AWCTTYPA@UIAMVS.BITNET ("David A. Lyons") (02/17/89)
>Date: Thu, 16 Feb 89 21:23:15 GMT >From: "Jeremy G. Mereness" <jm7e+@ANDREW.CMU.EDU> >Subject: Unidisks vs. Apple3.5 > >My //gs system came with Unidisk3.5 instead of the Apple3.5 > >I know that the Unidisk is not capable of 2:1 interleave and thus >some software packages will not run with the device. The UniDisk 3.5 _is_ capable of 2:1 interleave. For that matter, it's capable of any interleave from 1:1 to 12:1, just like the Apple 3.5. Interleave refers to the order in which blocks on a track are arranged. 1:1 interleave means that the blocks follow in sequence. This would be bad, because the computer would have extremely little time to process a block before the next one was ready to read--it would "miss" the block and have to wait nearly one full rotation of the disk to read the next block. With 2:1 interleave, there is one out-of-sequence block between every pair of sequential blocks, so the blocks might be in an order like 0 8 1 9 2 A 3 B 4 C 5 D 6 E 7 F. With 4:1 interleave, there are 3 blocks in between each pair of sequential blocks, like 0 4 8 C 1 5 9 D 2 6 A E 3 7 B F. The Apple 3.5 is directly controlled by the computer; the ROM reads straight from the disk into memory. The UniDisk 3.5 has a processor and ROM and RAM of its own; it reads the disk into internal RAM and then transfers the info to the computer. This isn't as fast. The result is that if you use 2:1 interleave with a UniDisk 3.5, the next block you want will have just gone by if you're reading the blocks in order (which is typical under ProDOS). 4:1 interleave is always recommended with UniDisk 3.5 drives; with 2:1 you can only read about 6 blocks a second. 4:1 interleave is generally recommended under ProDOS 8 even for Apple 3.5 drives, but in some cases 2:1 may be better. A block-by-block disk copy program has no trouble keeping up with 2:1 interleave under ProDOS 8, but file-level stuff has more housekeeping to do & doesn't always keep up (meaning things can go slow; they still work correctly). 2:1 interleave for Apple 3.5 drives is recommended under GS/OS. (4:1 is recommended for ProDOS 16, although ProDOS 16 is not recommended.) Programs which bypass the operating system and go right to the hardware to read the Apple 3.5 disk don't work with the UnidDisk 3.5, since the UniDisk 3.5 can't be directly accessed that way. >If I sell it, will it work on a Macintosh? Nope. (Even if you don't sell it, it won't work on a Macintosh.) >And how good are the Laser 3.5 drives? Are they compatible with the >Apple 3.5 drives or do they have similar problems with the Unidisk? Copy-protected software that doesn't work with the UniDisk 3.5 probably won't work with the Laser 3.5 drives either. Unprotected software should be fine. I don't know what interleave the Lasers can handle efficiently, but I'd be pretty surprised if it was different from the UniDisk--count on 4:1, not 2:1. >jerry >jm7e+@andrew.cmu.edu (arpa) >r746jm7e@cmccvb (bitnet) --David A. Lyons bitnet: awcttypa@uiamvs DAL Systems CompuServe: 72177,3233 P.O. Box 287 GEnie mail: D.LYONS2 North Liberty, IA 52317 AppleLinkPE: Dave Lyons