• System memory in the PC is broken into several areas

Logical Memory Organisation

Reserved area

Video RAM

User Programs(Conventional Memory)

1024KB

768KB

640KB

0KB

User Data and Program

TSD Program

Command.com

Device Drivers

DOS

Interrupt Vector

Conventional Memory

• The first 640kb of system memory is called “conventional memory”

• This is the area that is available for the use by the standard DOS program

• The bottom 1K area is used to store the interrupt vector pointer.

• Above is DOS itself.It’s hard to say exactly how much space DOS takes,it depends on version Of DOS

• Above DOS loads a special class of programs called device driver.

• Above the device driver COMMAND.COM is stored.It accept input from user and reformulate them in such a manner that the operating system can understand.

• Above the command loads TSR(terminate and stay resident) programs do the same things as device drivers.

• Above TSR,there is a space for user program and data.In this user can load the data and program.

Upper Memory Area

• It is the upper 384 KB of 1MB of system Memory(immediately above the conventional memory) is called “UMA (upper memory area)”.

• It is the very busy place• It is reserved for the use by system devices and

for special uses.

Higher Memory Area

• It is the first 64KB of the second megabyte of system memory.Technically this is the first 6KB of extended memory,but it can be accessed when the processor is in real mode.

Extended Memory

Reserved for BIOS

Reserved for display V RAM

Conventional Memory

4GB

1MB

640KB

0KB

Fig:Memory Map with Extended Memory

Extended Memory

• All of the memory above the first megabyte is called “Extended Memory”.• This is all the memory above the high memory area until the end of the system memory.•The memory above the 1MB is extended memory

It is used for program and data when using an operating system running in protected mode,such as any version of window.

• On 80286 or 80386 SX system,extended memory limit is 16MB(16 bit addressing).

• On a 80386 DX,80486,Pentium or Pentium pro system,extended memory limit is 4GB(32 bit addressing).

• The processor pentium –Iiand newer processor have a limit of 64KB(36 bit addressing).

• The Microsoft,Lotus developed the extended memory specification(XMS) to use extended memory

Expanded Memory

Extended memory

Motherboard ROM BIOS

EMS window

Adapter ROM

Video RAM

Conventional Memory

Expanded memory

Divided into logical pages andMapped into EMS window

16M/4G

1M

896K

832K

768K

640K

0K0K

32M

Conventional extended memory Expanded memory

fig: Conventional extended and expanded memory

• An older standard for accessing memory above 1MB which called “Expanded memory”.

• It uses a protocol called the “Expanded memory specification or EMS.

• EMS was originally created to overcome the 1MB addressing limitation of the first generation 8086 and 8088 CPU.

• Unlike conventional memory or extended memory ,expanded memory is not directly addressable by the processor.Instead of it ,it can only be accessed through a small 64k window established in UMA(upper memory area).

• This type of memory generally useful only in the system that do not have extended memory(advance processor) available to them.

• Expanded memory specification developed by Lotus,Intel and Microsoft(LIM).This is called LIM specification for expanded memory.

• It is the high speed memory,that store data temporarily.

• It is also called as “Intelligent Memory”• The processor retrive data faster than from

main memory.• It is made up of Static memory(SRAM).

Cache memory

L1(Internal cache):• Primary cache• Small in size• High speed• Placed at the right side of processor chip• Typical range of size of L1 cache 8KB to 6KB• Uses high speed SRAM(static RAM) instead of slower and cheaper

DRAM(dynamic RAM).

Levels of Cache

L1(LEVEL 1) (Internal cache)L2(LEVEL2) (External cache)L3 cache

• L2(External cache):• secondary cache• Bigger in size• Memory placed between CPU and main memory• Range 64K to 2MB

• L3(Old L2 cache):• L3 cache slowly replaced the L2 cache function• Extra cache built into the motherboards between

CPU and main memory is now being called “L3 cache”.

• Some manufacturer using L3 cache designs already,but most desktop and notebook computers do not offer this function yet.

Advantages of cache:• It is the high speed memory• Intelligent Memory• The processor retrive data faster than from main

memory.so improve the performance of system

• Small in size• Cost is very high

Disdvantages of cache:

Types of main memory

There are two types of main memory, Random Access Memory (RAM) and Read Only Memory (ROM)

Random Access Memory (RAM)

holds its data as long as the computer is switched on

All data in RAM is lost when the computer is switched off

Described as being volatile

Called as “Main memory” or “primary memory”.

Types of RAM

1. Dynamic Random Access Memory (DRAM)

• Uses capacitors for holding information.this makes DRAM slower and cheaper

• Contents are constantly refreshed 1000 times per second

• Access time 60 – 70 nanoseconds

• Note: a nanosecond is one billionth of a second!

1.DRAM

2.SRAM

2. Static Random Access Memory (SRAM)

• Uses on/off switches for holding information.this makes SRAM faster and expensive

Access time around 10 nanoseconds

• Used for cache memory

Types of Dynamic RAM (DRAM)

FPM(Fast Page Mode)

EDO(Extended Data Output)

ECC(Error correcting code)

SDRAM(Synchronous DRAM)

Traditional RAM for PC,before EDO was introduced.

Mounted on SIMM module

It is an improvement in FPM RAM

Data are faster

It is a special error correcting RAM.

Specially used in servers

Replacement of FPM,EDOSpeed of SDRAM 100MHZ and133MHZAll SDRAM for desktop PCs have 168 pins

DDR(Double data rate SDRAM)• It basically doubles the rate of data transfer of

standard SDRAM• Faster version of SDRAM• All DDR RAM chips have 184 pins• DDR RAM comes in different speed i.e. 100

MHZ,133MHZ,166MHZ,200MHZ

DDR2• Next generation of DDR SDRAM memory• Delivers data at twice speed of DDR.• Mounted on a 240-pin DIMM Modules• DDR2 to operate at data rates of 400 MHZ,533MHZ,667 MHZ

DDR3

• Technology used for high storage of working data of computer.

• DDR3 to operate at data rates of 800 mhz,1066 mhz,1600 mhz and above

BIOS

• BIOS(basic input output system) provides the processor with the information required to boot the system from a non-volatile storage unit(HDD,FDD,CD).

• BIOS is an electronic set of instruction that the computer uses to successfully start operating.

• Located on a chip inside of the computer and is designed in a way that protects it from disk failure.

Main function of BIOS

• Main function of BIOS is to give instruction for the POST i.e.power on self test.

• This self test ensures that the computer has all of the necessary parts and functionalities needed to successfully start itself,such as use of memory,keyboard and other parts