Let's take a look at the actual nuts and bolts that constituted each version of DOS.

DOS 1.x

PC DOS 1.0 was introduced along with the IBM PC on August 12, 1981, and supported only single-sided 5.25-inch drives. Floppy disks were formatted using 8 sectors per track (one side, 40 tracks) resulting in a capacity of only 160KB when formatted. There was no support for hard disks at all, which were generally quite rare for personal computers at the time. DOS was a text-based operating system, hence there was no graphical interface. Unlike CP/M, 8-bit ASCII characters were supported because the PC came with IBM 8-bit ASCII in ROM. This allowed the use of line drawing and other special characters to draw boxes and such on the screen.

For anyone who used the CP/M (Control Program for Microcomputers) operating system, DOS seemed pretty familiar. The "user interface," consisting of a prompt that indicated the logged drive, was very similar to CP/M. This similarity helped many people, including myself, make the transition from older CP/M systems to PC DOS a painless one.

When DOS 1.0 was introduced, it contained several commands and limited batch-processing facilities. About half of the commands were internally part of DOS, while the disk utilities were external assembly language programs. The commands in DOS at that time were:

DOS also came with interpretive BASIC, a text editor called EDLIN, a LINKer, a DEBUGger, and a series of BASIC programs. All of these programs were sold on one single-sided floppy diskette.

People immediately noticed that the IBM-PC and PC DOS could handle only single-sided diskettes. This was seen as a disadvantage at the time, and was an indication of the conservative nature of IBM. But everybody knew that a version supporting double-sided drives was coming. In reality very few PCs ended up being sold with DOS 1.0 as the PC was in pretty short supply for the first few months of its existence. It was really not until the early part of 1982 that just anybody could get one, and by then DOS 1.1 was available.

PC DOS 1.1 was introduced in May 1982. It was called 1.25 internally by Microsoft, and was later released by a number of OEMs under a variety of names and version numbers. DOS 1.1 supported double-sided drives with 8 sectors per track, resulting in a formatted capacity of 320KB. PC DOS 1.1 used 12KB of RAM, and was the last DOS written by Tim Paterson, based on the original Seattle Computer Products 86-DOS.

The major difference between DOS 1.0 and 1.1 is that the latter can operate double-sided drives. All of the external, and most of the internal commands, were rewritten to accommodate double-sided drives. There was still no support for hard disks explicitly, but several manufacturers supplied kits with software patches to DOS that allowed a large disk to be used. Actually many different companies offering expansion products for the PC emerged at this time, and many of these products came with software that patched DOS directly. I remember being concerned about this because the compatibility problems were enormous. I was getting the feeling that PC DOS was to go the route of Apple DOS at the time in that it would become so patched and hacked up by the aftermarket as to cease to be any kind of standard. Anyone who remembers the old Apple DOS for the Apple II series can tell you the problems with this methodology. Fortunately IBM and Microsoft came to the rescue with a new version (DOS 2.0) that was so changed and improved that it brought all of this patching to an abrupt halt. There was no need to patch it if it could already do what you wanted, or be easily adapted with extensions rather than patches.

DOS 2.x

PC DOS 2.0 was introduced on March 8, 1983 (along with the IBM PC XT), and was virtually a complete rewrite over the previous versions. DOS 2.0 added many new features and functions (mostly derived from UNIX), including a tree-structured (hierarchical) file system, support for hard disk drives up to 16.76MB (15.98 MiB) using FAT12, 5.25-inch 9-sector per track floppy formats resulting in 180KB/360KB for single/double sided drives, I/O redirection and piping, and background printing. Many new commands were added as well. PC DOS 2.0 used 24KB of RAM.

In achieving its objectives Microsoft tripled the size of DOS, and added 17 new commands. The major feature of DOS 2.0 was support for the use of a hard disk and a hierarchical file structure in order to support the new IBM PC-XT, which included a 10MB hard disk as a standard feature.

