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Apple DOS running a Hello World program
|Company / developer||Apple Computer|
|OS family||Apple DOS|
|Source model||Closed source|
|Initial release||Late 1978|
|Latest stable release||3.3 / 1980 (last released bug fix in 1983)|
|Kernel type||Monolithic kernel|
|License||Apple Software License Agreement|
Apple DOS refers to operating systems for the Apple II series of microcomputers from late 1978 through early 1983. Apple DOS had three major releases: DOS 3.1, DOS 3.2, and DOS 3.3; each one of these three releases was followed by a second, minor "bug-fix" release, but only in the case of Apple DOS 3.2 did that minor release receive its own version number, Apple DOS 3.2.1. The best-known and most-used version was Apple DOS 3.3 in the 1980 and 1983 releases. Prior to the release of Apple DOS 3.1, Apple users had to rely on audio cassette tapes for data storage and retrieval, but that method was notoriously slow, inconvenient and unreliable.
When Apple Computer introduced the Apple II in April 1977, the new computer had no disk drive or disk operating system (DOS). Although Apple cofounder Steve Wozniak designed the Disk II controller late that year, and believed that he could have written a DOS, fellow founder Steve Jobs decided to outsource the task. The company considered using Digital Research's CP/M, but Wozniak sought an operating system that was easier to use. On 10 April 1978 Apple signed a $13,000 contract with Shepardson Microsystems to write a DOS and deliver it within 35 days. Apple provided detailed specifications, and early Apple employee Randy Wigginton worked closely with Shepardson's Paul Laughton as the latter wrote the operating system with punch cards and a minicomputer.
There was no Apple DOS 1 or 2, per se. Versions 0.1 through 2.8 were serially enumerated revisions during development, which might as well have been called builds 1 through 28. Apple DOS 3.0, a renamed issue of version 2.8, was never publicly released due to bugs. To the dismay of many programmers, Apple published no official documentation until release 3.2. 
Apple DOS 3.1 was publicly released in June 1978 (slightly less than one year after the Apple II was introduced), becoming the first disk-based operating system for any Apple computer. A bug-fix release came later, addressing a problem with its MASTER CREATE utility, which was used to create Apple DOS master disks: The built-in INIT command created disks that could be booted only on machines with at least the same amount of memory as the one that had created them. MASTER CREATE included a self-relocating version of DOS that would boot on Apples with any memory configuration.
Apple DOS 3.2 was released in 1979 to reflect major changes in computer booting methods that were built into the successor of the Apple II, the Apple II Plus. Instead of the original Integer BASIC, the Apple II Plus firmware included the newer Applesoft II floating point BASIC. The new firmware also had an autostart feature which would automatically find a disk controller and boot from it when the system was powered up—earning it the name Autostart ROM.
Apple DOS 3.3 was released in 1980. It improved various functions of release 3.2, while also allowing for large gains in available floppy disk storage; the newer P5A/P6A PROMs in the disk controller could read and write data at a higher density, so that instead of 13 sectors (3.25 kB), 16 sectors (4 kB) of data could be stored per disk track, increasing the capacity from 113.75 kB to 140 kB per disk side — 16 kB of which was used by filesystem overhead and a copy of DOS, on a DOS 3.3-formatted disk, leaving 124 kB for user programs and data. DOS 3.3 was, however, not backwards compatible; it could not read or write DOS 3.2 disks. To address this problem, Apple Computer released a utility called "MUFFIN" to migrate Apple DOS 3.2 files and programs to version 3.3 disks. Apple never offered a utility to copy the other way. To migrate Apple DOS 3.3 files back to version 3.2 disks, someone wrote a "NIFFUM" utility. There were also commercial utilities (such as Copy II Plus) that could copy files from and to either format (and eventually ProDOS as well). Release 3.3 also improved the ability to switch between Integer BASIC and Applesoft BASIC, if the computer had a language card (RAM expansion) or firmware card.
Apple DOS 3.1 disks used 13 sectors of data per disk track, each sector being 256 B in size. It used 35 tracks per disk side, and could access only one side of the floppy disk, unless the user flipped the disk over. This gave the user a total storage capacity of 113.75 kB per disk side, of which about 10 kB were used to store DOS itself and the disk directory, leaving about 100 kB for user programs.
