The Apple 2's disk drive controller and 5¼" floppy disks have the following metrics:

  • The disks is spun at (roughly) 300 RPM (Revolutions per Minute), which means it takes = 1000 ms/sec / (300 RPM / 60 sec/min) = ~200 ms / revolution,
  • Beneath Apple DOS, Figure 3.3 states it takes 4 μs (microseconds) to read 2 consecutive bits off the disk. This metafilter discussion also mentions this.
  • Beneath Apple DOS, Figure 3.1, shows track zero is the outermost, but sadly doesn't give any dimensions. Loose empirical measurements show:
    • Track 0 (roughly) has a radius of 5"/2 = ~ 2.5"
    • Track 39 (roughly) has a radius of 1.5"/2 = ~ 0.75" (Center hole diameter measured at ~1½")

I've seen 6,250 nibbles given as the (theoretical) maximum number of nibbles on a track (encoded with 6+2) but never an explanation of where this number comes from. (Beneath Apple DOS on page 3-7 briefly mentions 50,000 bits but doesn't provide any details oh this number was calculated.) It is straight forward to see that a track has = 200 ms/revolution * 1000 μs/ms / 4 μs/bit = 50,000 bits / (8 bits/byte) = 6,250 nibbles/track.

However, since the radius (and therefore the circumference) of the outermost and innermost track differ, shouldn't track 0 be able to hold more bits then track 39? The circumference (C = 2 * π * r) for tracks:

  • Track 0 = 2*π*2.625 = ~15.7"
  • Track 39 = 2*π*0.75 = ~4.7"

shows track 0 has a whole lot more "space" then track 39.

This naturally raises a few questions. Namely,

  • Does the Apple 2 disk utilize CLV (Constant Linear Velocity) or CAV (Constant Angular Velocity)?

  • Assuming the same 4 μs/bit but the other speed type then what would be the maximum number of nibbles stored on an Apple II disk for:

    • Track 0, and
    • Track 39


  • While Apple II disk controller is designed to write one bit every four CPU cycles, the read speed can adjust slightly according to the flux patterns on the disk, so if a disk was written with custom hardware it would be possible for it to hold somewhat more data. I wouldn't be surprised if some copy-protection schemes took advantage of that. – supercat May 14 '16 at 22:05

The maximum is 8309 ($2075) nibbles for track 0. Well, according to a little experiment I did. ; - )

The Disk II uses Constant Angular Velocity standardized by Shugart at 300 RPM. The earlier 8-inch drives were 360 RPM, and since the physical media was the same it's likely that 300 RPM was chosen to increase data density on the smaller disks, and perhaps reduce controller throughput for use on early computers that were less powerful and cheaper. (But I can't find a reference for this.)

Disk II spindle speed can be manually adjusted on Apple's analog controller board between approximately 190 and 320 RPM. Slower speed results in higher data density, but this will be limited by physical parameters such as magnetic (iron oxide) particle density and read/write head characteristics - e.g. weaker magnetically induced current in the read head at low speed.

Track capacity is affected by its contents: since MFM clock pulses aren't used, the data written must be carefully chosen in order to synchronize with and be reliably read by the Disk II controller. This includes restrictions on consecutive zeros, and nibbles with trailing zeroes called self-sync bytes.

However in this experiment I didn't use sync bytes, instead relying on the controller sequencer's natural tendency to quickly sync to certain patterns. A simple nibble count measures the maximum theoretical capacity of the track, ignoring the required overheads of practical data storage. Here is the nibble count code:

      ORG $8000
      LDA #$30    ;WRITE LEN
      STA $01
      LDX #$50    ;SLOT * $10
      LDA $C08D,X ;LOAD WP
      LDA $C08E,X ;READ WP
      CMP $C08C,X ;4 - SHIFT
      BIT $FF     ;3
      LDY #$00    ;2
LOOP1 DEC $FF     ;5
      EOR #$7F    ;2 - D5^AA
LOOP2 DEC $FF     ;5
      CMP $FFFF   ;4
      NOP         ;2
      STA $C08D,X ;5 - LOAD
      CMP $C08C,X ;4 - SHIFT
      DEY         ;2
      BNE LOOP1   ;3 OR 2
      DEC $01     ;5
      BNE LOOP2   ;3 OR 2
      DEC $FF     ;5
      DEC $FF     ;5
MARK  LDA #$FF    ;2 - MARK
      STA $C08D,X ;5 - LOAD
      CMP $C08C,X ;4 - SHIFT
      JSR RTS     ;12
      LDA #$20    ;2 - ADDR
      STA STORE+2 ;4
      STA COUNT+2 ;4
      LDY $01     ;3
      STY $00     ;3
      LDA $C08E,X ;READ MODE
      BPL READ
      BNE READ
      INC STORE+2
      BPL READ    ;TO $8000
      STA $C088,X ;MOTOR OFF
      LDX #$00
      CMP MARK+1
      BNE NEXT
      JSR $FDDA
      JSR $FDDA
      LDA #$A0
      JSR $FDED
      LDA $01     ;1ST MARK?
      BEQ COPY
      TXA         ;NO
      SEC         ;CALC LEN
      SBC $00
      JSR $FE80   ;SETINV
      LDA COUNT+2
      SBC $01
      JSR $FDDA
      JSR $FE84   ;SETNORM
      LDA #$8D
      JSR $FDED
      STA $01
      STX $00
      INC COUNT+2
      BPL COUNT   ;TO $8000?
RTS   RTS         ;YES, DONE

