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 1 MB except the first 640 KB
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 64 KB of memory Beyond 1 MB
High memory area (HMA) - 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
CONVENTIONAL (640 KB)
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. The conventional memory is static and never changes no matter how much ram you have in the computer.