bheil@umaxc.weeg.uiowa.edu (Brian Heil) (11/17/90)
Hi, it's me again!
Can somebody explain quickly and briefly why Macs use that weird variable
speed deal on the floppies? I remember it was supposed to be to get more stuff
stored on the disk, but seeing as my amiga claims 880K (amigados info) or 837K
(SKsh info) and all I can get on the Mac disk is about 780K (730K usable) this
seems to be unreasonable. I suspect a marketing ploy.
Any comments?
Brian Heil ) University of Iowa
bheil@scout-po.biz.uiowa.edu ( College of Business Administration
bheil@umaxc.weeg.uiowa.edu ) Computing Services Organization
"Loni, I've told you a thousand times, BOOT CUT!" -Dave Letterman doing Burt
Reynolds.jma@beach.cis.ufl.edu (John 'Vlad' Adams) (11/18/90)
In article <yorkw.658880855@stable.ecn.purdue.edu> yorkw@stable.ecn.purdue.edu (Willis F York) writes: >bheil@umaxc.weeg.uiowa.edu (Brian Heil) writes: >>Can somebody explain quickly and briefly why Macs use that weird variable >>speed deal on the floppies? >>Any comments? > >Yea they did it so Amiga drives coulden't read Mac Disks. >(Or they was afraid IBM would sue them.) >Or for the Reason APPLE does all the stupid things they do. >Stupidity. Survey sez -- BUZZZZ you are the one exhibiting stupidity. Apple used variable-speed drives *BEFORE* the 1000 came out. It has to do with keeping the density of data the same over the entire disk surface. >Can a MAC drive read IBM disks? Where have you been? You honestly don't know about the SuperDrive. You should do youself a REAL favor and read some Sun Tsu (SP?). It is essential that one knows his enemy. >Macintosh... Proof that a Person can use a Computer all day and still >not know ANYTHING about computers. Or knows that there are SOME things the mac STILL does better. Don't get me wrong. I LOVE my Amiga. But the Mac cannot be beat for DTP work. -- John M. Adams --**-- Professional Student on the eight-year plan! /// Internet: jma@beach.cis.ufl.edu -or- vladimir@maple.circa.ufl.edu /// "We'll always be together, together in electric dreams" Moroder & Oakey \\V// Sysop of The Beachside. FIDOnet 1:3612/557. 904-492-2305 (Florida) \X/
DXB132@psuvm.psu.edu (11/19/90)
In article <1990Nov17.200840.22422@lavaca.uh.edu>, jet@karazm.math.uh.edu (J. Eric Townsend) says: >In article <3242@ns-mx.uiowa.edu> bheil@umaxc.weeg.uiowa.edu (Brian Heil) >writes: >>Can somebody explain quickly and briefly why Macs use that weird variable >>speed deal on the floppies? >The "Woz Machine" is proprietary Apple technology (patented as well, I >believe) that effectively prevents people from selling Apple compatible >drives. Yes, I know people sell Apple compatible drives, but it took >them awhile to ramp up and you don't save that much $$$. You still >can't go down to We-B-PCs and buy a 3.5" drive, however... The floppy controller ("Integrated Woz Machine") is simply the Apple II floppy controller on 1 chip. It is not special, it is not even good, it is not patented, and is 100% documented in Beneath Apple ProDOS and other books. The strange disk drives, by the way, are not made by Apple themselves and so may be purchased (off the shelf) by anyone. However they are quite non-standard . -- Dan Babcock
hwr@pilhuhn.uucp (Heiko W.Rupp) (11/20/90)
In article <yorkw.658880855@stable.ecn.purdue.edu>, Willis F York writes: >bheil@umaxc.weeg.uiowa.edu (Brian Heil) writes: [some stuff deleted] >Can a MAC drive read IBM disks? Yes,but only the new models -Heiko --- O|O Heiko W.Rupp Path: ...!ira.uka.de!smurf!flatlin!bagsend!pilhuhn!hwr | Gerwigstr.5 | There is someone in my head, but it't not me U FRG-7500 Karlsruhe 1 | - Pink Floyd Voice : + 49 721 693642| Do You know where Your towel is ???
252u3130@fergvax.unl.edu (Phil Dietz) (11/21/90)
In article <183c1119.ARN0dd5@pilhuhn.uucp> hwr%pilhuhn@bagsend.ka.sub.org writes: >In article <yorkw.658880855@stable.ecn.purdue.edu>, Willis F York writes: > >>bheil@umaxc.weeg.uiowa.edu (Brian Heil) writes: > [some stuff deleted] >>Can a MAC drive read IBM disks? > >Yes,but only the new models > >-Heiko By this one can see the FAULT the Apple has made. They realized that their mechanical and non-conformist drives were not all to compatible with anything. They now are trying to accomplish both by changing their drives from mechanically variable-speed to electronically non-speed. Now they can program what type of disk they want to read.....but these drives cost a pretty penny! Apple should have just stuck with the standard speed drives like IBM, Amiga, and the rest of the world uses! (Aside: although the Floppy speed that MAC's attain sure would be AWESOME compared to Amiga's CHUGGIN method....) Phil Dietz <<<=================--------- Cheap Ad ---------===================<<< Phil Dietz SWL Lincoln 565 MEGS! 2 lines 252u3130@fergvax.unl.edu (402)421-1963 AMIGA, IBM, MAC, GIFS IBM'ers and Mac'ers are shopping for a life. Amiga the best!
