Apple DOS - Technical Details

Technical Details

Apple DOS 3.1 disks used 13 sectors of data per disk track, each sector being 256 B in size. It used 35 tracks per disk side, and could access only one side of the floppy disk, unless the user flipped the disk over. This gave the user a total storage capacity of 113.75 kB per disk side, of which about 10 kB were used to store DOS itself and the disk directory, leaving about 100 kB for user programs.

Compared to the modern operating systems, Apple DOS was quite primitive. The first layer of the operating system was called the RWTS, which stands for "read/write track sector". This layer consisted of subroutines for track seeking, sector reading and writing, and disk formatting. An API called the File Manager was built on top of this, and implemented functions to open, close, read, write, delete, lock (i.e. write-protect), unlock (i.e. write-enable), and rename files, and to verify a file's structural integrity. There is also a catalog function, for listing files on the diskette, and an init function, which formats a disk for use with DOS, and stores a startup program (usually called HELLO) that will be auto-started when this disk is booted. On top of the File Manager API, the main DOS routines were implemented which hooked into the machine's BASIC interpreter and intercepted all disk commands. It provided BLOAD, BSAVE, and BRUN for storing, loading and running binary executables. LOAD, RUN, and SAVE were provided for BASIC programs, and an EXEC was provided for running text-based batch files consisting of BASIC and DOS commands. Finally, 4 types of files existed, identified by letters in a catalog listing.

Apple DOS File Formats:

• I: Integer BASIC programs (stored in a compact format, not plain-text).
• A: AppleSoft BASIC programs (also stored in a packed, space-saving format).
• B: Binary files, either executable machine-language programs, or data files.
• T: ASCII text files (or plain-text, unpacked batch files).

There were four additional file types; 'R', 'S' and an additional 'A' and 'B', none of which were fully supported. DOS recognized these types for catalog listings only, and there were no direct ways to manipulate these types of files. The 'R' type found some use for relocatable binary executable files. A few programs supported the 'S' type as data files.