The High Memory Area (HMA) is the first 64 KB of extended memory immediately above the 1 MB boundary. Compatible DOS systems could load DOS into this small region to leave more of conventional memory available for programs. It is not another name for all extended memory, XMS, or upper memory blocks.
Where is the High Memory Area?
The HMA sits just above the 1 MB boundary, at the beginning of extended memory. Microsoft defines it as the first 64 KB of extended memory; archived HIMEM.SYS documentation likewise describes a 64 KB area immediately above 1 MB that programs can use for code and data through XMS support. Microsoft’s HMA guidance and the archived HIMEM.SYS documentation describe the region in the context of DOS-era memory management.
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How did DOS use the HMA?
MS-DOS 5.0 and later could be configured to place part of DOS in the HMA. This could reduce DOS’s use of conventional memory—the area traditionally available to DOS programs below the 1 MB boundary—and leave more room for applications. Siemens’ S5-DOS/ST manual estimates that its described configuration released approximately 40 Kbytes of conventional user memory; that is a documented estimate for that system, not a guaranteed result for every DOS version or computer. Microsoft’s guidance explains the DOS configuration, while the Siemens manual gives its system-specific estimate.
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HMA use depends on the computer and its DOS configuration. Microsoft identifies an 80286-or-higher processor, extended memory, and correct configuration as prerequisites. For the MS-DOS configurations covered by its archived guidance, the DOS=HIGH line in CONFIG.SYS requests that DOS use the HMA. An XMS memory manager, such as HIMEM.SYS, must be available to provide the memory-management services DOS needs. The driver must be available before software that calls on those services. These names and steps describe the documented MS-DOS setup, not every DOS-compatible operating system.
- Make the XMS manager available: Configure
HIMEM.SYSor another compatible XMS manager inCONFIG.SYS. - Request HMA use: Add
DOS=HIGHtoCONFIG.SYSfor a compatible MS-DOS version and system. - Restart and check: Run
MEM. In the MS-DOS configurations addressed by Microsoft’s guidance, the message “MS-DOS is resident in the High Memory Area” confirms that DOS loaded there. See Microsoft’s archived instructions for the scope of those steps.
How is the HMA different from XMS, UMBs, and EMS?
| Term | What it refers to | How it relates to the HMA |
|---|---|---|
| HMA | The first 64 KB of extended memory just above 1 MB. | A specific memory region DOS could use. |
| XMS | A memory-management specification and interface for working with extended memory. | An XMS manager provides services used to request the HMA; XMS is not the region itself. |
| UMB | Upper memory blocks managed separately from the HMA. | Another way to manage memory for DOS, not another name for the HMA. |
| EMS | A distinct expanded-memory standard. | Related to DOS memory management, but not the HMA. |
Microsoft treats HMA use, XMS, EMS, and UMB management as related but distinct subjects in its MS-DOS memory-management overview.
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What the HMA does not mean
- It does not mean all memory above 1 MB; it is a specific 64 KB region at the start of extended memory.
- It is not a large, general-purpose memory pool for DOS programs.
- It is not synonymous with XMS, UMBs, or EMS. Those terms describe different interfaces, regions, or memory standards.
- It is not a universal DOS setting: processor support, available extended memory, DOS version, and configuration determine whether the documented method works.
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