Compared to the modern operating systems, Apple DOS was quite primitive. The first layer of the operating system was called the RWTS, which stands for "read/write track sector". This layer consisted of subroutines for track seeking, sector reading and writing, and disk formatting. An API called the File Manager was built on top of this, and implemented functions to open, close, read, write, delete, lock (i.e. write-protect), unlock (i.e. write-enable), and rename files, and to verify a file's structural integrity. There is also a catalog function, for listing files on the diskette, and an init function, which formats a disk for use with DOS, and stores a startup program (usually called HELLO) that will be auto-started when this disk is booted. On top of the File Manager API, the main DOS routines were implemented which hooked into the machine's BASIC interpreter and intercepted all disk commands. It provided BLOAD, BSAVE, and BRUN for storing, loading and running binary executables. LOAD, RUN, and SAVE were provided for BASIC programs, and an EXEC was provided for running text-based batch files consisting of BASIC and DOS commands. Finally, 4 types of files existed, identified by letters in a catalog listing.
Apple DOS File Formats:
- I: Integer BASIC programs (stored in a compact format, not plain-text).
- A: AppleSoft BASIC programs (also stored in a packed, space-saving format).
- B: Binary files, either executable machine-language programs, or data files.
- T: ASCII text files (or plain-text, unpacked batch files).
There were four additional file types; 'R', 'S' and an additional 'A' and 'B', none of which were fully supported. DOS recognized these types for catalog listings only, and there were no direct ways to manipulate these types of files. The 'R' type found some use for relocatable binary executable files. A few programs supported the 'S' type as data files.
Most 8-bit Apple II machine language programs did not concern themselves with code relocation since the Apple II usually did not run multiple programs at the same time like modern computers. Programs were generally written to always expect to find their machine code at the same memory location every time. Fully relocatable code was more complicated and difficult to implement.
Apple DOS, however, needed to accommodate varying amounts of memory installed in an Apple II — from 16 kB to 48 kB. DOS would load itself at the highest point possible, to free up the lower memory locations for BASIC programs and graphics capabilities. In machines with 48 kB or more memory, it occupied the region from $9600 to $BFFF (three file buffers + 10 kB of software), but could load in a lower memory area if less system memory were available. (In comparison, ProDOS always loads into language card RAM, thus requiring at least a 64 kB machine.)
A call vector table in the region of $03D0-03FF allowed programs to find DOS wherever it was loaded into the system memory. For example, if the DOS hooks into the BASIC CLI stopped functioning, DOS could be reinitialized by calling location $03D0 (976) — hence the traditional "3D0G" command to return to BASIC from the System Monitor.
The process of loading Apple DOS involved a series of very tiny programs, each of which carried the loading process forward a few steps before passing control to the next program in the chain.
- Originally, the Apple II ROM did not support disk booting at all. At power-up it would go straight to the System Monitor prompt. At this point, booting from disk required the user to type the slot number followed by Control-P then Return, which caused the next character printed to invoke the boot routine, or typing Cn00G (where n is the slot number) to invoke it directly.
- When the Apple II Plus was introduced, it included the ability to scan each expansion slot (working downward from slot 7 to slot 1) for an expansion card ROM with additional boot code, and automatically call it.
- The expansion card ROM boot code attempts to boot from drive 1 of the controller, by moving the read/write arm to track zero, and attempting to read 256 bytes from sector zero of that track. (If no readable disk is present the drive spins indefinitely until a readable disk is provided and the drive door is closed.)
- Sector zero contains a small program which tells the computer how to read sectors 0 to 9 of track zero (re-reading sector 0 in the process).
- The program in sectors 1-9 of track 0, including the complete RWTS code, then proceeds to load tracks 1 and 2, which contain the rest of DOS. On a "system master" disk, code is also included to relocate DOS as high in system memory as possible, up to the 48 kB limit of the Apple II's main memory ($BFFF).
- Once DOS is loaded into memory, it attempts to load and execute a startup program as indicated in the DOS program code. This was commonly a BASIC language program named HELLO (or some other name) but DOS could be modified to run other types of programs at startup, such as an executable binary file. The appearance of the right-hand bracket ( ] ) on the screen was an indication to the user that an Applesoft BASIC startup program was loading, while a greater-than symbol ( > ) indicated an Integer BASIC program was loading.
- The startup program would then begin executing.
Integer BASIC and Applesoft floating point BASIC support
The Apple II started out using a simple BASIC interpreter known originally as Apple BASIC and later as Integer BASIC. It could only handle integer numbers ranging from -32768 to 32767 and only had built-in support for low-resolution graphics. But it was the first version of BASIC available for the Apple II and is what the earliest BASIC programs used.
Only months after the Apple II's release, Apple commissioned Microsoft to develop a much more capable interpreter known as Applesoft BASIC, capable of handling floating-point real numbers with up to nine digits of precision and base 10 multipliers from -38 to +38, and with support for high-resolution graphics. While more capable, Applesoft could not run Integer BASIC programs, causing some users to resist upgrading to it.
