Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!ncrlnk!ncrwic!encad!ensub!dsleffel From: dsleffel@ensub.Wichita.NCR.COM (Don Sleffel) Newsgroups: comp.arch Subject: Re: desperately seeking explanation of typical SCSI configuration Message-ID: <196@ensub.Wichita.NCR.COM> Date: 9 Oct 89 16:56:52 GMT References: <8774@spool.cs.wisc.edu> Reply-To: dsleffel@ensub.Wichita.NCR.COM (Don Sleffel) Organization: NCR Corporation, Wichita, KS Lines: 80 Keywords: In article <8774@spool.cs.wisc.edu> shekita@provolone.cs.wisc.edu (Eugene Shekita) writes: > >From what I've read over the net, the way I view a typical SCSI configuration >is as follows: Should be more like: > > ------------- ---------- > | Processor | | Memory | > ------------- ---------- > | | > ================================== Main Bus > | > ------------------- ------- |Host Adapter or | |File Processor | ------------------- | |--------------|-------------|-------etc. --------------- ------------ ---------------- |Disk Controller| |Tape | |Disk Controller | --------------- |Controller | ---------------- | ------------ | |--Disk Drive | |--Disk Drive | |--Tape Drive | |--Disk Drive |--Disk Drive | | etc. etc. > > 1) How does DMA logic get added to the picture -- is it typically > a separate unit with its own buffer, thereby allowing full-word > transfers over the main bus instead of 8-bit SCSI words (ala Amiga)? > Or do DMA units typically have no buffer? DMA logic along with buffers and/or file processing software resides on the file processor. This also allows effecient matching of SCSI transfer rates to the system bus transfer rate. Of course not all systems contain all of the hardware options to configure something this extensive, depending upon the speed and price of the system. > > 2) Dual-ported memory systems let DMA transfers go straight into memory > without tieing up the main bus, right? Roughly, how much cost does > a dual-ported memory system add, and is that why they're usually only > found on *big iron*? And while we're at it, what level of multi-porting > do some of these *big iron* machines have (a Cray, for example)? Not neccessarily. There are a lot of different reasons why you might or might not want to use dual ported memory and why you might or might not want to use the side opposite the processor for IO operations. Keep in mind there are a lot of different classes and application types of machines. "Big Iron" does not neccessarily imply a Cray. > 4) Is it becoming common for SCSI controllers to have a track-size > buffer of their own and always do track-size reads? (I am told > that Conner contollers have this feature.) Yes, as well as various forms of caching. SCSI used on "Big Iron" will probably have from 2 to 200 Megabyte of buffer space, depending upon the type of usage. ---------------------------------------------------------------------- Don Sleffel 3718 N. Rock Rd. Wichita, KS 67226 <{ece-csc,hubcap,gould,rtech}!ncrcae!ncrwic!ensub!dsleffel> <{sdcsvax,cbatt,dcdwest,nosc.ARPA,ihnp4}!ncr-sd!ncrwic!ensub!dsleffel> -------------- Contempt Breeds Contempt