Computer Organisation Praxis F2'1UF1F2'1UH1 - PowerPoint PPT Presentation

Title: Computer Organisation Praxis F2'1UF1F2'1UH1


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Computer Organisation Praxis (F2.1UF1/F2.1UH1)
Lisha Ma
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Introduction

• Computer organization examines the computer as a
  collection of interacting functional units
• Functional units may be built out of the circuits
• Higher level of abstraction assists in
  understanding by reducing complexity

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What is a Computer?

• Based on what we learnt last time, a computer can
  be defined as
• A device that executes and follows a set of
  instructions to carry out some computing
  activity.
• To do this, we need to have the ability to input
  our problem to solve and all the information
  needed to solve it, actually do the problem
  solving part, and output the results in the end.

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What is a Computer?

• With this definition, there are a lot of
  different things that can be computers

• And, of course

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More
There are many different computers Multi-million
dollar supercomputers Million dollar
mainframes Minicomputers Workstations Laptops
Less than 100 hand held personal digital
assistants
Although the price tags on these and the speed,
capacity, and software differ significantly, MOST
are basically designed the same.
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Computer Hardware Elements
Input Devices
Secondary Storage
Buses
Communication Other Devices
Output Devices
Memory
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Von Neumann Architecture

• John von Neumann a Hungarian Mathematician,
• 1903- 1957.
• The term Von Neumann architecture refers to a
  computer design model that uses a single storage
  structure to hold both programs and data.
• This storage structure is main memory it
  contains programs written in machine language
  instructions and the data for the programs.
• This is as opposed to earlier fixed program
  computers whose programs were wired directly into
  the hardware.
• Some computers are still designed this way though
  (for example, simple desk calculators).

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Von Neumann Component

• Von Neumann architecture has four functional
  units
• Central Processing Unit (CPU)
• Memory (IAS or RAM)
• Input/Output
• Auxiliary Storage (Secondary Storage).
• Sequential execution of instructions
• Stored program concept

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The Von Neumann Architecture of a Computer
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CPU

• Typically contains
• Control Unit - exerts overall control
• Arithmetic and Logic Unit - does the sums
• Registers
• Clock

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Control Unit

• A Control Unit is the unit that handles the
  central work of the computer.
• There are two registers in the control unit
• The instruction register (IR) contains the
  instruction that is being executed
• The program counter (PC) contains the address of
  the next instruction to be executed
• Repeats the following three tasks
• Fetches an instruction from memory
• Decodes the instruction
• Executes the instruction

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How Does the Control Unit Work?
Once the instruction is fetched, the PC is
incremented.
The PC holds the address of the next instruction
to be executed. Whatever is stored at that
address is assumed to be an instruction.
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Arithmetic/Logic Unit (ALU)

• Performs basic arithmetic operations such as
  adding
• Performs logical operations such as AND, OR, and
  NOT
• Most modern ALUs have a small amount of registers
  where the work takes place.
• For example, adding A and B, we might find A
  stored in one register, B in another, and their
  sum stored in, say, A, after the adder computes
  the sum.

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The ALU Uses a Multiplexer
R
Register R
Other registers
AL1
ALU
AL2
condition code register
circuits
GT EQ LT
multiplexer
output
selector lines
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ADD X
f

D
ED
E
X
ADD X
E
D
ED
ALU1 ALU2
D
ED
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Registers

• In a computer, a register is one of a small set
  of holding places that are part of a central
  processing unit (CPU).
• Links to other subsystems
• Instruction decoder circuit
• A register may hold a software instruction, a
  storage address or other data, such as single
  characters or a bit sequence for a symbol, pixel
  anything.

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Example Register

• Memory Address Register (MAR)
• Memory Data Register (MDR)
• Program Counter (PC)
• Stores the memory address of the next instruction
  to be executed
• Instruction Register (IR)
• Stores the code for the current instruction
• Status register
• General purpose registers

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The Fetch-Execute Cycle
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The Fetch-Execute Cycle

• The CPU executes a program that is stored as a
  sequence of machine language instructions in main
  memory.
• It does this by repeatedly reading, or fetching,
  an instruction from memory and then carrying out,
  or executing, that instruction.

