IBM PC compatible
This was facilitated by IBM's choice of commodity hardware components, which were cheap, and by various manufacturers' ability to reverse-engineer the BIOS firmware using a "clean room design" technique.
Columbia Data Products built the first clone of the IBM personal computer, the MPC 1600[1] by a clean-room reverse-engineered implementation of its BIOS.
Other rival companies, Corona Data Systems, Eagle Computer, and the Handwell Corporation were threatened with legal action by IBM, who settled with them.
In a crucial concession, IBM's agreement allowed Microsoft to sell its own version, MS-DOS, for non-IBM computers.
IBM at first asked developers to avoid writing software that addressed the computer's hardware directly and to instead make standard calls to BIOS functions that carried out hardware-dependent operations.
At the same time, many manufacturers such as Tandy/RadioShack, Xerox, Hewlett-Packard, Digital Equipment Corporation, Sanyo, Texas Instruments, Tulip, Wang and Olivetti introduced personal computers that supported MS-DOS, but were not completely software- or hardware-compatible with the IBM PC.
[12] While admitting in 1984 that many PC DOS programs did not work on the computer, the company stated that "the most popular, sophisticated software on the market" was available, either immediately or "over the next six months".
To achieve such widespread use, and thus make the product viable economically, the OS had to operate across a range of machines from different vendors that had widely varying hardware.
Those customers who needed other applications than the starter programs could reasonably expect publishers to offer their products for a variety of computers, on suitable media for each.
Microsoft's competing OS was intended initially to operate on a similar varied spectrum of hardware, although all based on the 8086 processor.
However, as machines that were compatible with IBM hardware—thus supporting direct calls to the hardware—became widespread, it soon became clear that the OEM versions of MS-DOS were virtually identical, except perhaps for the provision of a few utility programs.
When PC Magazine requested samples from computer manufacturers that claimed to produce compatibles for an April 1984 review, 14 of 31 declined.
"[54] When a BYTE journalist asked to test Peachtext at the Spring 1983 COMDEX, Corona representatives "hemmed and hawed a bit, but they finally led me ... off in the corner where no one would see it should it fail".
[43] Zenith Data Systems was bolder, bragging that its Z-150 ran all applications people brought to test with at the 1984 West Coast Computer Faire.
[60][61][62] A September 1985 InfoWorld chart listed seven compatibles with 256 KB RAM, two disk drives, and monochrome monitors for $1,495 to $2,320, while the equivalent IBM PC cost $2,820.
[63] The Zenith Z-150[57] and inexpensive Leading Edge Model D are even compatible with IBM proprietary diagnostic software, unlike the Compaq Portable.
[72] After IBM announced the OS/2-oriented PS/2 line in early 1987, sales of existing DOS-compatible PC compatibles rose, in part because the proprietary operating system was not available.
However, as processor speed and memory width increased, the limits of the original XT/AT bus design were soon reached, particularly when driving graphics video cards.
Microsoft and Intel had become so important to the ongoing development of PC hardware that industry writers began using the word Wintel to refer to the combined hardware-software system.
To make things worse, IBM's choice of the Intel 8088 for the CPU introduced several limitations for developing software for the PC compatible platform.
A protected mode OS can also be written for the 80286, but DOS application compatibility was more difficult than expected, not only because most DOS applications accessed the hardware directly, bypassing BIOS routines intended to ensure compatibility, but also that most BIOS requests were made by the first 32 interrupt vectors, which were marked as "reserved" for protected mode processor exceptions by Intel.
Each manufacturer developed their own methods of accessing the screen memory, including different mode numberings and different bank switching arrangements.
Previously, the VGA standard had used planar video memory arrangements to the same effect, but this did not easily extend to the greater color depths and higher resolutions offered by SVGA adapters.
The Mac started out billed as "the computer for the rest of us", but high prices and closed architecture drove the Macintosh into an education and desktop publishing niche, from which it only emerged in the mid-2000s.
By the mid-1990s the Mac's market share had dwindled to around 5% and introducing a new rival operating system had become too risky a commercial venture.
During 2006 Intel began abandoning NetBurst with the release of their set of "Core" processors that represented a development of the earlier Pentium III.
[citation needed] This would include both the rapid growth of the smartphones (using Android or iOS) as an alternative to the personal computer; and the increasing prevalence of Linux and Unix-like operating systems in the server farms of large corporations such as Google or Amazon.
Until 2020 Macintosh computers shared the same system architecture as their Wintel counterparts and could boot Microsoft Windows without a DOS Compatibility Card.
The processor speed and memory capacity of modern PCs are many orders of magnitude greater than they were for the original IBM PC and yet backwards compatibility has been largely maintained – a 32-bit operating system released during the 2000s[update] can still operate many of the simpler programs written for the OS of the early 1980s without needing an emulator, though an emulator like DOSBox now has near-native functionality at full speed (and is necessary for certain games which may run too fast on modern processors).
Additionally, many modern PCs can still run DOS directly, although special options such as USB legacy mode and SATA-to-PATA emulation may need to be set in the BIOS setup utility.
