The 640K Barrier is an aspect of the IBM PC and compatible personal computers when running under MS-DOS or DR-DOS, which could only address up to 640KB of memory for running applications. This is due to limitations of the original IBM PC, which used the Intel 8088 CPU. IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5. ... Microsofts disk operating system, MS-DOS, was Microsofts implementation of DOS, which was the first popular operating system for the IBM PC, and until recently, was widely used on the PC compatible platform. ... DR-DOS is an DOS-family-compatible operating system for IBM PC-compatible personal computers, originally developed by Gary Kildalls Digital Research and derived from CP/M-86. ... The abbreviation KB can refer to: UK legal citation: denotes a case in the Kings Bench Division of the High Court, reported in the official Law Reports. ... IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5. ... An Intel 8088 Microprocessor The Intel 8088 is an Intel microprocessor based on the 8086, with 16-bit registers and an 8-bit external data bus. ...

The term refers to the size of the PC's Conventional memory. The 8088 and 8086 were only capable of addressing 1MB of memory, of which IBM reserved the upper 384K for system purposes - the Upper Memory Area. This left only the lower 640K for user programs and data. At the time of the PC's release in 1981, 640K would have seemed more than plenty for the typical user. Keep in mind that the most popular home computer of the time, the Commodore 64, had only 64K, of which only 38K were available to the user. Conventional Memory is the first 640 kilobytes of an IBM PCs memory. ... The 8086 is a 16-bit microprocessor chip designed by Intel in 1978, which gave rise to the x86 architecture. ... The Upper Memory Area (UMA) is a feature of the design of IBM PC-compatible x86 computers. ... 1981 is a common year starting on Thursday of the Gregorian calendar. ... The Commodore 64 (C64, CBM 64) was a popular home computer of the 1980s. ...

To maintain compatibility with older software, the 640K barrier remained part of the PC design even after the 8088 had been replaced with the Intel 286 processor, which could address up to 16MB of memory. Indeed it is still present in machines based on the 80386 and newer processors, even today, if they are running DOS. The Intel 80286 is an x86-family 16-bit microprocessor that was introduced by Intel on February 1, 1982. ... The Intel 80386 is a microprocessor which was used as the central processing unit (CPU) of many personal computers from 1986 until 1994 and later. ...

Note that it is neither a limitation of the PC compatible nor of MS-DOS itself, but only the combination of the two. Non-DOS operating systems such as Xenix and Netware did not suffer from the restriction, nor did MS-DOS running on Non-PC compatible X86 computers. One of the first PCs from IBM - the IBM PC model 5150. ... Microsofts disk operating system, MS-DOS, was Microsofts implementation of DOS, which was the first popular operating system for the IBM PC, and until recently, was widely used on the PC compatible platform. ... Xenix was a Unix-like computer operating system derived from the UNIX system developed by Microsoft in the 1980s. ... NetWare is a network operating system and the set of network protocols it uses to talk to client machines on the network. ... For much of the time that Microsoft operating systems have dominated business computing, it has been a given that all computers based on Intel x86 family CPUs were IBM PC Compatibles. ...

The barrier was only overcome with the arrival of DOS extenders, which allowed DOS applications to run in Extended Memory, but these were not very widely used. The first PC operating system to integrate such technology was Windows 2.0 in the Windows/386 edition of Windows 2.1, released in 1988, a year after Windows 2.0 itself. Windows applications did not suffer from the 640K barrier. DOS extender is a rather curious name invented in the 1980s for programs started from MS-DOS, but which actually need to run in protected mode. ... Extended memory refers to memory above the first megabyte of address space in an IBM PC with an 80286 or later processor. ... In computing, an operating system (OS) is the system software responsible for the direct control and management of hardware and basic system operations. ... Microsoft Windows 2. ...

Prior to DOS extenders, if a user installed additional memory and wished to use it under DOS, they would first have to install and configure drivers to create either EMS (Expanded Memory Specification) or XMS (Extended Memory Specification) RAM, and applications could only store data in the additional space, not executable code. EMS may stand for: Eastern Mountain Sports, an outdoor retailer The Edinburgh Mathematical Society Electromagnetic Spectrum Electronic Manual Special, a special edition Saab 99 automobile Electronic Music Studios (London) Ltd Element management system (telecommunications) Emergency medical service EMS Group or Ems-Chemie Energy Management System Enhanced Messaging Service Enterprise Messaging... XMS or Extended Memory Specification is the specification describing the use of IBM PC extended memory in real mode for storing data (but not executable code). ...

See also

• Unreal mode
• Real mode
• Protected mode
• Conventional memory
• Extended Memory Specification
• Expanded Memory Specification
• High Memory Area