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Signed By Woz

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What’s the perfect gift for a guy who’s obsessed with disk emulation for antique Apple computers? How about a 5 1/4 inch floppy disk signed by Steve Wozniak, inventor of the Apple II. Bonus points if you know what D5 AA 96 means!

Read 13 comments and join the conversation 

13 Comments so far

  1. Jordan Werthman July 10th, 2015 12:14 am

    That’s awesome! Woz is one of my hero, very jealous.

    P.S. Isn’t that a sector marker on Apple floppy disks, I want to say start of a block?

  2. Olivier Guinart July 10th, 2015 1:59 pm

    D5AA96 is a Twitter handle ;D

  3. Steve Chamberlin July 11th, 2015 7:05 am

    Yup, Jordan has it. The data for a single track on a floppy disk is essentially a ring buffer, with no external indicator of where the track data starts and ends, or where the individual sectors are located (or even the boundaries between the bits that make up separate bytes). D5 AA 96 is the magic byte sequence in that buffer that marks the beginning of a sector. This dates all the way back to Apple DOS floppy disks, but was also retained for ProDOS, Lisa, and Macintosh floppy disks. If you’ve spent a lot of time with the low-level details of floppy data layouts, or like Woz you designed that layout in the first place, then you’ll instantly recognize that byte sequence.

  4. Keith M July 13th, 2015 6:59 pm

    The Amiga’s magic sequence is 0xAAAA AAAA 4489 4489. While still being legal MFM, no combination of legitimate data could ever be encoded into this. As you said, this establishes bit-sync and tells you where the first bit of the sector header is located. Then it’s a simple byte counter until you get to the end of the sector. Each Amiga sector start off with these sync bytes.

  5. Todd A. Meyer July 18th, 2015 5:38 pm

    Steve, this is awesome!

  6. John D. Rogers August 17th, 2015 5:27 pm

    I think “Beneath Apple DOS” or possibly Beagle Brothers documentation might talk about the sequence D5 AA 96.

  7. Ralph Doncaster (Nerd Ralph) September 8th, 2015 7:04 am

    If you were into the C64/1541 back in the day, this will impress you:
    http://www.linusakesson.net/programming/gcr-decoding/index.php

    Since the 1541 has it’s own CPU, some games used non-GCR data. Epyx Winter Games is one I remember that loaded faster than anything else at the time. The non-standard encoding (except the directory track) not only allowed for extremely fast loading, it made the disks very difficult to copy.

  8. Krish Rapaka January 23rd, 2017 5:45 am

    Is ‘D5 AA 96’ being the magic byte sequence similar to ‘AUG’ being the “magic” start codon of a sequence of amino acids? Just thought it might be and I’m really proud that I still remember that haha.

  9. Sedentary April 25th, 2017 9:15 am

    What is the 96? I see the

    LDA #$D5 (2) 1ST DATA MARK
    LDA #$AA (2) 2ND DATA MARK.

    But can’t find any meaning in the 96.

    Thanks.

  10. Steve April 25th, 2017 9:28 am

    D5 AA 96 is the address mark, not the data mark. Each sector begins with a short address region, followed by the main data region, and the two regions use different marks. If I recall correctly, the data mark is D5 AA AD.

  11. Sedentary April 25th, 2017 10:09 am

    Yes, I saw that. Thanks!

  12. Sedentary April 25th, 2017 10:10 am

    Pretty cool artifact. Steve Wozniak is such an interesting guy.

  13. eric December 1st, 2017 2:46 am

    Tracks were divided in 16 sectors in Apple Dos (sometimes 17 or 18 in some commercial software not using apple DOS).
    Each sectors contained 256 bytes, and had a Header begining with “D5 AA 96”, and ended by “DE AA”, and a payload begining with “D5 AA AD”, also ending by “DE AA”.
    Due to some hardware restricion, not all 256 possible byte value could be stored in the floppy, so you first had to transform 8 bits bytes into 6 bits “nibbles”, and trasform the nibble value into one of the possible byte value storable on the floppy, so the actual size of the payload was 340 bytes.
    When reading you had to transform back bytes to nibble and then to orginal bytes, which requires so much time apple dos needed the disk to go around several times to real all data on a track.
    Eletronic Arts (EA – yes, they existed at that time) has developped an improved tricky algorythm to be able to read data from floppy track with only one revolution.

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