DOS 3.3 was released when Applesoft BASIC was standard in ROM on the Apple II Plus, so Apple designed it to support switching back and forth between the two BASIC interpreters. The DOS 3.3 System Master disk contained Integer BASIC and Applesoft on disk as a binary files which could be loaded into RAM, allowing whichever language was not resident in ROM to be used on any machine with enough RAM. The user could switch between BASICs by typing either FP or INT at either BASIC prompt. DOS automatically chose the needed language when running a saved program. Applesoft programs were saved as type A files, while Integer BASIC programs were saved as type I files.
Decline of Apple DOS
After 1980, DOS entered into a state of stagnation, along with all other Apple II products, as Apple concentrated its efforts on the ill-fated Apple III computer and its SOS operating system. After the Apple III had been abandoned by the company, two more versions of Apple DOS, both still called DOS 3.3 but with some bug fixes and better support for the new Apple IIe model, were released in early and mid-1983.
Without third-party patches, Apple DOS could only read floppy disks running in a 5.25-inch Disk II disk drive and could not access any other media, such as hard disk drives, virtual RAM drives, or 3.5-inch floppy disk drives. The structure of DOS was such that it was not possible to have more than 400kB available per drive without a major rewrite of almost all sections of the code; this was the main reason Apple abandoned DOS in 1983, when Apple DOS was entirely replaced by ProDOS.
ProDOS retained the 16-sector low-level format of DOS 3.3 for 5.25 inch disks, but introduced a new high-level format that was suitable for devices up to 32 MB in size; this made it suitable for hard disks and 3.5-inch floppies. All the Apple computers from the Apple II Plus onward can run both DOS 3.3 and ProDOS, the Apple II Plus requiring a "Language Card" memory expansion to use ProDOS; the Apple //e and later models had built-in Language Card hardware, and so could run ProDOS out of the box. ProDOS included software to copy files from Apple DOS disks. However many people who had no need for the improvements of ProDOS (and who did not like its much higher memory footprint) continued using Apple DOS or one of its clones long after 1983. The Apple convention of storing a bootable OS on every single floppy disk meant that commercial software could be used no matter what OS the user owned. A program called DOS.MASTER enables users to have multiple virtual DOS 3.3 partitions on a larger ProDOS volume, which allowed the use of many floppy-based DOS programs with a hard disk.
Performance improvements and other versions
DOS's RWTS could read or write a track in two revolutions with proper interleaving. A sector of the spinning disk would pass by under the read/write head while RWTS was decoding the just-read sector (or encoding the next one to be written), and if this missed sector was the next one needed, DOS would need to wait nearly an entire revolution of the disk for the sector to come around again. This was called "blowing a rev" and was a well-understood performance bottleneck in disk systems. To avoid this, the sectors on a DOS disk were arranged on the disk in this order:
0 7 14 6 13 5 12 4 11 3 10 2 9 1 8 15
Later on, ProDOS arranged sectors in this order:
0 8 1 9 2 10 3 11 4 12 5 13 6 14 7 15
When reading and decoding sector 0, then, sector 8 would pass by, so that sector 1, the next sector likely to be needed, would be available without waiting. When reading sector 7, two unneeded sectors, sectors 15 and 0, would pass by before sector 8 was available, and when reading sector 15, the drive would always have to wait a revolution for sector 0 on the same track. However, the sector 0 actually needed in most cases would be on the next higher track, and that track could be arranged relative to the last one to allow the needed time to decode the just-read sector and move the head before sector 0 came around. On average, a full track could be read in two revolutions of the disk.
Unfortunately, the DOS file manager ruined this efficiency by copying bytes read from or written to a file one at a time between the RWTS buffer and main memory, requiring more time and resulting in DOS constantly blowing revs when reading or writing files. Programs became available early on to format disks with modified interleaves; these disks gave DOS more time between sectors to copy the data, ameliorating the problem. Later, programmers rewrote the file manager routines to avoid making the extra copy for most sectors of a file; RWTS was instructed to read or write sectors directly to or from main memory rather than a buffer when at least a full sector was to be transferred. Speedups in the LOAD command of three to five times were typical.
This functionality soon appeared in commercial products, such as Pronto-DOS, Diversi-DOS, and David-DOS, along with additional features, but was never used in an official Apple DOS release. Similar functionality was, however, employed by Apple's successor operating system, ProDOS. The Apple IIGS operating system, GS/OS, would eventually employ an even more efficient "scatter read" technique that would read any sector that happened to be passing under the read head if it was needed for the file being read.
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- DOS 3.3, ProDOS & Beyond v1.3 bullet 07 Jul 01
- Paul Laughton's account of writing DOS 3.1
- Apple II History: DOS
- A2Central.com - Apple II news and downloads
- Everything2.com's DOS 3.1 Article