ggw@ethos.UUCP (Gregory Woodbury) (11/12/86)
In article <273@csustan.UUCP> smdev@csustan.UUCP (Scott Hazen Mueller) writes: >It's been done before. I don't think that it is quite done by changing the >clocking rate, but I do know of disk drives that vary the number of sectors >per track in order to fit the most data on the disk. I believe that the >lowly Commodore 64's disk drive is one such device. > Indeed, the Commodore 1540/1541/1571 5.25 in. floppy drives do indeed vary the number of sectors/track. There are three zones with from 17 to 21 sectors per track in each of the zones. Additionally, the drive controller changes the clocking rate as well. There are 4 clocking rates available to the DOS for use in the process. Some of the ``copy protect'' schemes on the CBM drives use variations in the clock rate to make copying difficult. One additional not about the CBM drives though, they use a 5-for-4 GCR encoding scheme to record the data, this supposedly allows them to get away with higher densities on cheaper media. -- ------------------------------------------ Gregory G. Woodbury The usual disclaimers apply {duke|mcnc|rti-sel}!ethos!ggw The line eater is a boojum snark! additional mail to ..ethos!dukcds!ggw
daveh@cbmvax.cbm.UUCP (Dave Haynie) (11/13/86)
> One additional note about the CBM drives though, they use a 5-for-4 > GCR encoding scheme to record the data, this supposedly allows them to get > away with higher densities on cheaper media. > -- > ------------------------------------------ > Gregory G. Woodbury The usual disclaimers apply > {duke|mcnc|rti-sel}!ethos!ggw > The line eater is a boojum snark! additional mail to ..ethos!dukcds!ggw The aforementioned 1540/1541/1571 disk drives, using 35 formatted tracks per side, can record at slightly more data then that of standard 40 track MFM; about 349K per (double sided) diskette, using the normal 256 byte sectors (the MFM encoding scheme results in about 328K using the same sector size). An older Commodore floppy drive, the IEEE-488 bus 8250, could store around 1.05 Megabytes per diskette, formatted. It used 77 tracks per side; the normal MFM formats get only around 720K per diskette using 80 tracks per side (and 512 byte instead of 256 bye sectors). The 8250 drive uses from 29 to 23 blocks per track, depending upon the track's position. The PC-AT drives can store a bit more than this, but they do it by formatting around 135 tracks per side, which requires special high-density diskettes instead of the standard DS/DD diskettes that an 8250 could use. Keeping in the Commodore tradition of weird disk formats, the Amiga has a few tricks of its own. It uses 80 track-per-side MFM encoding, but it ALWAYS reads and writes full tracks. This allows the positioning of adjacent sectors to be done tightly, resulting in 11 512 byte sectors per track, for a total of 880K of formatted storage versus the 720K norm. The MFM encoding is actually done in software, using the Amiga's blitter for coding (in 3 passes) and decoding (in 1 pass); the particular encoding scheme was in fact chosen because its easy to encode and decode; it is entirely possible to create higher density encodings with this machine, and it would be possible to vary the density with the track being written. -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Dave Haynie {caip,ihnp4,allegra,seismo}!cbmvax!daveh "Laws to supress tend to strengthen what they would prohibit. This is the fine point on which all the legal professions of history have based their job security." -Bene Gesserit Coda These opinions are my own, though for a small fee they may be yours too. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
phils@tekigm.UUCP (Phil Staub) (11/14/86)
In article <987@cbmvax.cbmvax.cbm.UUCP> daveh@cbmvax.cbm.UUCP (Dave Haynie) writes: > The PC-AT >drives can store a bit more than this, but they do it by formatting around >135 tracks per side, which requires special high-density diskettes instead >of the standard DS/DD diskettes that an 8250 could use. > >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ >Dave Haynie {caip,ihnp4,allegra,seismo}!cbmvax!daveh It is my understanding that the AT's high density drive is 80 tracks per side, but due to a higher bit density, made possible by the higher coercivity medium used, they are able to squeeze fifteen 512 byte sectors per track. This leads to the following calculation: 2 sides * 80 tracks per side * 15 sectors per track * 512 bytes per sector __________________________ 1228800 bytes (1.2 megabytes). By way of comparison, the standard PC/XT floppy format gives the following: 2 sides * 40 tracks per side * 9 sectors per track * 512 bytes per sector __________________________ 368640 bytes (360k). Phil Staub Tektronix, Inc. ISI Engineering P.O. Box 3500 Vancouver, Washington 98668 C1-904, (206) 253-5634 ..tektronix!tekigm!phils