COMPUTER MEMORY

• Modern computers use semiconductor memory

• It is made up of thousands of circuits (paths) for electrical currents on a single silicon chip

• Prices have risen and fallen over the past few years

• Semiconductor memory is VOLATILE ie it need continued electrical current to maintain data

MAIN MEMORY – What is does

• Special chips that hold data and instructions ready for use by the CPU

• Anything we want the computer to do must be brought into the computer’s memory first

MAIN MEMORY

• Main memory is limited in size (because of cost) and is measured in kilobytes, megabytes and gigabytes

• The purpose of Main Memory is keep needed data and instructions closer to the CPU

• The computer can then operate faster

MAIN MEMORY Temporary

• It is a TEMPORARY holding location for data and instructions

• Temporary – because there is too little of it!

• Since everything is lost when we close down the computer it is said to be VOLATILE

Permanent Storage• Some form of permanent

storage is needed or all our records will be lost when we switch off the computer

Permanent Storage

• Magnetic Storage– Hard disk– Floppy disk– Zip disk

• Optical Storage– CD– DVD

The CPU is linked very closely with Main Memory so that movement of data and instructions can take place very quickly along high-speed buses

The CPU and Main Memory are on the same circuit board called the MOTHERBOARD

Main Memory and the CPU

Types of Memory

• Registers – (On the CPU)• Level 1 (L1) memory (SRAM chips

on the CPU)• Level 2 memory (SRAM chips

nearer the CPU than Main Memory)• Main Memory chips (DRAM) –

normal memory

CPU and Memory Mismatch

• Clock speeds are increasing at about 60% per year

• Memory speeds are increasing at about 7% per year

• This results is a mismatch• A ‘Wait State’ occurs when the CPU

has to wait for the system to fetch the next instruction or data that it needs

Dealing with the Mismatch

• The types of memory we are discussing – registers, cache, SRAM etc are all attempting to deal with the mismatch, and to speed up the transmission of data and instructions between Main Memory and the CPU

Registers• High speed memory chips that are

located on the same chip as the CPU

• Hold next instruction or next piece of data needed by the CPU

• Hold the results of processing by the CPU

• Each CPU contains hundreds of registers

Registers - continued• There are registers in the ALU,

in the Control Unit etc• Special registers perform

special functions– Instruction register– Address register– Accumulator register– Storage register

CACHE Memory

• Keeps frequently needed instructions closer to the CPU

• Logs user’s activities to find out and anticipate the data and instructions the user might need and to have them ready

Level 1 (Internal) Cache

• This is a memory chip which is built into the processor chip

• The problem is to make the memory components small enough

• This is SRAM

Level 2 (External) Cache

• This is a separate SRAM chip which is placed between the CPU and regular DRAM or Main Memory to speed up processing

• Usually 256Kb to 512Kb in capacity

SIMM’s and DIMM’s• Memory chips come in multi-chip

modules called SIMM’s or DIMM’s (Single or Dual In-line Memory Modules) 16 MB 32MB, 64Mb, 128MB

• This makes the installation and handling of memory chips easier

Main Memory• After registers and cache the next

level of memory is referred to as Main Memory

• Two types– RAM– ROM

RAM – Random Access Memory

• Random Access - time to locate data is the same regardless of storage location in memory

• Volatile - It loses its contents when power is switched off

• RAM is read-write• Expensive• Different types (SRAM, DRAM, etc)

ROM• ROM is used to store programs

that perform specialised functions ie ‘boot’

• ROM is non-volatile - It retains its contents when power is switched off

• ROM is read-only but there are special chips which can be re-written (EPROM)

ROM - continued• ROM takes care of loading the

Operating system into Main Memory

• ROM ensures that the computer knows what to do when it is switched on

BUSES

• A BUS is a high speed circuit that provides a path for transferring information between 2 or more devices

• Brings data and instructions from Memory to the CPU for processing

• Brings results of processing to storage or output devices

Data Bus• There are special buses for special

functions– Data Bus – carries data– Address Bus – finds where things are

stored– Control Bus – controls the flow of data

to memory, CPU and the I/O devices

PROCESSOR POWER

• The speed of a computer will depend on the type of Processor Chip, the amount of RAM, cache etc. Factors include: – Word-size– Access rate/time– Transfer rate/time

Clock Speeds• Each CPU has a built in clock called the

‘System Clock’ which synchronizes all the activities of the CPU

• The type of chip dictates the speed at which the clock operates

• A clock cycle is needed to carry out one simple instruction

• Some instructions need more than one clock cycle to complete

Chip Speed• Measured in cycle per second (cps)• Called after a German scientist called

Gustav Hertz• We refer to chip speeds in Hertz (HZ)

KHz, MHz and GigaHz• Current Intel Pentium processors have

over 2GHz speed (2 thousand million cps)

WORD SIZE

– This is a measure of the amount of data that can be handled by the processor at one time

– It is determined by the size of the registers and the buses inside the CPU. We refer to 16-bit, 32 bit processors.

What does this mean?

• If the registers are bigger more data and larger instructions can be handled at one operation

• More complex operations can be performed

• The computer operates faster

ACCESS TIME

• The time taken for the processor to locate/find the data or instructions stored in memory that are needed by the CPU

TRANSFER TIME

• The time taken by the computer to carry the data and instructions along buses between the memory components and the CPU for processing

VIRTUAL MEMORY

• When data/instructions are not actually held in RAM because of lack of space

• Paging/Segmentation swaps data in and out of RAM

FLASH MEMORY

• Generally Main Memory is volatile• New non-volatile RAM memory chips

are now used in mobile-phones, notebook computers, digital cameras

• Reason for their use is they need less power so are suitable for portable devices and are faster than secondary storage devices

MEMORY HIERARCHY

• PROCESSOR - Registers• CACHE – (LI and L2)• RAM MEMORY – Main Memory• DISK - Flash Memory• MASS STORAGE – Permanent

storage