The track will then be read into memory at $2000-$7FFF and the data scanned for the mark value. The defaults as given will write $3000 nibbles of "D5AA" followed by one "FF" mark to the selected drive in slot 5. The routine assumes the drive is on and up-to-speed, and that the head is already positioned on the desired track. 8042: The page to start storing track data when read back and nibble counted (Default $20).8037: The nibble count mark value (Default $FF).801E: The XOR value bit 8 should be 0 (Default $7F).8010: The first nibble value this gives a pair sequence via XOR with $7F (Default D5/AA).8005: The drive slot number times $10 (Default $50 - safer than $60!). 8001: The number of nibbles to write to the track in units of $100 (Default $30 = $3000 nibbles).Here is the nibble count code: ORG $8000Īssembled at $8000, there are some defaults that can be optionally adjusted: A simple nibble count measures the maximum theoretical capacity of the track, ignoring the required overheads of practical data storage. 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. This includes restrictions on consecutive zeros, and nibbles with trailing zeroes called self-sync bytes. 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. weaker magnetically induced current in the read head at low speed. 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. (But I can't find a reference for this.)ĭisk II spindle speed can be manually adjusted on Apple's analog controller board between approximately 190 and 320 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. The Disk II uses Constant Angular Velocity standardized by Shugart at 300 RPM. Well, according to a little experiment I did. The maximum is 8309 ($2075) nibbles for track 0. Namely,ĭoes the Apple 2 disk utilize CLV ( Constant Linear Velocity) or CAV ( Constant Angular Velocity)?Īssuming 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: Shows track 0 has a whole lot more "space" then track 39. 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: (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. 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.
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