Editing Apple ProDOS (section)

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

Apple DOS only has built-in support for 5.25" floppy disks and requires [[patch (computing)|patches]] to use peripheral devices such as [[hard disk drive]]s and non-[[Disk II|Disk-II]] [[floppy disk]] drives, including 3.5" floppy drives. ProDOS adds a standard method of accessing ROM-based [[device driver|drivers]] on expansion cards for disk devices, expands the maximum [[Volume (computing)|volume]] size from about 400 [[kilobytes]] to 32 [[megabytes]], introduces support for [[hierarchical file system|hierarchical]] [[directory (computing)|subdirectories]] (a vital feature for organizing a hard disk's storage space), and supports [[RAM drive|RAM disks]] on machines with 128 KB or more of memory. ProDOS addresses problems with handling hardware [[interrupt]]s, 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|Apple II{{sc|gs}}]], the first in the Apple II series to include an RTC on board. Third-party clocks were available for the II Plus, IIe, and [[Apple IIc|IIc]], however. ProDOS shipped with a built-in clock driver for the Thunderware Thunderclock, which was a common real-time clock card in Apple II+ and IIe systems; other clock hardware required replacing the Thunderclock driver with a driver for the other clock.

ProDOS, unlike earlier Apple DOS versions, has its developmental roots in [[Apple SOS|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 are completely different from those of Apple DOS, and ProDOS cannot read or write DOS 3.3 disks except by means of a conversion utility; while the low-level [[track (disk drive)|track]]-and-[[disk sector|sector]] format of DOS 3.3 disks was retained for 5.25-inch disks, the high-level arrangement of files and directories is completely different. For this reason, most [[machine code|machine-language]] programs that run under Apple DOS will not work under ProDOS. However, most BASIC programs work, though they sometimes require minor changes. A third-party program called [[DOS.MASTER]] enables users to have multiple virtual DOS 3.3 [[disk partitioning|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 loads built-in support for BASIC programming, under ProDOS this job is given to a separate system program called {{mono|BASIC.SYSTEM}}, which one launches to run and write Applesoft BASIC programs. BASIC itself continued to be built into the Apple ROMs; {{mono|BASIC.SYSTEM}} is merely a [[command interpreter]] enhancement that allows BASIC programs to access ProDOS by means of the same "Control-D" text output they had used under DOS 3.3. {{mono|BASIC.SYSTEM}} alone requires about as much memory as the whole of DOS 3.3. Since the ProDOS [[Kernel (operating system)|kernel]] itself is stowed away in the "[[Apple_II_(original)#Programming_languages|Language Card]]" RAM, the usable amount of RAM for BASIC programmers remains 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 Apple DOS and in Apple-DOS-based software and data formats. A contributing reason was that ProDOS allows only 15 characters in a [[filename]] compared to Apple DOS's 30. 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 the older DOS (even the [[Apple IIGS|Apple II<small>GS</small>]] 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.

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

The original ProDOS was renamed ProDOS 8 when ProDOS 16 was released to support the 16-bit [[Apple IIGS|Apple II{{sc|gs}}]] computer, although ProDOS 16 (which was 8-bit at its core) was soon replaced by [[Apple GS/OS|GS/OS]].