Merced - and the ol' Unix story (was Re: OT, but info needed: , RAM uprade)

From: Don Maslin <donm_at_cts.com>
Date: Sat Jan 16 17:22:08 1999

On Sat, 16 Jan 1999, Pete Turnbull wrote:

> On Jan 15, 19:35, Max Eskin wrote:
>
> > In fact, I've always found the limitations of DOS and PCs curious. Does
> > anyone know why:
> >
> > *They refuse to boot from anything but A: or C:
> > *DOS FDISK refuses to create more than 1 primary partition
> > *DOS can't find an ATAPI cd-rom without a driver, even though it's an IDE
> > device
>
> Pass.
>
> > *Why we are limited to 2 floppy drives in DOS
>
> Actually, you can have 4, if you have a controller at the secordary floppy
> controller address as well as at the primaryfloppy controller address.
> Each controller can only have two drives because of a silly hardware setup
> chosen by IBM.

Actually, you can have a lot more than that, given the right
controllers. I have one machine with six different floppy drives operable.
With the MicroSolutions CompatiCard IV - regrettably, no longer
manufactured - you could have up to sixteen drives, four each controlled
by one of four of the cards. I don't really know why you might want that
many drives, but it is possible.
                                                 - don

> On a normal SA400-style interface, there are 4 drive selects, and one of
> each other control signal; each control signal is normally gated (on the
> drive) with the approriate select line. IBM chose to use an arrangement
> where completely separate motor-on signals were provided for the two
> drives. The way they did this was to arrange the signals in the cable such
> that one drive used a particular drive select and the motor signal in the
> standard place, and the other drive used a different select with the moror
> signal on a different wire. Hence the infamous cable twist, such that each
> drive sees a particular drive select (it's the second select line) and the
> motor-on signal in the standard place. However, from the point of view of
> either drive, there is another motor signal on a pin which would otherwise
> be a select line for another drive. Thus you can't use all 4 drive
> selects, and hence only two drives per controller cable.
>
> Another way to look at it, is to assume that IBM wanted all drives to have
> the same jumper settings, including the drive select jumper. But instead
> of putting a twist in the cable which would just swap some of the select
> lines, they used a twist over a wider section. Unfortunately that also
> moves the motor control line to where a select line would normally be.
> Thus they need another motor control line somewhere else (where a drive
> select would be, obviously), and end up only being able to put half as many
> drives on the bus.
>
> --
>
> Pete Peter Turnbull
> Dept. of Computer Science
> University of York
>
Received on Sat Jan 16 1999 - 17:22:08 GMT