PC compatible memory diagram

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

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

External memory (XMS) - directly accessible memory starting at 1 MB requires HIMEM.SYS as an external memory manager (loaded in config.sys) 80386 Limited to 16 MB 80286 Limited to 8 MB.

HMA - First 64KB of memory Beyond 1MB

High memory area (HMA) - first 64 KB of XMS also requires HIMEM.SYS as an external memory manager (loaded in config.sys) Only 1 program at a time can access HMA Commonly used for loading DOS "high", but also used for TSRs.

UMB - 384KB

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

CONVENTIONAL (640KB)

Conventional memory - traditionally was the only memory available to DOS It contains devices drivers, conventional variables, the DOS command Processor, TSRs application programs, an data. This is static and will never change no matter how much ram you have in the computer.