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Xref: sserve comp.unix.bsd:2824 comp.os.linux:6548 comp.periphs.scsi:6396 Newsgroups: comp.unix.bsd,comp.os.linux,comp.periphs.scsi Path: sserve!manuel!munnari.oz.au!news.hawaii.edu!ames!haven.umd.edu!darwin.sura.net!convex!egsner!adaptex!adaptx1!neese From: neese@adaptx1.UUCP (Roy Neese) Subject: Re: Query: Drivers for AHA >1522< ? Organization: Adaptec Inc., Texas Date: Wed, 29 Jul 1992 20:48:34 GMT Message-ID: <1992Jul29.204834.20882@adaptx1.UUCP> References: <adam.711725819@mcrware> <14hjqoINNr78@agate.berkeley.edu> <1992Jul23.232138.20172@colorado.edu> In article <1992Jul23.232138.20172@colorado.edu> drew@ophelia.cs.colorado.edu (Drew Eckhardt) writes: > In article <14hjqoINNr78@agate.berkeley.edu> wjolitz@soda.berkeley.edu (William F. Jolitz) writes: > >In article <adam.711725819@mcrware> adam@microware.com (Adam Goldberg) writes: > >>...The 1542 is a bus-mastering controller. This 1522 is not. That makes > >>the 1542 much faster (at least in a protected-mode OS like Linux, with > >>properly written drivers). > > > >One of the reasons that 386BSD works so well with the 1542B is the > >bus-mastering, the advantages of which are lost on DOS. There may > >be a false economy in getting a 1522, because the entire advantage > >of going to SCSI is the bandwidth improvement over the programmed I/O > >AT/IDE controllers. > > If you're talking about sustained transfer rate, it's going to be > faster to do polled I/O because you don't have the couple of clock > cycles associated with each bus on period - you just crank data > across the AT bus topspeed, 16 bits at a time. Nope. Polled I/O can never be faster than bus master DMA as it takes twicw as many cycles to move each word doing polled I/O as it does for a bus master to move the same word. The overhead you speak of is for acquiring the bus, wchi takes 2 cycles and then of course it takes another 2 cylces to release the bus. But to show a more subjective view. Let's make some assumptions, good assumptions. Let's say 1 cycle per word for bus master DMA and 2 cycles per word for polled I/O (you have to do a read and write for polled I/O, whereas bus masters do either a read or a write). Now let's move 512 bytes of data. Let's see,... for polled that would be 1024 cycles and for a bus master 512 + 2 + 2 = 516. Nuff' said. > However, if you can DMA to memory, and replace multiple interrupts with > a single interrupt, you'll come out ahead in terms of speed because > you're looking at ~200 clocks overhead for an interrupt on the i386 > in protected mode. True. > SCSI-I also allows you to have one outstanding command per LUN > (you can have one disk seeking, or transfering to local buffer > while yoou're transfering data to another disk), which IDE and > "traditional" disk controllers do not. Nope. SCSI has never forced command queueuing as mandatory. SCSI-2 makes it optional as well. Host adapters can do command queueing, but it doesn't have a thing to do with the SCSI spec. --- Roy Neese Adaptec Senior UNIX/SCSI Software Engineer UUCP: convex!egsner!adaptex!neese Inet: neese%adaptex@cirr.com