Higher Computing

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Higher Computing

• Computer structure

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What we need to know!

• Detailed description of the purpose of the ALU
  and control unit
• Description of the purposes of registers to hold
  data being processed, instructions being
  executed, and addresses to be accessed
• Description of the function of the data bus and
  the address bus
• Description of the read, write and timing
  functions of the control lines
• Identification of other control lines, including
  reset and interrupt lines
• Simple description, referring to the appropriate
  buses and control lines, of the steps in the
  fetch/execute cycle
• Description of the following elements of computer
  memory registers, cache, main memory,backing
  storage

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Organisation of a simple computer
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The purpose of the ALU

• The Arithmetic and Logic Unit (ALU) is the part
  of the Central Processing Unit (CPU) where the
  following take place
• Calculations
• Boolean logic (AND, OR, NOT)
• Comparisons (lt,, gt, lt, gt)

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The purpose of the Control Unit

• The Control Unit sends out control signals
• Within the processor to move data from one
  register to another and to activate specific ALU
  functions
• To the control bus to read or write from memory
• To Input/Output modules.

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The purpose of the registers

• Registers are storage locations that are internal
  to the processor.
• They are used to
• Hold data that is being transferred to or from
  memory
• Hold the address of the location in memory which
  the processor is accessing to read or write data
• Hold the instructions that are being carried out.

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The function of the data bus

• The lines on the data bus enable data to be
  transferred between the processor and the main
  memory.
• The width of the data bus is measured by the
  number of lines on the bus. Each line can carry
  one bit.
• A 32-bit data bus can transfer 32 bits at a time.
  
• The width of the data bus is important when
  determining how well a system performs.

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The function of the address bus

• This holds the address of the memory location
  being accessed.
• The more lines in the address bus the more
  locations the system can, in theory, access.
• The maximum number of location the system can
  access is
• 2width of the address bus

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The control lines

• The control bus is made up of several lines.
• The read line instructs the system to place data
  from the specified memory address on the data
  bus.
• The write line instructs the system to take the
  data from the data bus and place it in the
  location specified by the address bus.
• The Clock line provides the timing function of
  the system. The pulses from the clock line
  control when each step of the function takes
  place. Theses pulses control and regulate the
  activities of the processor.

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The control lines (Cont.)

• Other lines on the control bus include.
• The reset line. A signal on this line stops all
  processes, clears all registers and places the
  system back in its original state.
• The interrupt line. A signal here pauses the
  current process and allows another process to
  take control of the processor.

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Fetch execute cycle
A more detailed animation can be found here.
Read line

1. Set up the address bus by moving the address to
   the Memory Address Register.

1. Signal the read line on the control bus.

1. Transfer the data from memory to the Memory Data
   Register.

1. Decode and execute the instruction.

1. Reset all signals.

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What we should now know!

• Detailed description of the purpose of the ALU
  and control unit
• Description of the purposes of registers to hold
  data being processed, instructions being
  executed, and addresses to be accessed
• Description of the function of the data bus and
  the address bus
• Description of the read, write and timing
  functions of the control lines
• Identification of other control lines, including
  reset and interrupt lines
• Simple description, referring to the appropriate
  buses and control lines, of the steps in the
  fetch/execute cycle

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What we need to know!

• Description of the following elements of computer
  memory registers, cache, main memory, backing
  storage
• Distinction between the above elements of memory
  according to function and speed of access.
• The concept of addressability.
• Description and evaluation of the following
  measures of performance clock speed, MIPS,
  FLOPS, and application based tests.
• Description of the effect the following factors
  have on system performance data bus width, use
  of cache memory, rate of data transfer to and
  from peripherals.
• Description of current trends in computer
  hardware, including increasing clock speeds,
  increasing memory and backing storage capacity.

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Registers

• Registers are storage locations that are internal
  to the processor.
• They are used to
• Hold data that is being transferred to and from
  memory.
• Hold the address of the memory location that the
  processor is reading/writing to/from.
• Hold the instructions that are being carried out.

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Cache memory

• This is a small and very fast memory unit which
  is located close to the processor.
• When the processor attempts to read from main
  memory, the cache is checked first. If the data
  is already stored there then it is transferred
  directly to the processor.
• This saves a read from main memory which is much
  slower than cache memory.

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Main Memory

• This is the main internal storage area for the
  computers instructions.
• It is divided into ROM and RAM sections.
• Reading from main memory is slower than reading
  from registers or cache.

