Apple ProDOS - Background

Background

ProDOS was released to address shortcomings in the earlier Apple operating system (called simply DOS), which was beginning to show its age. DOS only had built-in support for 5.25" floppy disks and required patches to use peripheral devices such as hard disk drives and non-Disk II floppy disk drives, including 3.5" floppy drives. ProDOS added a standard method of accessing ROM-based drivers on expansion cards for disk devices, expanded the maximum volume size from about 400 kilobytes to 32 megabytes, introduced support for hierarchical subdirectories (a vital feature for organizing a hard disk's storage space), and supported RAM disks on machines with 128kB or more of memory. ProDOS addressed problems with handling hardware interrupts, and included a well-defined and documented programming and expansion interface, which DOS had always lacked. Although ProDOS also included 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. Third-party clocks were available for the II Plus, IIe, and IIc, however.

ProDOS, unlike earlier Apple DOS versions, had its developmental roots in SOS, the operating system for the ill-fated Apple III computer released in 1980. Pre-release documentation for ProDOS (including early editions of Beneath Apple ProDOS) documented SOS error codes, notably one for switched disks, that ProDOS itself could never generate. Its disk format and programming interface were completely different from those of DOS, and ProDOS could not read or write DOS 3.3 disks except by means of a conversion utility; while the low-level track-and-sector format of DOS 3.3 disks was retained for 5.25 inch disks, the high-level arrangement of files and directories was completely different. For this reason, most machine-language programs that ran under DOS would not work under ProDOS. However, most BASIC programs would work, though they sometimes required minor changes. A third-party program called DOS.MASTER enabled users to have multiple virtual DOS 3.3 partitions on a larger ProDOS volume.

With the release of ProDOS came the end of support for Integer BASIC and the original Apple II model, which had long since been effectively supplanted by Applesoft BASIC and the Apple II Plus. Whereas DOS 3.3 always loaded built-in support for BASIC programming, under ProDOS this job was given to a separate system program called BASIC.SYSTEM, which one launched to run and write Applesoft BASIC programs. BASIC itself continued to be built into the Apple ROMs; BASIC.SYSTEM was merely a command interpreter enhancement that allowed BASIC programs to access ProDOS by means of the same "Control-D" text output they had used under DOS 3.3. BASIC.SYSTEM alone required about as much memory as the whole of DOS 3.3. Since the ProDOS kernel itself was stowed away in the "Language Card" RAM, the usable amount of RAM for BASIC programmers remained the same under ProDOS as it had been under DOS 3.3.

Despite ProDOS's many advantages, many users and programmers resisted it for a time because of their investment in learning the ins and outs of DOS and in DOS-based software and data formats. A contributing reason was that ProDOS allowed only fifteen characters in a filename compared to DOS's thirty. But Apple's integrated software package AppleWorks, released in 1984, proved a compelling reason to switch, and by the end of 1985 few new software products were being released for the older operating system. Apple IIs continued to be able to boot DOS (even the Apple II could boot DOS floppies) but as 3.5" floppies and hard disks became more prevalent, most users spent the bulk of their time in ProDOS.

The Apple IIe, also released in 1983, was the first Apple II computer to have 64kB of memory built in. For a while, Apple shipped both DOS 3.3 and ProDOS with new computers.

ProDOS was renamed ProDOS 8 when ProDOS 16 was released to support the 16-bit Apple IIGS computer, although ProDOS 16 was soon replaced by GS/OS.