Computer Memory

Topics

History of memoryMemory Types of memory

History of memory

In the early 1940s, memory technology mostly permitted a capacity of a few bytes.

ENIAC(Electronic Numerical Integrator And

Computer): Firs electronic general-purpose computer . That is capable of being reprogrammed to solve a large class of numerical problems.

 Vacuum tubes.The first electronic programmable digital computer the ENIAC, using thousands of octal-base radio vacuum tubes could perform simple calculations involving 20 numbers of ten decimal digits 

delay line memory:

 developed by J. Presper Eckert in the early 1940s.

It is also used in digital computers. Delay line memory would be limited to a

capacity of up to a few hundred thousand bits to remain efficient.

random access computer memory

the Williams tube and Selectron tube originated in 1946.

 in 1947  Jay Forrester, Jan A. Rajchman and An Wang developed magnetic core memory which allowed for recall of memory after power loss.

random access computer memory.

the Selectron was limited to 256 bits, while the Williams tube could store thousands

Magnetic core memory

•It was the computer memory of choice throughout the 1960s, until it was replaced by semiconductor memory.

Memory

.

Electronic components that store instructions, data, and results

Consists of one or more chips on motherboard orother circuit board

Each byte stored in unique location called an address.

Memory

It holds the data and instructions that the Central Processing Unit (CPU) needs.

Before a program can be run the program is loaded from the storage medium into memory.

It allows the CPU to direct interact with the program.

Memory is a need of every computer

Memory

Memory chips almost always come as part of a card called a module.

Memory's usually listed with two numbers, such as 8x32 or 4x16.

The first number represents the number of chips on the module.

The second one tells the capacity of each individual chip, measured in megabits (Mb), or one million bits.

Types of memory

1.ROM (Read Only Memory) 2.RAM (Random Access Memory)3.Cache4.Flash memory

ROMstores data without electrical current; it is the ROM (Read Only Memory)

or is sometimes called non-volatile memory as it is not erased when the system is switched off

Read Only Memory

Types of ROM

PROM (Programmable Read Only Memory)EPROM (Erasable Programmable Read Only

Memory)EEPROM (Electrically Erasable Read Only

Memory .

PROM

Programmable read-only memory (PROM) is a blank ROM chip on which a programmer can write permanently, but once written cannot be changed or modified

ROM cannot be written when the computer is running.

ROM use for important program like BIOS or the firmware of a device.

PROM.

Once a program has been written onto a PROM, it remains there forever.

But most ROM chips designed to be modified by users, use

Erasable programmable read-only memory ( EPROM) or electrically erasable programmable

read-only memory ( EEPROM).

EPROM

An EPROM or erasable programmable read

only memory, is a type of memory chip

Once programmed, an EPROM can be erased with the help of ultraviolet light

EEPROM

Electrically Erasable Programmable Read-Only Memory

Type of PROM that can be erased by exposing it to an electrical charge.

Similar to EPROM.

Memory

How is memory measured?

Term Abbreviation Approximate Size

Kilobyte KB or K 1 thousand bytes

Megabyte MB 1 million bytes

Gigabyte GB 1 billion bytes

Terabyte TB 1 trillion bytes

By number of bytes available for storage

RAM

Random Access Memory, can be accessed randomly. Any byte of memory can be accessed without touching the preceding bytes. RAM is the most common type of memory found in computers and other devices, such as printers.

Memory

What is random access memory (RAM)?

p. 195

The more RAM a The more RAM a computer has, the computer has, the faster it respondsfaster it responds

Also called Also called main memorymain memory

or or primary primary storagestorage

Most RAM is Most RAM is volatilevolatile, it is lost , it is lost when computer’s when computer’s

power is power is turned offturned off

Memory chips that can be Memory chips that can be read from and written read from and written

to by processorto by processor

RAM

RAM is mostly sold in multiples of 16 megabytes: 16, 32, 64, 128, 256, 512 and 1,024 (the same as 1GB) .

DDR2 and DDR3 RAMs.DDR2Appears in 2004 (speed: delivering

bandwidth of up to 8.5 GB per second)DDR3Appears in 2007(speed : can transfer data at

rates up to 12.8 GB per second)

DDR2DDR2 DDR3DDR3

RAMs

Types of RAM

There are two different types of RAM: DRAM (Dynamic Random Access Memory) SRAM (Static Random Access Memory).DRAM: Needs to be refreshed thousands of times per

second while.Dynamic RAM needs to automatically refresh

thousands of times per second; that's where it gets the name dynamic. Otherwise, it will forget the information it's holding.

DRAM

SRAMDoes not need to be refreshed, which is what makes it faster than DRAM.

Both types of RAM are volatile, meaning that they lose their contents when the power is turned off.

Memory

Where does memory chips reside?

Memory slots on motherboard hold memory modules

memory chip memory slot

dual inline memory module

Cache memory

Helps speed computer processes by storing frequently used instructions and data

The cache is a smaller, faster memory which stores copies of the data from frequently used main memory locations.

Cache memory

When the processor needs to read from or write to a location in main memory, it first checks whether a copy of that data is in the cache. If so, the processor immediately reads from or writes to the cache, which is much faster than reading from or writing to main memory.

Flash Memory

Non volatile memory can be erased electronically and rewritten similar to EEPROM.

Introduced by Toshiba in 1984Most computers use it to hold up their

startup instructionsAlso used in many mobiles , smart phones , digital cameras and PDA

Flash memory

There are two main types of flash memory.1. NAND2.NOR NAND type flash memory may be written

and read in blocks. NOR type flash allows a single machine

word(byte) to be written to an erased location or read independently.

What is capacity

.

Kilobyte (KB) 1 thousand

Megabyte (MB) 1 million

Gigabyte (GB) 1 billion

Terabyte (TB) 1 trillion

Petabyte (PB) 1 quadrillion

Number of bytes (characters) a storage medium can hold

Exabyte (EB) 1 quintillion

Zettabyte (ZB) 1 sextillion

Yottabyte (YB) 1 septillion

Memory

Smallest unit: bit8 bit = 1 byte 1024 byte = 1 kb (kilo byte)Each byte is able to represent 255 different

number (0 to 255)