The difference between Commodore and Atari disk drives is interesting - both were intelligent drives, receiving commands from the host and dealing locally with the details of reading magnetic fields on the disk media. It'd be interesting to see a deeper exploration of those differences - why the OS seems to be mostly in the Commodore drive with the computer issuing file operations to the drive while on the Atari the computer issues low level block IO commands.
I suspect it was RAM-related. The Commodore drive was designed to work with the PET and VIC-20, both of which didn’t really have enough RAM to load DOS, so they built it into the drive ROM instead and stuck with that design.
It is thought that perhaps ill-fated $1500 Atari Accountant software (tied to the never-released 815 dual double-density drive) might have stored user data at the sector level rather than the file level.
I had/have a (physical paper) copy of the official DUP.SYS listing published publicly by Atari https://www.atarimania.com/documents/DOS_2_Source_Listings.pdf
I see a 1982 version of DOS.SYS original (i.e. not disassembled) source code is available at https://atariwiki.org/wiki/attach/Atari%20DOS%20II%20Assembly%20Source%20Listing/Atari%20DOS%20II%202.0S%20DOS.SYS%20Assembly%20Source%20Listing.txt
The difference between Commodore and Atari disk drives is interesting - both were intelligent drives, receiving commands from the host and dealing locally with the details of reading magnetic fields on the disk media. It'd be interesting to see a deeper exploration of those differences - why the OS seems to be mostly in the Commodore drive with the computer issuing file operations to the drive while on the Atari the computer issues low level block IO commands.
I suspect it was RAM-related. The Commodore drive was designed to work with the PET and VIC-20, both of which didn’t really have enough RAM to load DOS, so they built it into the drive ROM instead and stuck with that design.
It is thought that perhaps ill-fated $1500 Atari Accountant software (tied to the never-released 815 dual double-density drive) might have stored user data at the sector level rather than the file level.