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| High Memory Area | |
|---|---|
| Name | High Memory Area |
| Type | memory region |
| Introduced | 1980s |
| Predecessor | conventional memory |
| Successor | extended memory |
| Used by | MS-DOS, DR-DOS, Windows 3.1, ROM BIOS |
| Architecture | x86 |
High Memory Area The High Memory Area (HMA) is a specific region of addressable x86 memory located just above the first megabyte boundary in real mode. It enabled early personal computer operating systems and utilities such as MS-DOS, DR-DOS, and PC DOS to access additional RAM without switching to protected mode, influencing designs by vendors including IBM, Compaq, and Microsoft. The HMA played a notable role during transitions involving Intel 80286 and Intel 80386 processors, affecting bootstrapping, memory managers, and compatibility strategies used by vendors like Novell and Phoenix Technologies.
The HMA consists of the first 64 KB minus 16 bytes of memory above the 1 MB boundary, reachable in real mode by exploiting the A20 line addressing behavior. This region became significant for operating systems such as MS-DOS and extensions like DOS4GW and device drivers from firms like Microsoft Windows Team and Seagate Technology. Hardware implementations by manufacturers including Intel Corporation and firmware from American Megatrends controlled access to the A20 address line, affecting how software from entities like Lotus Development Corporation and Borland used the HMA for resident system code.
After the release of the Intel 80286 CPU, vendors sought ways to extend usable memory while maintaining compatibility with real-mode software from suppliers such as IBM PC Group and software houses like Microsoft. The A20 address line gating behavior, originally a quirk tied to the IBM PC/AT design, was popularized by makers including Compaq Computer Corporation and was formalized into software interfaces used by Microsoft MS-DOS and DR-DOS memory managers. Third-party memory managers and utilities from companies like Qualcomm and Quarterdeck Office Systems exploited the HMA to load parts of COMMAND.COM and TSRs, mirroring strategies used by Windows 3.0 and later by Windows 95 boot processes.
Access to the HMA depends on enabling the A20 address line, which lets real-mode addressing produce physical addresses above 1,048,576 bytes (1 MB). The mechanism interacts with hardware components designed by firms such as Intel Corporation and National Semiconductor, and with BIOS implementations from vendors like Phoenix Technologies and American Megatrends. Software interfaces to control A20 were provided by MS-DOS and later standardized by utilities and drivers from vendors including Microsoft and DR-DOS Library. The HMA spans addresses from 0x100000 to 0x10FFEF (64 KB − 16 bytes) in typical implementations and required careful handling of wrap-around behavior that affected software from Microsoft Visual Basic runtime libraries and compilers such as those by Borland International.
Operating systems and applications used the HMA to relocate parts of the kernel, resident utilities, and startup components. For example, MS-DOS variants moved portions of COMMAND.COM and memory-resident programs into the HMA to reclaim conventional memory beneath 640 KB for applications like Lotus 1-2-3, WordPerfect Corporation word processors, and games by Sierra On-Line. Compatibility techniques were necessary due to differences among BIOS implementations by IBM, Compaq, and clone makers; memory managers such as EMM386 and HIMEM.SYS provided consistent access and were included with DR-DOS and MS-DOS distributions. Third-party utilities from Norton Utilities and PC Tools also used HMA to optimize resource usage on systems from Tandy Corporation and Gateway 2000.
Memory managers and DOS extenders handled HMA allocation via interfaces that cooperated with XMS and EMS services, produced by organizations including Microsoft and Lotus Development Corporation integrations. The A20 gate could be toggled through keyboard controller tricks originating in IBM PC/AT designs or via chipset-controlled ports used by systems from Intel and AMD. Software such as EMM386 on MS-DOS or memory control in DR-DOS invoked BIOS hooks and driver routines to enable HMA use, affecting interaction with protected mode features present in Intel 80386 and later processors. Conflicts with device drivers and BIOS shadowing required coordination among companies like AMI and Award Software.
BIOS vendors and operating system developers implemented support to enable and manage HMA for end-users and OEMs. MS-DOS shipped HIMEM.SYS to control A20 and grant HMA access; DR-DOS implemented similar functionality through its kernel and memory subsystems. Firmware from Phoenix Technologies and American Megatrends provided A20 control methods via POST and interrupt services used by system integrators including Dell and Hewlett-Packard. Later operating systems such as Windows 3.1 and early Windows 95 boot loaders interacted with HMA during initialization before switching to protected mode.
Although modern 32-bit and 64-bit systems from Intel Corporation and AMD no longer rely on HMA, its legacy influenced memory management, BIOS interfaces, and compatibility decisions by corporations such as Microsoft and IBM. Concepts developed around HMA and the A20 line informed design choices in system firmware by Phoenix Technologies and influenced documentation from standards bodies like IEEE and consortiums involving MPEG LA (in other domains). Collectors and preservationists of vintage computing from organizations such as Computer History Museum study HMA behavior to understand software by Microsoft, Borland, and Lotus and hardware from Compaq and IBM PC Company.
Category:Computer memory