ProDOS

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Template:Infobox OS Image:Beneathappleprodos.jpg

ProDOS (the Professional Disk Operating System) became the most popular operating system for the Apple II series of computers not long after its release in 1983.

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, and introduced support for subdirectories, a vital feature for organizing a hard disk's storage space. ProDOS also addressed earlier DOS issues with hardware interrupts and included a well-defined and documented programming and expansion interface, which DOS had always lacked, and added support for a real-time clock, though this did not become a standard hardware feature of the Apple II until the release of the Apple IIgs. ProDOS also supported a RAM disk on machines with 128K or more of RAM.

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 program called DOS.MASTER enabled users to have multiple virtual DOS 3.3 partitions on a larger ProDOS volume.

The enhanced feature set of ProDOS meant that the operating system required a minimum of 64K RAM. The Apple IIe, also released in 1983, was the first Apple II computer to have this amount of memory built in. For a while, Apple shipped both DOS and ProDOS with new computers.

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+]]. Whereas DOS 3.3 included built-in support for BASIC programming, under ProDOS this job was given to a separate 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 that allowed BASIC programs to access ProDOS by means of the same "print Control-D" method 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 IIgs 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.

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.

Apple II computers with a 65C02 chip can run ProDOS 8 version 2.0 or later, which is included on Apple II System Disk 4.0 or later; the utility programs on that disk add the additional requirement of at least 128KB of RAM. Systems without a 65C02 must use ProDOS 8 versions prior to version 2.0.

ProDOS system disk images can be downloaded legally from a number of user group web sites. It can also be purchased on disk from Syndicomm, which distributes it under license from Apple Computer.

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