The memory system is divided into three main parts: TPA, system area, and XMS. The first 1MB of memory is called real or conventional memory. It is divided into TPA (640KB) and system area (384KB). TPA holds the operating system, active programs, and inactive programs. The system area is smaller and contains programs like video control programs in ROM/flash. Memory above 1MB is available for use by expanded memory systems.

2.  The memory system is divided into three main parts.
They are:-
1. TPA (Transient Program Area)
2. System Area
3. XMS (Extended Memory System)

4. The first 1 M byte of memory is called the
real memory or conventional memory. It is called so
because each microprocessor is designed to function in
this area by using the real mode of operation.

5.  The conventional memory of 1 M byte is comprised of
two portions.
They are:-
TPA – 640k
System Area – 384k

6.  The Transient program area holds the dos operating
system and other programs that control the computer
system.
 The TPA also stores any currently active or inactive
DOS application programs.
 The size of TPA is 640KB.
 DOS requires a portion of TPA to function.

7.  Both PCDOS and MSDOS are compatible with TPA.
Windows and OS/2 are other two operating systems that
are compatible with DOS programs and executes it.

9.  Interrupt vectors access various features of the
DOS, BIOS and applications. System BIOS is a collection
of programs stored in ROM or Flash Memory.

10.  Contains the transient data used by the program to access
I/O devices and internal features of the computer system.
These are stored in the TPA so they can be changed as the
system operates.

11.  It is a program that loads into TPA whenever the MSDOS
or PCDOS system is started. This program allow DOS to
use keyboard, printer, video display and other I/O devices
often found in computer system.

12.  The size of driver area and number of drivers changes
from one system to another. Drivers are programs that
control installable I/O devices. Drivers are files normally
they have extension .SYS and sometimes it will be .EXE.
REGEDIT is a program that contains information about
system and the drivers.

13.  Controls the operation of the computer when keyboard is
operated in DOS mode.
 Incase if it is erased the computer cannot be used with
keyboard in DOS mode.

14.  This area holds DOS application programs as they are
executed. This application includes word processor and
spreadsheets. Also it holds TSR (terminate and stay
resident) programs that remain in memory in an inactive
state until activated by interrupt.

15.  It occupies 2 area of memory. One area of 16 bytes of
memory at the top and another large area of memory located
near the bottom of TPA. Size depends on version of DOS
installed. If DOS is installed in High Memory, most of the
TPA is free to hold Application program.

16.  It is smaller than TPA and it is of 384 kb in size. It
contains program on either a read only memory or flash
memory and RAM for data storage.

18.  It is the first part in system area, it contains video control
programs on ROM or flash memory. The size and amount
of memory used depends on the type of video display
adapter.
 The Video Display Ram is Classified into 3 parts:-
 Video RAM (Graphics)
 Video RAM (Text)
 Video BIOS ROM

19.  Video RAM (Graphics ) used to store graphical or bit-
mapped data.
 Video RAM (Text area) stores Text.
 Video BIOS ROM contains program that control the DOS
Video display.

20.  C8000H – DFFFFH is free. This area is used for expanded
memory systems. It allows a 64-k byte page frame of
memory to be used by application programs.

22.  The memory region between E0000H – EFFFFH contains
the BASIC language on ROM found in early IBM
personal computer systems. In newer systems this area is
free and referred as upper memory, which is used as extra
RAM for the use of DOS programs.

23.  It controls the operation of Basic I/O devices connected to
the computer. The Video system is not controlled by this.
I/O space in a computer system extends from 0000H to
FFFFH which allows the microprocessor to communicate
between itself and the outside world.

25.  This memory from 0000H to FFFFH is divided into
two regions. 0000H to 03FFH is reserved for system
devices and other for expansion on newer system that
extends from I/O port 0400H through FFFFH.