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The Fetch-Execute Cycle

• The components of a processor
• DR - Data Register
• M - Memory
• MAR Memory Address Register
• MBR Memory Buffer Register
• AC - Accumulator
• PC - Program Counter
• IR - Instruction Register
• ALU - Arithmetic Logic Unit (including
  arithmetic logic circuits)
• PCU - Program Control Unit

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The Fetch-Execute Cycle

• Fetch operation
• The address of the desired memory cell is moved
  into the MAR
• Fetch/store controller signals a fetch,
  accessing the memory cell
• The value at the MARs location flows into the MDR

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The Fetch-Execute Cycle

• Store operation
• The address of the cell where the value should go
  is placed in the MAR
• The new value is placed in the MDR
• Fetch/store controller signals a store, copying
  the MDRs value into the desired cell (CIR)

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Load Fetch/Execute Cycle
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Store Fetch/Execute Cycle
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ADD Fetch/Execute Cycle
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Memory
CPU
Memory
Control info
Write data
Read data

• CPU Interacts with the memory in 3 ways
• fetches instructions
• loads the value of a variable
• stores the new value of a variable
• Memory is capable of only 2 operations
• reads a load or a fetch
• writes operation of a storing the value of a
  variable

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DDR DIMM Memory Slot
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Memory

• Memory is a collection of cells, each with a
  unique physical address
• The size of a cell is normally a power of 2,
  typically a byte today.

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Memory

• A cell is the smallest addressable unit of memory
  i.e. one cell can be read from memory or one
  cell can be written into memory, but nothing
  smaller.

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Memory

• Memory types in a hardware system
• Registers
• Cache
• RAM (Random Access Memory, main memory)
• ROM (Read Only Memory)
• Also Flash Memory
• Hard disk ('C Drive')
• CD - CD ROM or DVD

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Cache Memory

• Cache memory is Random Access Memory (RAM) that a
  computer CPU can access more quickly than it can
  access 'main memory' RAM.
• As the microprocessor processes data, it looks
  first in the cache memory and if it finds data
  there, where it was placed from a previous
  reading of data, the CPU saves time in not having
  to transfer data from RAM.

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Cache Memory

• Cache memory is sometimes described in levels of
  closeness and accessibility to the CPU. A Level 1
  (L1) cache is on the same chip as the CPU and
  can, typically, store 32 kilobytes.
• Level 2 (L2 cache) is usually a Static RAM (SRAM)
  chip, near to the CPU but separate from it. The
  CPU can access the L2 if the L1 happens to be
  empty, still saving the transfer from RAM.

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RAM

• RAM Random Access Memory serves as a
  short-term memory for the computer. (The hard
  disk is usually described as the long-term memory
  for the computer.)
• While the computer is in use, the application
  program instructions and data pass continuously
  through the processor.
• They are stored, most immediately and for a short
  time, in RAM.
• RAM also stores many operating system
  instructions during this same time.
• The main RAM is usually a Dynamic RAM (DRAM)
  chip.
• DRAM is 'dynamic', unlike Static RAM that is used
  in the cache. DRAM needs to have its storage
  cells refreshed with electrical charge every few
  milliseconds.

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RAM

• Static RAM does not need refreshing because it
  uses continually moving current , switched in one
  of two directions. DRAM cells hold a charge in
  place. Static RAM can be accessed more quickly
  than DRAM.

A photo of RAM
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ROM (Read Only Memory)

• ROM is 'built-in' computer memory containing data
  and instructions that normally can only be read,
  not written to.
• ROM code holds start-up instructions - i.e. ROM
  contains the programming that allows your
  computer to be "booted up" or regenerated each
  time you turn it on.

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ROM (Read Only Memory)

• ROM chips contain instructions which are specific
  for that particular motherboard. Those programs
  and data will remain in the PC throughout its
  life usually they are not altered.
• Unlike RAM, the data in ROM is not lost when the
  computer power is turned off.
• The ROM is sustained by a small long-life battery
  in the computer.

