Remembering RAMAC

[Vintage Computer Festival]

On Sat, 28 May 2005, Ethan Dicks wrote:

> Yes... 4 bit rates - two bits from a TTL 'divide-by-N' counter chip
> can be written to by the 650x processor on the drive.  The drive
> firmware looks at the track number and stuffs the two bits out an I/O
> port before reading/writing.  ISTR one of the 4 available bit rates
> happens to match Apple II data rates, but I don't think anyone ever
> wrote a program for a 4040 or 1541 to attempt to decode Apple data.

A long, long, long time ago I put a Commodore formatted disk into an Apple
drive and ran a nibble editor on it and I could swear I saw valid disk
bytes.  I didn't really pursue it much further.

> Not sure how the Apple detects sync marks, but Commodore drives have
> an 8-input-NAND attached to a shift register that will go 'true' when
> 8 bits of ones goes past the read head.  The output of that NAND goes
> to the pin on the 650x that fiddles the overflow bit... one sees tight
> loops in the firmware where the CPU loops at the same address sampling
> the overflow bit, and what happens is things (other than IRQs) stop
> happening until the sync detector triggers, then the code flow
> resumes.  I'm sure one could rig up another method of sync detection,
> but that one is quite efficient.

To explain how the Apple does it would take 43 pages :)  But suffice it to
say, it is extremely clever.

> It was not as catastrophic as the Apple 13 to 16-sector conversion,
> but for a brief time, there was some media chaos in the PET world.

Interestingly enough, I found an Apple disk recently that was published in
1981 in 13-sector format(!)

-- 

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  ]