DOS 3.3 Nibble Encoding

[gregorya@cs.curtin.edu.au (Andrew Gregory)]

Hi!

A little while back I posted a question to do with the Disk ][ Controller
Interface.  I haven't got anything in reply and I am still interested in
any info.

This time I'm also wanting to know how DOS 3.3 and ProDOS 8 encode their
data into something the disk drive can understand.  My understanding is that
they use something called '6 and 2' which encodes 256 bytes of 'real' data
into 342 (or thereabouts) of disk data.  I would  like to know the details
of that process.

I have heard that a book called 'Beneath Apple DOS' would explain all this
but none of the bookshops here in this backwater of Perth, Western Australia
carry it.

The reason for this question is I'm writing my own Apple ][ emulator for the
PC.  Yes, I know it's been done.  BUT NOT BY ME!  I would like my emulated
disk drives to be able to read the disk images that apl2em uses so I need
to encode the image to something the emulated drive controller will like.

I've tried to figure out the //c disk ROM code without success (some comments
with the source would be nice).

Any help would be greatly appreciated.

Thanks in advance,

Andrew Gregory

gregorya@lillee.cs.curtin.edu.au