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From: edhall@rand.org (Ed Hall)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Unix on 486 Machines
Message-ID: <1991Jun17.230100.6934@rand.org>
Date: 17 Jun 91 23:01:00 GMT
References: <9106110417.AA06408@w20-575-117.MIT.EDU> <1991Jun11.062033.5218@kessner.denver.co.us>
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In article <1991Jun11.062033.5218@kessner.denver.co.us> david@kessner.denver.co.us (David Kessner) writes:
>The REAL benifit of EISA comes into play when a multitasking system is being
>used.  When a UNIX task requests a block from the disk (on an ISA bus), the
>CPU goes off and grabs that block-- it takes 100% of the CPU time to get that
>block since the ISA bus and hard drive controllers are brain-dead.

No, *some* ISA controllers are brain-dead.  Not all.

>When an EISA based UNIX system requests a disk block, the OS tells the 
>controller to read a block into memory.  It then puts that task on hold
>(until the block is read in), meanwhile it goes off and runs another task.
>So, while the one task is waiting for the disk controller, others are still
>being executed-- where as an ISA based machine will pause all tasks since the
>CPU is tied up doing the disk transfer.  

Not with a bus-mastering controller.  Most popular in the UNIX world
is the Adaptec 1540-series SCSI controller.  This type of controller
will grab the bus and run it at full speed while the CPU goes off and
does other things.  It can easily transfer data at the fastest speeds
SCSI can deliver.

		-Ed Hall
		edhall@rand.org