MAIN MEMORY

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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
Main Memory and the
CPU
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
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 rewritten (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
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