DOS 2.0 also increased the storage capacity of single-sided floppies from 160KB to 180KB, and double-sided floppies from 320KB to 360KB. This was achieved by increasing the number of sectors on each track of the disk from 8 sectors to 9. DOS 2.0 could also read and write any of the older formats, ensuring backwards compatibility.

Another major feature of DOS 2.0 was support for device drivers. This meant that there was a provision for new software routines that supported various hardware and software to be installed into DOS directly. I like to think of this as software "slots" for adding components to DOS, without patching DOS directly.

IBM introduced PC DOS 2.1 on November 1, 1983 (along with the IBM PCjr). It added no new commands or functions, but fixed bugs and altered timing parameters for the half-height floppy drives used in the PCjr and IBM Portable PC. PC DOS 2.1 used 24KB of RAM, same as the previous version. For the most part, DOS 2.1 was considered a maintenance release from 2.0, with no new functionality added.

DOS 3.x

PC DOS 3.0 and 3.1 were both introduced on August 14, 1984 (along with the 286 processor based IBM PC AT). Although both were introduced at the same time, only version 3.0 was available immediately while version 3.1 became available a few months later, in October 1984.

DOS 3.0 was basically an unfinished version of 3.1, designed to get the necessary support in place for the PC AT. DOS 3.0 added support for a virtual disk (VDISK) using memory greater than 1MB, and added FAT16 support for hard drives supporting a single partition of up to 32 MiB (33.55 MB). DOS 2.x and earlier only supported FAT12, even on hard disks. Support was also added for high-density 1.2MB 5.25-inch floppy drives using 15 sectors per track. PC DOS 3.0 used 36KB of RAM.

PC DOS 3.1 was introduced on August 14, 1984 (same as 3.0), but wasn't made available to the public until October 1984. Because 3.0 was really an unfinished 3.1, IBM offered the first DOS upgrade (from 3.0 to 3.1) for $30. International keyboard support was added to 3.1, but the main additions were network printer and file redirection as well as file sharing support. This was designed to support the new IBM PC Network hardware and software that was also released at the same time. PC DOS 3.1 used 36KB of RAM.

PC DOS 3.2 was released on March 18, 1986 (along with the Token Ring Network interconnect program). DOS 3.2 supported the Token Ring Network, and added support for 3.5-inch double-density (720KB) floppy drives. The first 3.5-inch drives for PCs were coming in the PC Convertible, also the first IBM laptop computer (which was introduced less than a month later on April 2, 1986). DOS 3.2 also added the XCOPY and REPLACE commands. PC DOS 3.2 was the first DOS available on both 3.5-inch and 5.25-inch diskettes, and used 44KB of RAM.

After the release of PC DOS 3.2, Microsoft made the first non-specific OEM (generic) packaged version of MS-DOS available. The packaged version of MS-DOS 3.2 was designed for smaller system builders who did not have the ability to produce a finished product from the raw code supplied with the large-scale OEM licenses. As such, the OEM-packaged MS-DOS 3.2 was essentially the first release of MS-DOS available in Microsoft packaging from MS directly. This was also the beginning of Microsoft's OEM program for small system builders, and OEM versions of its software were only sold to OEMs, who then could only resell the software with a system.

Both IBM and Microsoft issued a minor release update to DOS 3.2 called 3.21. This new version was available free of charge if you had 3.2, and corrected problems with BASIC and the keyboard of the IBM Convertible PC, as well as some other minor bugs.

PC DOS 3.3 was released on April 2, 1987, along with IBM's line of PS/2 systems, more than a year after DOS 3.2. DOS 3.3 introduced the extended partition, which could internally support up to 23 sub-partitions (logical drives) of up to 32 MiB (33.55 MB) each. Combined with the primary partition on a disk, this allowed for a total of 24 partitions of up to 32 MiB each, which would be seen by the operating system as logical drives C through Z. Support was also added for 1.44MB high-density 3.5-inch floppy drives.

