ProDOS is the name of two similar operating systems for the Apple II of personal computer.

[citation needed] The other, ProDOS 16, was a stop-gap solution for the 16-bit Apple IIGS that was replaced by GS/OS within two years.

[3] ProDOS was released to address shortcomings in the earlier Apple operating system (called simply DOS), which was beginning to show its age.

ProDOS adds a standard method of accessing ROM-based drivers on expansion cards for disk devices, expands the maximum volume size from about 400 kilobytes to 32 megabytes, introduces support for hierarchical subdirectories (a vital feature for organizing a hard disk's storage space), and supports RAM disks on machines with 128 KB or more of memory.

ProDOS addresses problems with handling hardware interrupts, and includes a well-defined and documented programming and expansion interface, which Apple DOS had always lacked.

Although ProDOS also includes support for a real-time clock (RTC), this support went largely unused until the release of the Apple IIGS, the first in the Apple II series to include an RTC on board.

A third-party program called DOS.MASTER enables users to have multiple virtual DOS 3.3 partitions on a larger ProDOS volume.

A contributing reason was that ProDOS allows only 15 characters in a filename compared to Apple DOS's 30.

Apple IIs continued to be able to boot the older DOS (even the Apple IIGS can boot the older DOS floppies) but as 3.5" floppies and hard disks became more prevalent, most users spent the bulk of their time in ProDOS.

With the release of ProDOS version 1.01 and higher, a check was added to see if it was running on an official Apple-manufactured computer.

If the word "Apple" is found in the computer's ROM firmware, ProDOS will load up as normal.

If anything else is found (e.g. "Golden", "Franklin", "Elite") ProDOS refuses to run, locking up at the boot splash screen.

ProDOS 8 natively supports Disk II-compatible floppy drives, a RAM drive of approximately 59 KB on computers having 128 KB or more RAM, and block devices whose controllers support the Pascal firmware protocol, a standardized method of accepting block reads and writes originally introduced for use with the UCSD p-System.

The root directory on most disks is initialized to 4 blocks, allowing 51 entries (excluding the volume header).

When the Apple IIgs was introduced, a new storage format was introduced for files with two forks, as was typical for IIgs system and program files; the directory entry points to an informational block that tells the computer the storage format of the two forks.