22 Mar
2005
22 Mar
'05
1:57 p.m.
On Tue, 22 Mar 2005, Eric Smith wrote:
Ok, this is what I thought. And I haven't read far enough into
UNDERSTANDING THE APPLE II to have dispelled that rumor.
Ok, cool.
--
Sellam Ismail Vintage Computer Festival
------------------------------------------------------------------------------
International Man of Intrigue and Danger http://www.vintage.org
[ Old computing resources for business || Buy/Sell/Trade Vintage Computers ]
[ and academia at www.VintageTech.com || at http://marketplace.vintage.org ]
I wrote:
Yes, the drugs you are taking are so good that you're reading the title of
your book, which is "Understanding the Apple II" by Jim Sather, as, "The
Apple II Circuit Description". Pass the mushrooms!
> You want the book "The Apple II Circuit
Description", or "The Apple IIe
> Circuit Description", by Winston Gayler.
Sellam wrote:
While that's a good book, it has nothing on
the disk controller. See
my previous message for a better selection.
Oh. I suppose I must be hallucinating when I look at this copy in
front of me. The table of contents lists chapter 9 as being "The
Disk Controller". And pages 9-1 through 9-45 seem to give a detailed
explanation of how it works. But I guess this must just be a
particularly strange side effect of the drugs that I'm not taking. I see. And
this is where I'm confused. Beneath Apple DOS shows diagrams
that indicate there's a clock pulse between every data bit. So either
that diagram is wrong, and there are no clock pulses,
The authors of Beneath Apple DOS had no clue whatsoever as to what was
going on at that level, so they apparently just assumed that it worked
the same as other disk controllers in that regard.
There are no clock bits. That's the whole point of GCR. Fortunately they understood the higher-level stuff, so
the rest of
the book is much more accurate.
Right.
According to
my reading (in Beneath Apple DOS), you can only have *one
pair* of consecutive zeros in any one byte, but it was not clear if that
was a hardware constraint or an artificial constraint to pare down the
number of "legal" values.
Artificial constraint. The hardware (w/ the 16 sector state machine)
can deal with any nybble that has the MSB set, and no more than two
consecutive zeros.