mueller@hatteras.cs.unc.edu (Carl Mueller) (11/22/90)
In article <3242@ns-mx.uiowa.edu> bheil@umaxc.weeg.uiowa.edu (Brian Heil) writes: >Can somebody explain quickly and briefly why Macs use that weird variable >speed deal on the floppies? I remember it was supposed to be to get more >stuff stored on the disk, but seeing as my amiga claims 880K (amigados info) >or 837K (SKsh info) and all I can get on the Mac disk is about 780K (730K >usable) this seems to be unreasonable. I suspect a marketing ploy. The Mac and the Amiga use radically different techniques to obtain their higher data capacity on 3.5" disks. I don't really think you can put the blame on any marketing ploys, though, since Apple has been using their variable speed drive since prior to 1984 (when the Mac came out) and has been using GCR recording since the Apple Disk II came out in ~1978. When the Mac first came out, hardly anybody was using 3.5" disks. Amigas weren't even seen in public until the following year (?). A (long) explanation of the technologies: A disk lets you store magnetic signals. When you design a drive system, you have to determine how you are going to encode the bytes into signal and then how you put this signal on the disk. There are a couple of fundamental problems you must overcome in doing this. The signal can be looked at as digital data (bits). However, it's not quite so simple: this signal is coming at a continuous rate as the disk spins. All disk drives do not turn at exactly the same rate, so the first problem is detecting when the bits start and end. Once you have the bits, you need to know where the bytes start and end, and then you have blocks.... Let's look at the first problem, of identifying the bits. What is needed is a method of synchronization, and this can be obtained by looking for transitions in the digital signal. Each transition tells where a bit boundary is, so the disk controller can synchronize with the media by timing these. Problems can occur when you try to write too many contiguous 1's or 0's - you will have no transitions and thus the controller may become out of sync. There are different methods of overcoming this problem, and some are known as FM, MFM, and GCR. FM inserts a sync bit between every bit of data (simple, but inefficient). MFM uses slightly more intelligence, and I forget the details, but it still operates on the 'bit stream' principle. GCR (group-code recording) uses a different approach. Instead of looking at individual bits, it looks at groups of bits. Suppose you look at all the 'legal' 8-bit combinations which can appear on a disk (the ones without too many contiguous like bits). There may turn out to be about 70 or so (again, I forget the details). This is enough to encode a 6-bit number and have a few left over. So you can break your data up into 6-bit chunks, use a lookup table to encode these into the 8-bit sequences which you then write out to disk. That's GCR. With the extra 8-bit combinations you have left, you can mark the beginning of sectors and other such goodies. Now, going back to the fundamental problem of applying the signal to the disk. There is a maximum density at which you can write on the disk. As the disk spins, this means that you can write the signal out only so fast. Most disk drives spin at a constant rate, and most disk controllers write data at a constant write. The upshot of this is that data is written more densely on the smaller inner tracks than on the longer outer tracks. Several people saw that the outer tracks weren't being used to their optimal capacity. What was needed was a way to write more data to the outer tracks than the inner ones. There are two ways to do this: either slow down the disk or speed up the controller. Apple took the former approach since it let them keep the same controller design at the expense of needing a custom, variable-speed drive, which SONY was happy to make for them. So how did Commodore store more data on a constant speed, MFM drive? They did so by looking at another disk drive problem: data sectoring (or 'blocking' since block and sector are about equivalent terms). This is more of a software issue, but still tied in to the hardware in some cases (if GCR is done in hardware, for instance). For a variety of reasons, computers like to deal with data in nice, small blocks of 256, 512 (most common), or 1024 bytes. Most disk systems are designed to read and write single blocks at a time. With disks, you have to be careful as you write out a block so that you don't overwrite the beginning of a subsequent block (once again, this problem is due to each drive not turning at exactly the same speed). To this end, there are gaps written out between blocks. Note that useful data could have been written where the gaps are if you didn't have to worry about them. Commodore saw this and basically did away with physical blocking. Instead of writing 9 or so individual blocks along with 9 gaps, they write one big block with only 1 gap. Area that was previously used for gaps is now used for data, and thus you can store more bytes. The disadvantage of this method comes when you look at performance. You can no longer read and write a block at a time. Each time you want to read or write a logical block, you must read or write the entire track. Track buffers are also necessary, since writing means you must first read the track, modify the buffer, then write the track. Commodore did incorporate several optimizing techniques, though, to help things. For instance, instead of waiting for the beginning of the track to spin around and start reading from there, one can start reading immediately and continue for an entire revolution. The software can then figure out where the start of the track is. So, in summary: (the number in parens is # sectors/track) IBM PC: MFM, constant speed, sector-based (9): 720K MAC: GCR, variable speed, sector-based (varies): 800K AMIGA: MFM, constant speed, track-based (11): 880K Higher density disks (1.4M) are just that - higher density. They use a more precise read/write head, lower current, and more sensitive media. There! That's all I plan to say for now. I hope I haven't bored you or made too many mistakes. Perhaps someone who is more of an expert on this can correct me or fill in the details. -Carl (mueller@cs.unc.edu)