Assembled at $8000, there are some defaults that can be optionally adjusted:

  • 8001: The number of nibbles to write to the track in units of $100 (Default $30 = $3000 nibbles)
  • 8005: The drive slot number times $10 (Default $50 - safer than $60!)
  • 8010: The first nibble value; this gives a pair sequence via XOR with $7F (Default D5/AA)
  • 801E: The XOR value; bit 8 should be 0 (Default $7F)
  • 8037: The nibble count mark value (Default $FF)
  • 8042: The page to start storing track data when read back and nibble counted (Default $20)

The routine assumes the drive is on and up-to-speed, and that the head is already positioned on the desired track. The defaults as given will write $3000 nibbles of "D5AA" followed by one "FF" mark to the selected drive in slot 5. The track will then be read into memory at $2000-$7FFF and the data scanned for the mark value. Mark locations will be printed in normal text, and length between marks in inverse.

Here is an image of a sample run which uses boot 0 to seek track 0 of drive 1 in slot 6 in AppleWin:

Sample run of nibble count routine

Adjusting a real drive's speed (measured with Copy II Plus 8.4) I obtained the following results:

Speed (ms)  Speed (RPM)  Max length  Length (Hex)
188         319          6015        177F
200         300          6400        1900
210         286          6715        1A3B
220         273          7045        1B85
230         261          7360        1CC0
240         250          7680        1E00
250         240          8004        1F44
260         231          8309        2075
270         222          -           -
280         214          -           -
317         189          -           -

At 270ms and above the sequencer would not auto-sync to simple nibble patterns, though Copy II Plus could still measure drive speed. Its exclusive use of selected sync byte values would likely explain this.

Tracks written at one speed can be read at another speed that is +/-10ms. I didn't test how far this tolerance extends. Roland Gustafsson (who wrote the copy protection for many Brøderbund titles) says he tried slowing the drive speed but "never used that technique due to compatibility problems. In fact, [he] wrote a speed calibrator that was used at Broderbund to keep the disk drives in spec".

When speculating about an alternative Constant Linear Velocity capacity you would need to specify the exact hardware being used, since that big a change would probably extend from the controlling software to the magnetic media - and everything in between. There is a good list of existing floppy disk formats on Wikipedia.

  • Hi Nick! :-) Thanks for the nibble code -- with ADTPro I'll get that transferred to a real disk image. What should $FF be initialized with? $00? – Michaelangel007 May 29 '16 at 1:12
  • 2
    Hi Michael. References to $FF or $FFFF are time-wasting code. All disk accesses are in 32-cycle blocks. – Nick Westgate May 29 '16 at 2:02
  • 1
    In trying to work out why everyone was talking about nibbles (I'd never heard the term in this context before) I came across diskii.nibbles which goes into all sorts of gory details that someone may find useful. – TripeHound Apr 20 '18 at 12:57
  • 1
    Woz called them disk nibbles because originally the disk controller would write 4-bits as an 8-bit value with every other bit being one (for timing.) – Michaelangel007 Apr 23 '18 at 5:48

Apple ][ used an ordinary disk transport and fixed bit-rate in the controller. In order to fit more bits on the outer track it would need to vary the spin rate of the disk so that the controller had time to put the extra bits on.

CD drives use variable spin rate, but floppy disks (mostly) use a fixed spin rate (like gramophone records).

Early Apple Macintosh's used a custom disk transport with variable spin rate, so it could fit more data on the outer tracks of the disk, getting 800K on a "720K" (3.5" DSDD) floppy.

  • 3
    Actually, the 1.4 MB format was the same on the Mac as on PCs. The variable rate was used on double-density 720K disks to get 800K onto them on the Mac (and the Apple IIgs). – Eric Shepherd May 17 '16 at 21:48
  • @Jasen there is quick discussion of zone bit recording that alludes to Apple's techniques to get single and double density 3.5" floppies to store 400/800k instead of 360/720k. Unfortunately modern USB 3.5" drives can't read these disks since they require a Mac/IIgs disk drive. – Michael Shopsin May 19 '16 at 18:50

You might also find this discussion interesting: https://groups.google.com/forum/#!topic/comp.sys.apple2/7srpWGp1pCs

Although the Apple II could only write at 4 cycles per bit, it can read faster: if you slow the drive down, you can write more densely.

  • Already participating there, but thanks. :-) – Michaelangel007 May 19 '16 at 14:33
  • Your answer would be more useful if you could summarize the discussion from that group that's relevant to the question here. – cjs Jul 16 '19 at 8:56

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