When formatting a blank 5¼ disk on an 8-bit Apple II using the INIT command, was there only ever one variation of the boot sector code that was written, or was it updated from time to time over the years?

I'm interested in identifying all the standard boot sectors apart from any customized ones so if there were more than one standard variation I'd like to find a list of the changes from version to version.

For the purposes of this question I'm not asking about ProDOS or 3½ inch disks. For now I'm only asking about the first sector of the first track, not other sectors that are written to during the INIT process. Also I'm aware that some bytes such as the volume ID probably change from disk to disk.

1 Answer 1


This is a little tricky to answer, because only about 30% of a DOS 3.3 boot sector is actually boot code. The BOOT0 ROM has everything needed to read a sector from track 0, so BOOT1 (T0/S0) just needs to call it with physical sector numbers and buffer addresses to load the full RWTS.

The DOS INIT command needs a T0/S0 image handy when formatting a disk, so the boot sector lives in memory at $B600. Software expands to fill all available space, so the unused area is occupied by patches to other parts of DOS.

It's reasonable to expect that you'd find different boot sector images on the various "enhanced" DOS disks. So a list might include:

  • Vanilla (DOS 3.3 system master)
  • Diversi-DOS
  • Pronto-DOS
  • Hyper-DOS
  • David-DOS

According to wikipedia, there were two releases of Vanilla DOS 3.3.

  • I wonder why the I/O card did a JMP to $0801 rather than a JSR followed by a looping branch, since the latter would have avoided any need to have the BOOT1 code be reliant upon the read-sector routine being precisely at offset $5C of the boot ROM.
    – supercat
    Commented Nov 30, 2020 at 22:11

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