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HMA

(High Memory Area) In PCs, the first 64K of extended memory from 1024K to 1088K, which can be accessed by DOS. It is managed by the HIMEM.SYS driver. It was discovered by accident that this area could be used by DOS, even though it was beyond the traditional one-megabyte barrier. See PC memory.



PC memory

(1) The main memory in a PC, which is made up of dynamic RAM chips. Definition #2 outlines the structure of memory in a PC, much of which was of importance to users back in the 1980s, but has long since been an issue. See memory module, SDRAM and dynamic RAM.

(2) The original PC design was constrained to one megabyte of memory. In addition, certain parts of the operating system were placed into fixed locations in the upper part of memory without any method for cooperatively storing additional drivers and programs. This design gave rise to the most confusing platform in history. Windows 3.0 became very popular because it handled all of the memory types much better than DOS.

Following are the different types of memory in a PC. In other computers, there is just plain memory. In mainframes and supercomputers, there are also large, auxiliary memory banks that function as caches between disk and RAM. See PC memory map.


 Memory Type              Location

 Conventional             First 640K

 UMA (Upper Memory Area)  Next 384K

 HMA (High Memory Area)   Next 64K

 Extended Memory          From 1MB up

 EMS (Expanded Memory)    Early spec
                          (memory above
                           1MB bank
                           switched
                           into UMA)





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