Extended memory

IBM PC Memory areas

Extended memory refers to a memory management scheme used in IBM PC compatible computers to access memory beyond the first 1 megabyte (MB) of address space. This concept became particularly relevant with the introduction of the Intel 80286 and later processors, which could address more than 1 MB of memory.

History

The original IBM PC and IBM PC/XT were based on the Intel 8088 processor, which could address up to 1 MB of memory. However, the top 384 KB of this address space was reserved for system use, leaving only 640 KB of conventional memory available for applications. This limitation became a significant bottleneck as software grew more complex.

With the introduction of the Intel 80286 processor in the IBM PC/AT, the addressable memory space was expanded to 16 MB. This additional memory was referred to as extended memory. However, the MS-DOS operating system, which was widely used at the time, could not directly utilize this extended memory due to its reliance on the real mode of the processor, which was limited to 1 MB of addressable space.

Extended Memory Specification (XMS)

To address this limitation, the Extended Memory Specification (XMS) was developed. XMS is a standard that allows DOS programs to access extended memory. It was first introduced by Microsoft and AST Research in 1988. The XMS standard defines a set of functions that can be used to allocate, release, and manage extended memory. These functions are provided by an XMS manager, such as HIMEM.SYS, which is a device driver included with MS-DOS.

Usage

Extended memory is primarily used by applications that require more memory than what is available in conventional memory. Examples include RAM disks, disk caches, and certain types of data-intensive applications. Extended memory can also be used to load parts of the operating system or drivers, freeing up conventional memory for applications.

Related Technologies

Extended memory is often confused with expanded memory, which is a different memory management scheme used to extend the capabilities of the original IBM PC architecture. Expanded memory uses a technique called bank switching to access additional memory, whereas extended memory relies on the processor's ability to address more memory directly.

See Also

• Conventional memory
• Expanded memory
• High memory area
• Memory management
• MS-DOS
• Intel 80286
• HIMEM.SYS