DOS 3.3 also added support for nested batch file commands (using the new CALL command), and the DATE and TIME commands would finally update the CMOS RTC chip directly. Foreign language support was enhanced with support for Code Pages (alternate international character sets), and the FASTOPEN and APPEND commands were added as well. 128KB of RAM was advertised as the minimum memory required for DOS 3.3.

DOS 4.x

In order to take full advantage of FAT16 and allow for much larger drives and partition sizes, Microsoft collaborated with Compaq, and Compaq introduced Compaq DOS 3.31 in November 1987. Compaq DOS 3.31 was the first DOS to use 32-bit sector addressing internally and in the BPB (BIOS Parameter Block), which when combined with the 16-bit File Allocation Table (FAT16) file system, allowed for a single partition to be supported up to 2GiB in size.

The rest of the PC world followed suit on July 19, 1988, when IBM and Microsoft released PC/MS-DOS 4.0. The use of 32-bit sector addressing meant that FAT16 could now handle partition sizes up to 2 GiB (2.15GB) using 64 sectors per cluster. DOS 4.0 also added support for Lotus, Intel, Microsoft (LIM) Expanded Memory Support (EMS), as well as an optional graphical user interface shell. The MEM command was added. 256KB of RAM was advertised as the minimum memory required for DOS 4.0.

After the initial 4.0 release, IBM followed up with six sets of CSDs (Corrective Service Diskettes), each of them reporting 4.01 as the version. MS followed the IBM CSDs with some (but not all) of the 4.01 corrections and enhancements in the following MS-DOS 4.01 release.

DOS 5.x

After switching initial DOS development back to Microsoft, MS-DOS 5.0 was introduced by Microsoft on June 6, 1991, and PC DOS 5.0 was introduced by IBM on June 11, 1991.

MS-DOS 5.0 was a significant release in that for the first time Microsoft was feeling some competitive heat from Digital Research Corporation, which sold an alternative operating system called DR-DOS. Microsoft was compelled to add several additional useful features to MS-DOS 5.0, including a full-screen editor, significantly improved memory management, and an improved BASIC language interpreter.

By this point, too, it could be assumed that most PCs had an Intel 80286 or better processor, so DOS could take advantage of this processor's ability to "map" extended memory into unused parts of the upper 384KB address range visible to DOS. These memory segments were called upper memory blocks. In addition, programmers had by now figured out how to use a programming trick to gain access to the first 64KB of extended memory (called the high memory block) from real mode without using the mapping features. These two techniques let MS-DOS locate parts of itself, device drivers and "terminate and stay resident" accessory programs outside the lower 640KB address range, leaving more of the precious lower 640KB memory range available to application programs. MS-DOS 5.0 thus introduced the DOS=HIGH,UMB configuration option and the loadhigh command.

Many commands were added, including: HELP (online help), DOSKEY, SMARTDRV (disk cache), EDIT (full-screen editor), EXPAND (file extractor), FC (file compare), LOADHIGH/DEVICEHIGH (load resident programs or drivers into UMBsUpper Memory Blocks), MIRROR (backup FAT and directory structures for later UNDELETE or UNFORMAT), QBASIC (a version of BASIC that works on all IBM and IBM-compatible hardware, without the requirement of an IBM ROM as with BASICA), RAMDRIVE (replaces VDISK), SETVER (report different DOS versions to "fool" older programs that check versions before running), UNDELETE (undelete files), and UNFORMAT (unformat disks/partitions). A full-screen editor called EDIT was added to DOS 5.0 in addition to the EDLIN line editor.

DOS 5.0 also added support for 2.88MB ED 3.5-inch floppies, and the FORMAT command used the media sense capability present in most 3.5-inch drives to automatically format 720KB, 1.44MB, or 2.88MB media to the correct capacity. 512KB of RAM was listed as the minimum memory requirement for DOS 5.0.

