China DOS Union

Memory classification:

1---------Conventional memory Conventional
Every computer has conventional memory, with a capacity of 256-640KB. DOS itself manages conventional
memory on its own, so there is no need to load programs like HIMEM.SYS/EMM386.EXE to manage it. Programs
under DOS all use conventional memory.
2---------UMA UMB
A PC can have at most 384KB of UMA. The UMA area comes right after conventional memory and is generally reserved for system hardware
drivers. The space remaining in UMA after being used by system hardware drivers is called UMB. On 386/486
systems, UMB can be used to store device drivers or TSRs. Before using UMB,
you need to set DOS=UMB in CONFIG.SYS. UMB is shown as upper in the computer's memory display.
3---------XMS extended memory （Extended）
On 2/3/486 systems, memory above 1M generally belongs to extended memory. The operation of WINDOWS
requires extended memory. Before using extended memory, you need to load HIMEM.SYS.
In the WINDOWS environment, if there is insufficient memory, most of the time it is caused by insufficient extended memory.
Under the DOS environment, if there is insufficient memory, most of the time it is because there is not enough conventional memory.
4--------HMA
The first 64KB of extended memory is also called HMA. If you use DOS=HIGH, then most of the MSDOS operating system's
own programs can be moved to HMA, so as to free up more available conventional memory space for the user to
use. If DOS=HIGH is not set in advance, the MSDOS operating system itself will be stored entirely in conventional
memory and occupy a certain amount of storage space.
5--------EMS
Expanded memory. Programs developed early on for MSDOS not only used conventional memory, but might also use
expanded memory. For various reasons, most computers at present no longer use expanded memory to increase memory
capacity. DOS provides EMM386.EXE, which can simulate extended memory as expanded memory.

"Expanded memory" and "extended memory" got reversed. Usually XMS is what is called "extended memory"。。

Let me add a few more kinds of DOS memory:

DPMI memory: DOS Protected Mode Interface memory;
DPMS memory: DOS Protected Mode Services memory;
VCPI memory: Virtual Control Program Interface memory;

Thanks, collected.