The memory system is divided into three main parts.

They are:-

1. TPA (Transient Program Area)

2. System Area

3. XMS (Extended Memory System)

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.

The conventional memory of 1 M byte is comprised of

two portions.

They are:-

TPA – 640k

System Area – 384k

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.

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.

Interrupt vectors access various features of the

DOS, BIOS and applications. System BIOS is a collection

of programs stored in ROM or Flash Memory.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

This memory from 0000H to FFFFH is divided intotwo regions. 0000H to 03FFH is reserved for systemdevices and other for expansion on newer system thatextends from I/O port 0400H through FFFFH.