*BSD News Article 26376


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From: ivie@cc.usu.edu
Newsgroups: comp.os.386bsd.development,comp.unix.bsd
Subject: Re: Late comments on the DMA problem.
Message-ID: <1994Jan21.095505.8375@cc.usu.edu>
Date: 21 Jan 94 09:55:05 MDT
References: <jmonroyCJy610.HB7@netcom.com>
Followup-To: comp.os.386bsd.development,comp.unix.bsd 
Organization: Utah State University
Lines: 21

In article <jmonroyCJy610.HB7@netcom.com>, jmonroy@netcom.com (Jesus Monroy Jr) writes:
>>> For example, it is not possible to implment "scatter/gather" with the FDC by
>>>                    ^^^
>                       |||
>         I think he meant to say that "it is possible", because I know
>         it is.   Evidence is the the FDC (flexible drive controller)
>         has a command for read-a-track.

Yes, but "scatter/gather" has nothing to do with the FDC's read-a-track
command. Scatter/Gather involves organizing the DMA such that DMA happens
from virtually contiguous addresses when pages are crossed; in other words,
at a page boundary you have to be able to switch to some page other than
the next physical page.

Since you can't get an interrupt to tell you that the DMA controller has
finished doing a page, you can't reload the DMA addresses when the end of
a page is reached.
-- 
----------------+------------------------------------------------------
Roger Ivie      | Don't think of it as a 'new' computer, think of it as
ivie@cc.usu.edu |     'obsolete-ready'