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The High Memory Area (HMA) is the first 64 KB of extended memory just above the 1 MB boundary. Compatible DOS versions could load part of DOS there, freeing some conventional memory for programs. The HMA is a specific address range—not a synonym for all extended memory, XMS, or upper memory blocks.
Where is the High Memory Area?
In the IBM PC memory map, the HMA sits immediately above the first megabyte of address space, at the start of extended memory. Microsoft defines it as the first 64 KB of extended memory. Its precise size and location make it a small, distinct region rather than a general-purpose large memory pool. Microsoft’s MS-DOS support article describes the HMA and the conditions for using it.
How did DOS use the HMA?
MS-DOS 5.0 and later could place DOS in the HMA when the system supported it and was configured to do so. Moving DOS out of conventional memory left more of that limited memory available to applications. Siemens’ S5-DOS/ST manual estimates that its described configuration releases approximately 40 Kbytes of conventional user memory; that is a manual’s estimate for that configuration, not a guaranteed result on every DOS system. Siemens S5-DOS/ST manual
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What is needed to use it?
Microsoft’s documentation identifies an 80286-or-higher processor, extended memory, and appropriate system configuration as prerequisites. On MS-DOS systems covered by its guidance, a 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 services DOS uses to request it. Microsoft’s HIMEM.SYS and HMA guidance
In the relevant MS-DOS configurations, check the result by running MEM. Microsoft’s documented confirmation is the message “MS-DOS is resident in the High Memory Area.” Driver names, commands, and configuration details can differ among DOS-compatible systems, so these steps should not be assumed to apply to every derivative.
How does the HMA differ from XMS, UMBs, and EMS?
| Term | What it refers to | Relationship to the HMA |
|---|---|---|
| HMA | The first 64 KB of extended memory immediately above the 1 MB boundary. | A specific memory region DOS could use to reduce its conventional-memory footprint. |
| XMS | A memory-management interface for accessing extended memory. | An XMS manager provides services through which software can request HMA use; XMS is not the HMA itself. |
| UMB | Upper memory blocks managed separately from the HMA. | UMB management is a distinct way of making certain memory areas available; it is not another name for the HMA. |
| EMS | A separate expanded-memory standard. | EMS is distinct from both the HMA and XMS. |
Microsoft’s MS-DOS memory-management overview treats HMA use, XMS, EMS, and UMB management as related but separate concepts.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why does the HMA matter today?
The HMA is mainly relevant when understanding or configuring historical DOS systems and software. The memory-saving technique depended on compatible hardware, an XMS manager, and the DOS configuration; it is not general advice for modern operating systems.
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