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How does the memory unit work?
Trace the following operation
s
Store data D in memory location 0.
D
0
0
D
D
0
D
D
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How does the memory unit work?
Trace the following operation
f
1) Fetch data D from memory location 1. 2) Obtain
an instruction I from memory location 7.
D
1
How does the computer distinguish between 1)
and 2) above?
D
I
We need to look at the Decoder Circuit.
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Using the Decoder Circuit To Select Memory
Locations
0
4 x 24 decoder
0
1
0
0
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The decoder circuit doesn't scale well--- i.e. as
the number of bits in the MAR increases, the
number of output lines for the decoder goes up
exponentially.
Most computers today have an MAR of 32 bits.
Thus, if the memory was laid out as we showed it,
we would need a 32 x 232 decoder!
Note 232 is 22 230 4 G
So most memory is not 1 dimensional, but
2-dimensional (or even 3-dimensional if banked
memory is used).
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2-D Memory
Note that a 4 x 16 decoder was used for the 1-D
memory.
2 x 4 decoder
columns
2 x 4 decoder
rows
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Input/Output Units

• An input unit is a device through which data and
  programs from the outside world are entered into
  the computer
• Keyboard, the mouse, and scanning devices
• An output unit is a device through which results
  stored in the computer memory are made available
  to the outside world
• Printers and video display terminals

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The I/O Devices
Pictorially, these look the simplest, but in
reality, they form the most diverse part of a
computer. Includes
keyboards, monitors, joysticks, mice, tablets,
lightpens, spaceballs, ....
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I/O Units
Memory
Processor
I/O buffer
Control-logic
Each device is different, but most are interrupt
driven. This means when the I/O device wants
attention, it sends a signal (the interrupt) to
the CPU.
I/0 device
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IN X
s

D
X
D
IN X
D
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OUT X
f

D
X
D
OUT X
D
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Secondary Storage

• Because most of main memory is volatile and
  limited, it is essential that there be other
  types of storage devices where programs and data
  can be stored when they are no longer being
  processed
• Secondary storage devices can be installed within
  the computer box at the factory or added later as
  needed

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Magnetic Tape

• The first truly mass auxiliary storage device was
  the magnetic tape drive

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Magnetic Disks

• A read/write head travels across a spinning
  magnetic disk, retrieving or recording data

The organization of a magnetic disk
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Compact Disks

• A CD drive uses a laser to read information
  stored optically on a plastic disk
• CD-ROM is Read-Only Memory
• DVD stands for Digital Versatile Disk

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Flow of Information

• The parts are connected to one another by a
  collection of wires called a bus

Data flow through a von Neumann architecture
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Buses

• Cables/Wires that connect the various components
  of the computer together.
• Used for control messages and data flows.
• Important in determining the overall speed of the
  machines
• 200Mhz CPU clock speed is pointless if the bus
  can only operate at 50MHz, and vice versa.

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Buses

• The three buses connecting the processor to other
  components, called system buses, are
• control bus
• address bus
• data bus
• Each bus is made of lines, with each line capable
  of carrying a 0 or 1 value bit at a time

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Control Bus

• The control bus carries bi-directional signals
  that control the operation of the system
  components.
• status signals enable sharing of the address and
  data buses
• bus request a component needs control of
  bus
• bus grant a component is granted
  control of bus
• command signals specify operations to perform
• memory write data on data bus written to
  location given by address bus
• memory read data in location given by address
  bus read into data bus
• timing signals synchronize components.
• Clock provides regular pulses (giving speed of
  processor)

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Address bus

• The address bus carries address signals in one
  direction only from the processor to a
  component.
• The address bus carries the address of the
  location of an instruction, or data, to be
  written to or to be read to memory or to an I/O
  controller.
• The width of the address bus determines the
  maximum memory size,
• for example if an address bus had a width of 3
  lines, the number of locations that can be
  addressed would be 8 locations.
• (at least 20 lines would be more realistic)

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Data bus

• The data bus is bi-directional and carries
  program instructions or data.
• The number of lines determines the amount of data
  that can be transferred at a time (in a single
  fetch)

Word size

• The word size of a computer is the number of bits
  the processor can process simultaneously.
• This may be the same or a multiple of the data
  bus size

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Central Processor Unit
System Buses
Processor
Control Bus
Address Bus
Main Memory
Data Bus
Keyboard Input Controller
Keyboard
VDU Output Controller
VDU
(Disk) I/O Controller
Secondary Storage
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Peripheral Devices

• Attached to motherboard via ports
• Examples
• keyboard
• mouse
• modem
• external drive
• printer
• scanner
• ...

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Motherboard

• The nervous system of the computer.
• Houses the main buses, CPU, primary memory and
  expansion slots for interface cards.
• Interface cards help communication between the
  peripherals and the motherboard.
• Connected to the secondary storage and
  peripherals.

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Thank You!