Extended memory

Extended memory is located above 1 MiB.

In computing, extended memory refers to memory above the first megabyte of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems. DOS programs, running in real mode or virtual x86 mode, cannot directly access this memory, but are able to do so through an application programming interface called the eXtended Memory Specification (XMS). This API is implemented by a driver (such as HIMEM.SYS) or the operating system, which takes care of memory management and copying memory between conventional and extended memory, by temporarily switching the processor into protected mode. In this context the term "extended memory" may refer to either the whole of the extended memory or only the portion available through this API.

Extended memory should not be confused with expanded memory, an earlier method for expanding the IBM PC's memory capacity beyond 640 KiB using an expansion card with bank switched memory modules. Because of the available support for expanded memory in popular applications, device drivers were developed that emulated expanded memory using extended memory. Later two additional methods where developed allowing direct access to a small portion of extended memory from real mode. These memory areas are referred to as the high memory area and the upper memory blocks.

Overview

On x86-based PCs, extended memory is only available with an Intel 80286 processor or higher. Only these chips can address more than 1 MB of RAM. The earlier 8086/8088 processors can make use of more than 1 MB of RAM, if one employs special hardware to make selectable parts of it appear at addresses below 1 MB (paging).

On a 286 or better PC equipped with more than 640 KB of RAM, the additional memory would generally be re-mapped above the 1 MB boundary, since the IBM PC architecture mandates a 384 KB "hole" in memory between the 640 KB and 1 MB boundaries. This way all of the additional memory would be available to programs running in Protected mode. Even without such remapping, machines with more than 1 MB of RAM would have access to memory above 1 MB.

Extended memory is not directly accessible in real mode (except for the a small portion called the high memory area); only applications executing in protected mode can use extended memory directly. In this case, the extended memory is provided by a supervising protected-mode operating system such as Microsoft Windows. The processor makes this memory available through the Global Descriptor Table and one or more Local Descriptor Tables (LDTs). The memory is "protected" in the sense that memory segments assigned a local descriptor cannot be accessed by another program because that program uses a different LDT, and memory segments assigned a global descriptor can have their access rights restricted, causing a hardware trap (typically a General Protection Fault) on violation. This prevents programs running in protected mode from interfering with each other's memory.

A protected-mode operating system such as Microsoft Windows can also run real-mode programs and provide expanded memory to them. The DOS Protected Mode Interface is Microsoft's prescribed method for an MS-DOS program to access extended memory under a multitasking environment.

eXtended Memory Specification (XMS)

The eXtended Memory Specification or XMS is the specification describing the use of IBM PC extended memory in real mode for storing data (but not for running executable code in it). Memory is made available by extended memory manager (XMM) software such as HIMEM.SYS. The XMM functions are accessible through interrupt 2Fh.

XMS version 2.0 allow for up to 64 MiB of memory, with XMS version 3.0 this increased to 4 GiB. To differentiate between the possibly different amount of memory that might be available to applications, depending on which version of the specification they were developed to, the latter may be referred to as super extended memory or SXMS.

The extended memory manager is also responsible for managing allocations in the high memory area and the upper memory blocks. In practice the upper memory blocks will be provided by the expanded memory manager, after which DOS will try to allocate them all and manage them itself.

See also

• Conventional memory
• Expanded memory (EMS)
• High Memory Area (HMA)
• Upper Memory Area (UMA)
• Unreal mode

References

Specifications

• Microsoft, Lotus, Intel, and AST Research (July 19, 1988). eXtended Memory Specification (XMS), ver 2.0.
• Microsoft, Lotus, Intel, and AST Research (January 1991). eXtended Memory Specification (XMS), ver 3.0.

Microsoft Knowledge Base

• Microsoft. KB37242: A General Tutorial on the Various Forms of Memory
• Microsoft. KB95555: Overview of Memory-Management Functionality in MS-DOS
• Microsoft. KB121008: Microsoft Diagnostic Utility: Definition of SXMS

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.