Re: Hardware musings

[Ed Eastman <noone@nowhere.net>]

mdj wrote:
> Personally I'm not interested in doing the project unless I can come up
> with a way to access the disk at the nibblized level to handle copy
> protection,

I've put a bunch of thought into this myself.

Lets see, 4us per bit, .2s per track = 50k bits +3% means we would need 
about 6.25k per track, or 25 256 byte pages.  256k would be the most one 
would need to hold a 40 track file of A2 nibbles.  Of course at the 
front end you would want to place a table of all the tracks 0 through 40 
including the  quarter tracks.  Each entry should have a track length (2 
bytes) of formatted or 0 of not in use.  You could build the disk 
sequentially starting with track 0, followed by the data from the next 
viable track, etc.  The file would vary in length in 6.25k increments. 
Standard disk image would be 36 track * 6.25k length + .25 header or 
231k  Plus or minus.

SST would need to be rewritten to save data in this format.  Emulators 
would probably also want to incorporate this new format.  Maybe make a 
new extension of apple2 nibble or a2n?  How about .bst for bitstream?


The hardware would need several things:
- an asynchronous 4us timing circuit to 'rotate the disk' and point into 
the track bit stream buffer for a read/write operation.

- A latch to push/ pull bits to/ from the bit stream.

- Have the bit read routing 'randomly' change the read more than 2 
sequential 0s as 1s.  (if easy)

- Have the write routine place the bits in the latch into the track bit 
stream

- Have a timing circuit emulate the timing of a track seek and set the 
bit stream to the proper place.

So the device would work something like this:

Timer advances at 4us rate (rotate disk bits)

On Read or write command: Add timer + Mod tracklength + track offset = 
bits to start reading.  Load bits into the latch as timer advances. 
Clock them in until a read is requested or stop and 'latch' if you get 
high bit to MSB, or just write the 8 bits into the stream at the proper 
offset.

On Track change command, recalculate track offset, add seek delay to 
timer for 'synchronizing' the bit stream.

Hardware should account for these asynchronous commands:
Motor on/ off
Stepper motor phase 0-4
Rotate bits into latch on read
Clear latch (even if a other operation is in progress)
Write latch into bitstream

Signals to send back:
Latch MSB is 1
Data in the latch