PC compatible memory diagram

Updated: 05/21/2018 by Computer Hope
PC compatible memory

Below is an illustration of the four sections of memory. The following specifications are conclusive for all IBM compatible computers.

XMS - Remainder of all Memory above 1 MB except the first 640 KB

XMS (external memory) is directly accessible memory starting at 1 MB requires himem.sys as an external memory manager (loaded in config.sys) on early MS-DOS computers.


The 80386 is limited to 16 MB and the 80286 is limited to 8 MB.

HMA - First 64 KB of memory Beyond 1 MB

HMA (high memory area) is the first 64 KB of XMS also requires himem.sys as an external memory manager (loaded in config.sys). Only one program at a time can access HMA commonly used for loading DOS "high," but also used for TSRs.

UMB - 384 KB

UMA (upper memory area) is located between 640 KB and 1 MB. Contains the ROM BIOS, device controller ROM, and video controller ROM and RAM.


Originally the only memory available to DOS, conventional memory contains devices drivers, conventional variables, the DOS command processor, and TSRs application programs. The conventional memory is static and never changes no matter how much RAM you have on the computer.

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