CompuServe Messages

Dan, I know of no consumer type HD units that can read more than one head at a time. I do know of some *very* expensive commercial units that can do that. These drives, can read the data as the equivalent of a parallel read rather than a serial read with each head reading one bit of a byte at a time (thus 8 heads all being read at once read a byte at a time). Thus you can read 8 times as much data in one rev of the platter. The type of drive you'll find on a personal computer can only read the data as fast as the data passes under any one head, as I've already explained. But what it does with that data in the way of presentation to the host computer, and how intelligently it gets the data can make a large difference in performance. For example, if the drive is real dumb, and every time it reads a byte…it sends it down the bus and then every time the host adapter gets a byte…it passes it to the CPU then the CPU is going to get interupted 512 times per sector. It'll have to stop what it's doing and move that byte into RAM. However, if that same drive cached the data until it got say 64 bytes, and then dumped it all at once…and the host adapter were DMA capable…then the CPU would never get interupted until the whole read was done (whatever size of read it had requested). Also, another way to up the apparent speed of a drive is an intelligent read-ahead scheme. Let's say the CPU asked for sectors # 25 and #26. It's a fair bet that the next request will be for sectors #27 and #28…so the drive goes ahead and reads them into an on-board cache. IF the CPU does in fact ask for 27 and 28, they are already in the cache and can be dumped immediately…whereas if the request is for some other sectors…nothing is lost…because no matter what, those other sectors have to be read…they're just not already in memory. Another way is simply ot have a large cache of recently read sectors on the drive. If the next request from the CPU is for one of those sectors…again, it is sipmly a matter of dumping what's already in the cache with no read required from the heads.