of
your physically drives look like one single drive. Therefore if
you had say 31 drives on a single U3W SCSI controller, you could
make these drives appear as a single drive.
Early IDE did this when you connected more than one drive to a
single controller. This was done in the controller itself. This
was one of the first methods used in IDE to have more than one
drive.
> Personally, I see no use for so many drives/devices except in
> a network server, and then it would have to have a pretty big ,
> fast CPU, or 2/4/6/9 CPU's, and 500+ MGB fast memory just to
Not really. RAID systems can use hundreds of drives and not have
to have multiple CPUs and a ton of memory. For years, they used
RAID systems with 1GB hard drives installed, because at that time
it was the fastest set up. Also, you are thinking in terms of
limited IDE. You should read up on SCSI and RAID systems.
You can get a copy of the SCSI specs, but these are too technical
for most people. I would recommend on getting some of the Linux
HOWTO's releated to SCSI, hardware, drives, system design, etc.
These are pretty easy to read. You may need a glossary to
understand parts of it, simply because you may be unfamiliar with
some of the terminogy used.
SCSI is a completely different world than IDE and is not limited
like IDE is. SCSI is intelligent, whereas, IDE is dumb.
Pat
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