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From: penev@rockefeller.edu (Penio Penev)
Subject: Re: IDE vs SCSI
Message-ID: <1993Apr22.043745.23130@sol.ctr.columbia.edu>
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Date: Thu, 22 Apr 1993 04:37:45 GMT
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On Mon, 19 Apr 1993 03:45:17 GMT Wayne Smith (wlsmith@valve.heart.rri.uwo.ca) wrote:
| In article <RICHK.93Apr15075248@gozer.grebyn.com> richk@grebyn.com (Richard Krehbiel) writes:

| >So, when you've got multi-tasking, you want to increase performance by
| >increasing the amount of overlapping you do.
| >
| >One way is with DMA or bus mastering.  Either of these make it
| >possible for I/O devices to move their data into and out of memory
| >without interrupting the CPU.  The alternative is for the CPU to move
| >the data.  There are several SCSI interface cards that allow DMA and
| >bus mastering.
|  ^^^^^^^^^^^^
| How do you do bus-mastering on the ISA bus?

As an earlier post noted - through DMA.

| >IDE, however, is defined by the standard AT interface
| >created for the IBM PC AT, which requires the CPU to move all the data
| >bytes, with no DMA.

| If we're talking ISA (AT) bus here, then you can only have 1 DMA channel
| active at any one time, presumably transferring data from a single device.
| So even though you can have at least 7 devices on a SCSI bus, explain how
| all 7 of those devices can to DMA transfers through a single SCSI card
| to the ISA-AT bus at the same time.

Any one time means IMHO a single byte xfer. If I have four sources of
DMA requests ready, the DMA would service the one after the other. If
the bandwidth for the four together is lower than the ISA/DMA
bandwidth, this will work.

Note that the bus mastering here is the priority mechanism in the DMA
controller.

--
Penio Penev  x7423 (212)327-7423 (w) Internet: penev@venezia.rockefeller.edu

Disclaimer: All oppinions are mine.
