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From: rdt@houxv.UUCP
Newsgroups: comp.arch
Subject: Re: Using a DMA chip in strange ways
Message-ID: <540@houxv.UUCP>
Date: Thu, 19-Feb-87 09:51:54 EST
Article-I.D.: houxv.540
Posted: Thu Feb 19 09:51:54 1987
Date-Received: Sat, 21-Feb-87 10:22:50 EST
Distribution: world
Organization: ATT Information Systems, Holmdel, N.J.
Lines: 28
Summary: Re) System buffer space to user space memory to memory  copies via specialized DMA hardware rather than a tight loop.


	 simple answer. 

	 There is a distinct tradeoff between the size of the
	 block transfer (how large a chunk) and the amount of
	 indirection overhead in:

	 1) making a system call, 
	 2) being authorized to use the DMA in the fashion you want,
	    (permission checks via probes and address translations;
	    recall that the DMA works in physical space where as the
	    CPU tends to work in virtual spaces) 
	 3) and then setting up the DMA context registers with the
	    necessary pointers, sizes and configuration information.
	
	CONCLUSION:

	 for transfers under 32-64 words, the overhead time to setup
         the dma may swamp the more efficient block move capabilities
	 of the specialized dma hardware. 

	 mileage (breakeven point) may vary with OS system call structure, 
	 and manufacturers MMU and DMA hardware.

Richard Trauben
ATTIS, Holmdel, New Jersey
WE32x00 Processor Development