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From: bairds@eecs.cs.pdx.edu (Shawn L. Baird)
Newsgroups: comp.sys.amiga.programmer
Subject: Direct hardware drive access
Message-ID: <2161@pdxgate.UUCP>
Date: 31 Mar 91 22:57:13 GMT
Sender: news@pdxgate.UUCP
Lines: 43

Having finished in my attempts at stepping in and out tracks I have decided
to move on to the more difficult prospect of reading and (eventually)
writing tracks. With this move I have several questions:

   1. The documentation for DSKBYTR states that in read mode data is placed
      here one byte at a time. I assume this is before DMA is started? For
      the purposes of syncing or somesuch?

   2. DMAON is DSKBYTR does not refer to actual DMA activity, but rather
      that DMA is enabled?

   3. Is there any way to busy loop on the completion of disk DMA? (I
      realize that an interrupt is more elegant, but this is just a test
      program.)

   4. The hardware manual mentions bugs in both reading and writing data.
      Now, how am I expected to compensate for these bugs? For reading, I
      assume I request an extra word?

   5. I use DSKSYNC on $4489. Is this correct, or at least for most disks
      such as AmigaDOS disks? If $4489 isn't found does it keep searching
      forever?

   6. The ADKCON register mentions precomposition. Is there a value for
      precomposition that I should use with standard MFM data?

   7. Does anyone have examples of blitter routines which both encode and
      decode raw MFM tracks? The hardware manual mentions this as a
      possibility as have several people on Usenet. I suspect this
      involves some tricky operation with the line mode, with which I'm
      just not well enough adjusted to figure it out for myself.

If you are looking for a good article on just what MFM encoding itself is,
and GCR for that matter, take a look at _Transactor for the Amiga_ Vol. 3,
Issue 1, "Amiga Disk Encoding Schemes" by Betty Clay, pp. 70-73.

Oh, one last question. Has anybody taken a look at AC Tech? Is it very
technical? Are they planning on moving from quarterly to bi-monthly or
somesuch?

---
 Shawn L. Baird, bairds@eecs.ee.pdx.edu, Wraith on DikuMUD
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