Path: utzoo!attcan!uunet!husc6!rutgers!mailrus!utah-gr!stride!mitch
From: mitch@Stride.COM (Thomas Mitchell)
Newsgroups: comp.arch
Subject: Re: Sw vs. Hw BitBlit.
Message-ID: <840@stride.Stride.COM>
Date: 5 Aug 88 21:00:18 GMT
References: <399@ma.diab.se> <1988Jul28.173301.7275@utzoo.uucp>
Reply-To: mitch@stride.stride.com.UUCP (Thomas Mitchell)
Organization: MicroSage, 680 S. Rock Blvd, Reno, NV 89502
Lines: 33

In article <1988Jul28.173301.7275@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>In article <399@ma.diab.se> pf@ma.UUCP (Per Fogelstr|m) writes:
>>... all bus cycles doing anything else than movin data is overhead.
>>Agree ??.  Okay, so even if a 68k is doing a "Blit" in straight code
>>Correct me if i'm wrong...
>Almost right, which means "wrong".  Take out the word "much" and I'll go
>... People designing things like
>Blitters, DMA interfaces, etc., consistently ignore just how quickly a
>modern CPU can move data if the programmer really sits down and thinks

This is true, and a common surprise.  Many 'DMA' processors are not
as fast as the main processor. They commonly do not have, a
bus interface equal to the processor or instruction cache or other
goodies we now expect in a micro-processor.  

In a system the DMA processor also must arbitrate with the processor
for control of the bus.  Then communicate (message?) with the
processor ....  Well--

The result is that DMA processors are a loss except to the sales
department.   

If I was careful -- I use the words DMA processor and not DMA
device.  It is possible to build custom hardware (a device) that
does DMA to or from main memory vastly faster than a 'programed'
transfer but such things are today rare.


-- 
Thomas P. Mitchell (mitch@stride1.Stride.COM)
Phone: (702)322-6868	TWX: 910-395-6073	FAX: (702)322-7975
MicroSage Computer Systems Inc.
Opinions expressed are probably mine.