greg%brl-bmd@sri-unix.UUCP (10/17/83)
From: Gregory Hogg <greg@brl-bmd> Hello, Can a Double Sided Double Density disk drive read/write Single Sided Single Density disks? This question is for both 8 inch and 5 1/4 inch drives. Greg the Hogg
ABN.ISCAMS%usc-isid@sri-unix.UUCP (10/17/83)
Greg: Mine can -- donno if that's a function of my wonderful drives (Morrow Decision I with 8"DSD - I think a Tandon, but forget the model #), or a function of the Morrow CBIOS that has all the different density formats built in. Not quite sure how the CBIOS decides what it's reading, but I cheerfully read and write to IBM SSSD and SSDD disks I've formatted on my own system, AND other disks formatted and written to by some sort of 8" drive in a Santa Clara 10Meg hard disk. Now the Santa Clara (with their CP/M for a 56K Apple) won't read mine, no way, not even in SSSD (something funny about my sectors, I think - I can only go down to 256-byte sectors with my disk formatter), but I have no trouble reading it. If I format the disks on the Santa Clara, I can fill the disk on my drive and the Santa Clara can read it just fine. Hope that's an answer. PS: I got some Cromemco 5 1/4 disks in from Langley AFB (I think Cromemco), supposedly SSSD soft sector, but be damned if my CP/M Apple can read them! Donno why - I don't like the Apple so very much, and my disassembly of Santa Clara's CP/M and the original Microsoft 48K CP/M is not yet complete, so I don't have a clear picture of the disk drivers yet. Regards. David Kirschbaum SGM, USA (civilian persona Toad Hall)
towson%amsaa@sri-unix.UUCP (10/18/83)
From: David Towson (CSD) <towson@amsaa>
Greg - Yes, a double-side double-density DISK DRIVE can read and write
single-side single-density disks. HOWEVER, there is more to this than
the disk drive alone. The controller and software must also work with
the single-side single-density format. As far as the drive itself is
concerned, single-side single-density just uses one head and a different
encoding scheme. The bit-rate is the same for both single and double
density, but the bits are used "more economically" in double density,
giving an increase in storage capacity by a factor of about 1.8(not
actually 2). It is the controller that encodes and decodes the data, and
it is also the controller that determines which head is used at any
given time. Some double-side systems treat each side as a separate disk
having its own separate directory. Other double-side systems treat both
sides as a single volume having a single directory. The difference is
determined by the software that does disk I/O. In summary then,the drive
is able to handle either format, and the single-chip disk controllers
that I know of for double-density can also handle single density(however,
I don't claim to know about all available single-chip controllers), but the
software in a particular system may or may not be able to switch the disk
controller to either format.
Daveciaraldi@Rochester.ARPA (10/18/83)
From: Mike Ciaraldi <ciaraldi@Rochester.ARPA> The question can be divided into several sub-questions, with different answers: 1) The drives themselves--I have never encountered a drive which works for DSDD than would not also work on SSSD. The problem we used to have, before DD became so widespread, is that some drives could not handle the DD, but that was several years ago. 2) The controller. The way data is stored on the disk, i.e. the pattern of data and clock pulses going to the read/write head and ultimately onto the media, is controlled by the circuitry of the controller and the controller chip. The most common chips, e.g. Western Digital 179x series, can handle both, and most modern controllers handle both, under software control. In short, older controllers could often handle only SD, almost all modern contollers can handle both DD and SD. Concerning SS vs. DS., this is again a chip/board question, and the controllers that have DS should be able to run SS with no problem. Aside--J. Pournelle's column in the new Byte asks if you can use SS 8-inch drives on a Zenith Z-100. I have certainly used SSSD diskettes on such a system, but the drives themselves were DS. 3) Software. The IBM PC always uses DD, whether SS or DS. In gneral, drivers can be written to handle both, and, in fact, the newest versions of CP/M have explicit places in the Disk Parameter Header and Disk Parameter Block for recording number of tracks/sector, etc. These data tables are within the operating system, but the actual driver could ignore them, I suppose. The most common way to hnadle the differing densities and other formats is something like this: the driver itself can handle several types of disk formats, and uses the info in the DPH and DPB to decide things like how many bytes to transfer in each read or write operation. How do the data tables get set up? An older way was to make it an explicit command. e.g. a keyboard command that made drive B: DSDD. Woe unto you if you stuck a SS or SD disk in that drive! Another way was to give alternate names to the drives, e.g. A: and B: would be physical drives 1 and 2; (assumed SSSD) referring to C: and D: gave you the same physical drives, but formatted DSDD. Again, if you stuck the wrong disk in you would be in trouble. More modern software switches automatically whenever you log in a diskette. It works like this: the driver reads the first track and sector of the new disk. (and possibly other sectors, too). If it cannot read them SD, it tries DD. If it cannot read the second side, it must be SS, etc. A more common way is to encode the format info in the first sector of the first track, e.g. by storing the data from the DPB and DPH, or by just storing a code number. The problem there is that different manufacturers' formats and code numvbers are not the same. Since the first sector usually contains the boot loader, this format code starts several bytes in, leaving room for a jump instruction. Other wrinkles: for compatibility with older formats which do not have encoding, e.g. CP/M standard 8-inch SSSD (same as IBM 3740), there has to be some default. Also, on an 8-inch diskette, the index hole is in a different place depending on whether the diskette itself is SS or DS. This is a little long-winded, I'm afraid, but I hope it answers your questions. I just got done writing drivers for the Z-100, that's why I know some of the tricks they use. If anyone else has more info, I'm sure the net would like to hear about it. Mike Ciaraldi ciaraldi@rochester
Platteter.Henr@PARC-MAXC.ARPA@sri-unix.UUCP (10/18/83)
Most certainly can. At least the Shugert and Remex versions I know for sure.