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Backing Storage

• This is the slowest form of data used to store
  data or software.
• It retains its data when the power is switched
  off, unlike the RAM in the main memory.

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Comparing different types of memory.
Memory Function Speed of access
Registers Internal to the processor. Holds data while being processed, e.g. Instruction Register Fast access time internal to the processor.
Cache Stands between the processor and main memory. Processor checks the cache memory for data before accessing main memory.   Slower access than a register but faster than accessing main memory.
Main Memory Stores user data and software in RAM and some system software in ROM. Accessing data in main memory is slower than accessing either cache memory or registers.
Backing Storage Stores data, software. Retains the data when power is off. Slowest of all the types of memory.
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Addressability

• In your street all the houses are given different
  numbers so the postman knows where to deliver his
  mail.

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Addressability

• Computers also give each location in memory a
  different number so it knows where to place each
  piece of data, this number is known as a Memory
  Address. The computer can use this Memory Address
  to find the correct location when accessing its
  memory.

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Addressability

• The number of locations that a processor can
  address is, in theory, limited by the number of
  lines on the address bus.
• If the address bus is 16 bit wide there can, in
  theory, be
• 216 65536 different memory locations.
• If each memory location stores 32 bits of memory
  then the total memory capacity is
• 65536 x 32bits 65536 x 4bytes
• 262144bytes 256KB

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Measuring performance

• There are several different way to measure the
  performance of a processor. You need to know
  about
• Clock speed.
• MIPS
• FLOPS
• Application-based tests.

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Clock speed

• One way to compare computers is to compare the
  clock speed.
• The clock pulses regulate the activities in the
  processor.
• The pulses are measured in Megahertz (MHz) and
  Gigahertz (GHz)
• 1 MHz 1 million pulses a second.

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Clock speed

• The clock speed gives you an indication of the
  performance of the processor, which is at the
  heart of the processor, but do no over emphasize
  the importance of clock speed.
• The performance of the processor is not dictated
  by the processor alone. Other factors are also
  important such as the data bus width.

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MIPS

• MIPS stands for Millions of Instructions Per
  Second.
• This approach is based on the measurement of the
  number of machine code instructions that can be
  executed per second.
• MIPS doe not take into account the size and
  complexity of the instructions being carried out
  and so is generally seen as a rough indication of
  performance.

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FLOPS

• FLOPS stands for Floating Point Operations Per
  Second.
• This measures how many floating point operations
  a processor can carry out in a second.
• This is generally seen as a more reliable
  indicator of performance, as it measures the
  amount of clearly definable arithmetical tasks
  that can be carried out in a second.

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Application-based tests

• Most computer magazines use application-based
  tests (benchmark tests) to compare system
  performance.
• They set out a series of practical tasks using a
  range of standard application packages, award
  scores for the performance in each task and then
  use these scores to make overall comparisons.

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Application-based tests

• These are typical sets of application-based tests

Application Details Package
Spreadsheet Trigonometrical changes to 200 rows of data each with related graphs. Excel
Word Processing Spell check and reformat a 145 page document Word
2-D Graphics Open 25 high resolution photos, rotate and apply filters Photoshop
3-D Graphics 3-D graphics run at 1024x768 resolution with 32-bit colour Unreal Tournament
Database Run Queries on 3 tables with over 120,000 records and produce reports Access
Media creation Convert 25 minutes of WAV to MP3 then WMA dBPowerAM
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Application-based tests

• How good are they?
• Other measures of performance are reasonable but
  they dont provide us with evidence of how the
  system will actually perform.
• Application-based tests provide us with
  reproducible, evidence of performance when
  carrying out complex operations.
• For this reason they are very reliable in
  measuring system performance.

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How transfer rate affect performance.

• All peripherals are slower than the processor.
  This can slow the processing down if the CPU
  needs to read from CD/DVD etc.
• The faster the data transfer rate, the faster the
  overall performance of the system.

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Transfer Rate
CD transfer Rate Transfer rate in Kilobytes per second Time take to read a 10Mb file
52x 7800 1.31 seconds
32x 4800 2.13 seconds

• This difference can quickly effect the system
  performance.

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Comparing CPUs

• When looking at processor developments look at
  key features
• Clock Speed (now measured in GHz)
• Address bus
• Data Bus
• When looking at storage developments look at
• Capacity
• Data Transfer rates