From: pekangas AT sci DOT fi (Petteri Kangaslampi)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Need help on DMA transfer
Date: 28 Apr 1997 15:12:16 GMT
Organization: Scifi Communications International Oy, http://www.sci.fi/, helpdesk AT sci DOT fi, 03 2899111
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References: <5E6E9801EF3 AT brain1 DOT lstm DOT ruhr-uni-bochum DOT de>
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On Mon, 28 Apr 1997 12:39:59 GMT, Tom Demmer <DEMMER AT brain1 DOT lstm DOT ruhr-uni-bochum DOT de> wrote:
> - Can the DMA buffer be above 1 MB, or _must_ I transfer from/to
>   DOS memory?

I could be wrong, but I have a feeling that the DMA controller could
access the first 16 megabytes of memory, but not more. I stick to the
first megabyte though, it always works.


> - If I have to go over DOS mem, how can I ensure the buffer does not
>   cross a 128kB boundary? Or is the information, DMA buffers must not
>   cross such a boundary, outdated?

No, you certainly cannot cross the boundaries. In addition, the page size
is 128k for the 16-bit DMA channels, and 64k for the 8-bit ones. I'd
suggest you always stay inside a 64k page.


>And the toughest thing ;-) this is from the Linux code:
>I know what most of the functions are doing, but I don't have the 
>slightest idea _how_:
[clip]

Most of the functions seem to map pretty 1-to-1 to the DMA controller
registers. I suggest you grab some reference about the DMA controller
(maybe HelpPC has some information), and a good example DMA source code -
the Allegro DMA code is probably a good place to start.

Some comments on the functions though:


>   disable_dma( ibdma );                /* ibdma is the DMA channel */ 

Probably masks out the DMA channel.


>   clear_dma_ff ( ibdma );              /* ???? */

Clears the DMA controller word flip-flop - now, when writing/reading a
16-bit register, the next write/read will go to the lower byte.


>   /* These ones are self-explaining, but how to do it? */
>   set_dma_count( ibdma, rwop->io_cnt );
>   set_dma_addr ( ibdma, rwop->io_pbuf);
>   if (rwop->io_flags & IO_READ) {
>      set_dma_mode( ibdma, DMA_MODE_READ );
>   }
>   else {
>      set_dma_mode( ibdma, DMA_MODE_WRITE );
>   }
>   enable_dma( ibdma );/* enable Host side DMA transfers */
>   sti();
>   /*
>   ** Let IO card initiate DMA
>   */
>   bdDMAstart(rwop);
>   /*
>   ** Wait until card finished or error
>   */
>   bdDMAwait(rwop, 0);
>   /*
>   ** Stop IO card and get amount xfered
>   */
>   resid = bdDMAstop(rwop);
>   /*
>   ** Disable DMA channel. How?
>   */
>   disable_dma( ibdma );
>   return resid;
>
>   
>Any help appreciated, TIA.
>
>Ciao
>Tom
>
>  ******************************************************************
>  *  Thomas Demmer                     * Phone : +49 234 700 6434  *
>  *  Universitaetsstr. 150             * Fax   : +49 234 709 4162  *
>  *  Lehrstuhl fuer Stroemungsmechanik *                           *
>  *  D-44780 Bochum                    *                           *
>  ******************************************************************
>  *  Email: demmer AT LStM DOT Ruhr-Uni-Bochum DOT De                         *
>  *  WWW:   http://www.lstm.ruhr-uni-bochum.de/~demmer             *
>  ******************************************************************
>
>    Anatomy (n): something everyone has, but which looks
>                 better on a girl.      -- Bruce Raeburn.
>