The disk controller hardware had some limitations where it could drift out of sync if a byte had too many zero bits in a row. In DOS 3.2 there were 32 values available that met the criteria. So each byte was split up into groups of 5 and 3 bits, groups of 5 3-bit chunks were put together into 5-bit values, and all the resulting 5-bit values were mapped to the 32 available values for writing. On reading, the process had to be reversed and the original bytes reassembled.
Clever Wozniak... this was a big improvement over existing disk controllers which split bytes into two 4-bit chunks and mapped these to 16 values. This got you about 88K per disk; Wozniak's scheme got you about 113K. The reason other manufacturers used 4-bit chunks was because they did all this in hardware and it was a lot easier that way. Wozniak figured since he was doing it in software he'd do it the most efficient way possible and get higher denisty.
A later upgrade to the controller hardware, introduced with DOS 3.3, allowed two zero bits in a byte, which enabled 64 different values to be written to disk. This further increased disk storage to 140K. Apple provided new ROMs for the controller, which could be installed by the user.
Some pretty significant portions of it were published in Nibble magazine, which had a column called "Disassembly Lines" that reverse-engineered BASIC and DOS. (So, not the original Apple source, but another programmer's version that would assemble to the same object code.) And while the book Beneath Apple DOS didn't have source code, it had just about everything else you'd want to know.
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The disk controller hardware had some limitations where it could drift out of sync if a byte had too many zero bits in a row. In DOS 3.2 there were 32 values available that met the criteria. So each byte was split up into groups of 5 and 3 bits, groups of 5 3-bit chunks were put together into 5-bit values, and all the resulting 5-bit values were mapped to the 32 available values for writing. On reading, the process had to be reversed and the original bytes reassembled. Clever Wozniak... this was a big improvement over existing disk controllers which split bytes into two 4-bit chunks and mapped these to 16 values. This got you about 88K per disk; Wozniak's scheme got you about 113K. The reason other manufacturers used 4-bit chunks was because they did all this in hardware and it was a lot easier that way. Wozniak figured since he was doing it in software he'd do it the most efficient way possible and get higher denisty. A later upgrade to the controller hardware, introduced with DOS 3.3, allowed two zero bits in a byte, which enabled 64 different values to be written to disk. This further increased disk storage to 140K. Apple provided new ROMs for the controller, which could be installed by the user.
VTOC = Volume Table of Contents, a bitmap that keeps track of which sectors are used.
Some pretty significant portions of it were published in Nibble magazine, which had a column called "Disassembly Lines" that reverse-engineered BASIC and DOS. (So, not the original Apple source, but another programmer's version that would assemble to the same object code.) And while the book Beneath Apple DOS didn't have source code, it had just about everything else you'd want to know.