Editing Atari DOS (section)

==={{anchor|3}}DOS 3===
[[File:Atari Master Diskette 3 DOS 3 Floppy Disk in Sleeve.jpg|thumb|DOS 3 diskette as supplied with a 1050 disk drive]]
The 1050 was the first drive from Atari to offer higher recording density. For reasons unknown, they did not take advantage of the 180 kB "double density" mode the hardware in the 1050 was capable of using, and instead introduced a new "Dual Density" mode. To avoid confusion, Atari users soon began referring to this as [[#Disk formats|Enhanced Density]] to differentiate it from real double density systems.

Initially, this increased density mode was not able to be used as Atari shipped the drives with 2.0, which only supported the original 90 kB mode. This was addressed not long after with the release of DOS 3.0. When formatted with DOS 3.0, the disk held 40 tracks of 26 sectors, up from 18 on DOS 2.0. Each sector holds 128 bytes, for a total of 133,120 bytes of storage, up from DOS 2.0's 92,160.

In DOS 2.0, a 10-bit number was used to store sector numbers in the directory. This limited disks to a maximum of 1024 sectors. The new format had 1040 sectors, so 2.0 would not be able to use all of its capability. To address this, and to offer the possibility of working with even larger formats in the future, DOS 3.0 grouped sectors together in groups of eight, known as a [[Block (data storage)|block]], each holding 1,024 bytes. The number in the directory was reduced to 8-bits, meaning it could address up 256 kB on a single disk. The boot information and directory used three blocks, leaving 130 kB free for user storage on the 1050.{{sfn|Wilkinson|1982|p=23}}

Unfortunately, the new directory format made the disks unreadable on DOS 2.0. The DOS 3.0 Master Diskette included a small utility program to copy a 2.0 disk to 3.0, but not one to copy it back. A separate utility allowed disks to be formatted in 2.0 format if compatibility was needed.{{sfn|Wilkinson|1982|p=25}} As a result of this decision, DOS 3 was extremely unpopular and did not gain widespread acceptance amongst the Atari user community.

DOS 3 provided built-in help via the Atari HELP key and/or the inverse key. Help files needed to be present on the system DOS disk to function properly. DOS 3 also used special XIO commands to control disc operations within BASIC programs.