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From: bright@dataio.UUCP (Walter Bright)
Newsgroups: comp.sys.ibm.pc
Subject: Re: DMA
Message-ID: <1196@dataio.UUCP>
Date: Thu, 13-Nov-86 16:02:20 EST
Article-I.D.: dataio.1196
Posted: Thu Nov 13 16:02:20 1986
Date-Received: Sun, 16-Nov-86 20:09:57 EST
References: <1189@dataio.UUCP> <197@oliveb.UUCP>
Reply-To: bright@dataio.UUCP (Walter Bright)
Organization: Data I/O - FutureNet Corp., Redmond, WA
Lines: 23

In article <197@oliveb.UUCP> spud@oliven.UUCP (John Purser) writes:
>In article <1189@dataio.UUCP> bright@dataio.UUCP (Walter Bright) writes:
>>I am interested in copying pixel data from one page to another on the
>>IBM EGA. This involves moving 128k bytes of data. Doing it with a
>>REP MOVSW takes about 1/2 second (on an AT), which is too slow.
>>Does anyone know
>>how the DMA channel could be programmed to do this? It is not clear
>>from the documentation how to program the DMA chip, or even if
>>it is capable of memory-to-memory transfers.
>
>How did you arrive at the time of 1/2 second? The way I figure this
>it should only take about .05 seconds. According to the 286 programmers
>referance guide a REP MOVSW takes 5+(4*CX) clocks. In your example that would
>be 64k words times 4 plus 5 or a total of 262,149 clocks. The clock speed
>of the AT is 6Mhz so dividing the 262,149 by 6,000,000 leaves us with .045
>seconds.

I did some checking. First off, the EGA is 8 bit ram, so the 128kb move
should take .09 seconds. Second, the EGA needs 4 out of 5 memory cycles
to do refresh, so the copy winds up taking about 1/2 seconds. DMA obviously
wouldn't help much here.

Also, nobody replied with a method of doing memcpy()s with the DMA.