Apple 2 Floppy Disk Codes - Computerphile

Steve Wozniak got creative with the Apple ][ floppy drive, creating a system that used less complicated hardware and was cheaper, accomplishing much in software instead. Dr Steve Bagley explains.

The graphic showing where sector holes were read is incorrect. Sector hole was seen where Dr Bagley showed. Earlier 8 inch floppy disks had different standards for hard sector disks, some at the hub, some near the outside of the disk- which is what the graphic was trying to illustrate.

This video was filmed and edited by Sean Riley.

[bobwashburn]

Back in July 1978, as I was leaving Data General, Westborough MA, to lead the Apple distributing company, Larry Seligman, DG's top computer designer (he designed the Supernova 16 bit minicomputer), told me how impressed he was with the minimalist design that Woz had done on the DOS hardware controller. Larry was a thesis away from his MIT PHD, gave high praise to a UC Berkeley dropout at the time.

[StevenWeyhrich]

Part of the genius of Woz's design was that he eliminated the electronics that used that timing hole, and did the syncing via software, as you said. You could put a piece of tape over that hole and an Apple II floppy would still work.

[jamesslick]

This design was pure genius, It also explains why I saw many more Apple ][s with FDDs in the early "home computer" era, Almost everyone else put up with Cassettes. On many early "home computers" the FDD often exceeded the price of the whole rest of the set-up!

[KuraIthys]

mmh. Wozniak. The true legend of Apple.

Also, stiffies. Lol. I remember hearing about that...

[thoughtyness]

Steve was very helpful to Apple. He envisioned the products and their design, but Wozniak was the true genius.

[Mew]

And once again we see who the real genius behind Apple actually was.

[DrRChandra]

A little extra information for the interested: Using a sequence which is not legal to find in the middle of payload data is fairly common in storage and communications systems, called a "preamble." When the preamble pattern goes by the receiver or read head, the circuit "knows" it's the beginning of something, like a sector or frame. It can be implemented in very high speed hardware with a shift register and logic gates. The logic gates are wired in the pattern of the preamble, and constantly compare the value in the shift register with the preamble...and the output goes "true" when they match. Then the circuitry can begin collecting the bits of the payload.

[menachemsalomon]

I remember seeing that modern disks use a similar pattern. Something like 8 bits encoded in a series of 15 pits and lands on a CD. I don't remember the exact numbers, but I'm sure they'll be in the next video.
Much appreciated this. Keep 'em coming, +Computerphile.

[AntoineVignau]

At 3:30, there is a wrong statement. You can have two consecutive zeroes (the nibble shown here, #$9F is an authorized value) but no more. This is rule #2.

As with rule #1 (nibbles start with bit 7 at 1), these can be bypassed to create interesting on-disk protection schemes.

[DasIllu]

I remember GCR was also used in the Commodore floppy drives back in the days. It was a real pain in the rear end to transfer data between MFM-coded systems like IBM or Kaypro and the Commodore family. In the end i found my self soldering a little pcb with some transistors and resistors and hook up those systems directly via parallel port. Big plan was to use e decommissioned 286 board as "hard drive controler" for my beloved C=128. Never completly finished it though. But i used it for cross assembler stuff.

[NeilRieck]

It's been a while since I read "Beneath Apple DOS" but IIRC, 5-and-3 encoding supported 13 sectors per disk (Apple DOS 3.2) while 6-and-2 encoding supported 16 sectors per disk (Apple DOS 3.3). I recall taking my Apple ][ to a local retailer to have the floppy controller updated from 3.2 to 3.3 (IIRC they just replaced a ROM). After that you could deal with 3.3 formatted disks natively or run a small utility which would allow you to read 3.2 disks. There also existed many copy-protection schemes based upon partially written tracks as well as data written between tracks. Many people got around this (for backup purposes only) with a utility call Locksmith.

[nixdorfbrazil]

I remember when I had to use the other side (apple drives had just one head), we had to punch a hole on the floppy jacket in order to make the drive to read it properly when we flipped the disk. Also, disks weren't designed to be flipped, and with time you had problems with disks that were flipped, as the fibers inside the jacked got sticky with dust.

Nostalgia! :)

[zxb]

That beeping when the machine booted the nostalgia....

[iseslc]

old tech is so much more interesting than new one!

[wisteela]

Cool to see an Archimedes A310 with twin floppy drives. I've got one like that, and had never seen another one.

[ScottLahteine]

I once had to write custom floppy read/write routines for the Amiga. On that system you could use the "blitter" to do the encoding and decoding. I believe it used "Modified Frequency Modulation" (MFM) for the encoding system.

[ewlloyd]

Steve implied that the index hole was used by the disk ][ hardware to find Sector 0 -- it was not. Woz didn't use the index hole for anything at all. In fact, had the index hole been used, it would've made it impossible for third-party formatters to optimize the disk. They worked by staggering the zero sector of adjacent tracks to reduce the number of total revolutions needed to read the disk sequentially.

Oh, and the sectors were laid out on each track non-sequentially so the RWTS routine had enough time between sectors to process the data it had read.

[julesverne]

call the police, he's storing cp on his floppies :P

[ChristopherdeVilliers]

Yes in South Africa they were called Stiffy Disks. And USB memory sticks are sometimes called a Stokkie

[wattage]

Great topic and explanation. Love hearing about the genius of Woz and other pioneers. Please keep the vids like this coming. Loved it!