On November 11, 1991, Microsoft released DOS 5.0a to address data-corrupting bugs in the original DOS 5 CHKDSK and UNDELETE commands. IBM on the other hand was much more diligent in finding and reporting bugs, and released five sets of CSDs (Corrective Service Diskettes) and two sets of IFDs (Interim Fix Diskettes) between July 1991 and September 1992, fixing not only the CHKDSK and UNDELETE problems, but more than 50 other individual problems in DOS 5.

PC DOS 5.00.1 was introduced on April 28, 1992, and is the first IBM version specifically supported on non-IBM hardware (although previous versions ran on all PC-compatible systems). One change was that QBASIC was now included, which no longer depended on the ROM BASIC as with previous DOS versions.

Version 5.00.1 also added refreshed code that included the latest CSD fixes, as well as a new SETUP module that installed over all IBM and IBM-compatible PC/MS-DOS Versions 2.1 and higher, and even across a LAN.

PC DOS 5.02 was introduced on October 20, 1992. It incorporated all of the previous DOS 5 fixes, as well as added APM (Advanced Power Management) support for laptops via the new POWER command. Also added were the INTERLNK and INTERSVR commands supporting file transfer between systems over a parallel or serial cable. Version 5.02 also added support for electrically ejectable/lockable drives and ISO screen fonts. IBM also released various retail bundles with 386Max & Stacker using the 5.00.1 package.

MS-DOS 6.x

Microsoft introduced MS-DOS 6.0 (codename Astro) on March 30, 1993. Again, in response to competitive pressure, Microsoft copied disk data compression technology created by Stac Corporation as the new DoubleSpace feature.

Microsoft introduced MS-DOS 6.2 (codenamed Elroy) in November 1993. It included bug fixes to the DoubleSpace (Stacker) disk compression as well as upgrades to the SCANDISK, DISKCOPY, and Smartdrive (disk cache) programs.

Later it was ruled that Microsoft illegally used the Stac Electronics Stacker disk compression, which they were forced to remove in a 6.21 release. Finally in 1994 Microsoft released MS-DOS 6.22, with disk compression added back in, but using different (noninfringing code) and renamed as DriveSpace. 6.22 was the last standalone version of MS-DOS released by Microsoft.

PC DOS 6.x

IBM introduced PC DOS 6.1 on June 19, 1993 as its version of MS-DOS 6.0, but with some improvements. IBM skipped the 6.0 release number to avoid confusion with Microsoft's 6.0 release, and to indicate to the public that IBM's version included more.

PC DOS Version 6.1 included a library of integrated DOS utilities from other software vendors and IBM Research. The utilities included the following:

• Disk compression (licensed from Stac Electronics)

• Antivirus

• Full-screen backup

• New full-screen editor (called "E")

• Program scheduler

DOS 6.1 also included support for Pen-based systems as well as PCMCIA (PC Card) slots in laptop systems, plus an automatic memory configuration program called RAMBoost.

IBM then released PC DOS 6.3 on April 27, 1994, and included the MSCDEX program for CD-ROM support, plus many minor updates to the various utility programs.

IBM continued to improve DOS, and released PC DOS 7.0. This was followed by PC DOS 2000, which was really 7.0 with updates added to fix certain Y2K issues. PC DOS 2000 was the last standalone version of DOS ever released, and can still be purchased from IBM today.

Windows 9x/Me DOS

Windows 95, 98, and Me all include subsets of MS-DOS as their base, and those versions offer enhancements over the standalone releases from both Microsoft and IBM.

The DOS included with Windows 95 is internally coded as MS-DOS 7.0, and includes support for long filenames. MS-DOS 7.1 was included with Windows 95B (OEM Service Release 2), Windows 98, and Windows 98SE (Second Edition), which added support for FAT32. Windows Me included MS-DOS 8.0, which had some bug fixes for supporting larger drives.