Higher Grade Computing Studies
2. Computer Structure
Computer Structure
• The traditional diagram of a computer ...
INPUT
DEVICES
keyboard
+ mouse
etc.
CPU
CONTROL
ALU
UNIT
REGISTERS
RAM
R
O
M
MAIN MEMORY
BACKING STORAGE
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Higher Computing
Computer Systems
S. McCrossan
OUTPUT
DEVICES
monitor
+ printer
etc.
Higher Grade Computing Studies
2. Computer Structure
The Structure of the CPU - The Control Unit
The control unit is like the CPU’s manager. The main functions of
the control unit are:
• To send out the signals to fetch instructions from memory.
• To interpret those instructions.
• To send out signals to enable instructions to be carried out.
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Computer Systems
S. McCrossan
Higher Grade Computing Studies
2. Computer Structure
The Structure of the CPU - The ALU
The arithmetic and logic unit, the ALU, performs the following
functions;
• Calculations such as addition, subtraction, multiplication and
division.
• Logic functions, such as comparing values.
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Computer Systems
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Higher Grade Computing Studies
2. Computer Structure
Main Memory
4
Address
Contents
Address
Contents
00000001
0111010011110000
00001010
0111010011111000
00000010
0011001101101111
00001011
0001000111100000
00000011
1111000011101111
………
00000100
0011001100110011
………
00000101
1111000010101010
………
00000110
1100000000111111
………
00000111
0001110010110010
………
00001000
0101000000110000
………
00001001
0110101100001101
………
•Address
The main
Contents
………
memory
part
………
can
be visualised
………
as
a series of
………
………
individual
boxes
……… their own
with
………
unique
address.
………
………
Higher Computing
Computer Systems
S. McCrossan
Higher Grade Computing Studies
2. Computer Structure
The Size & Number of Memory Locations
• The word length of a computer is
the size of the data, in bits, which
can be manipulated as a single unit
by the processor. Logically, it is also
the number of bits that a single
memory location can store.
• The size is determined by the
width of the data pathways (or
buses) within the computer, between
the processor and the memory.
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Address
Contents
Ad
00000001
0111010011110000
0000
00000010
0011001101101111
0000
00000011
1111000011101111
……
00000100
0011001100110011
……
00000101
1111000010101010
……
00000110
1100000000111111
……
00000111
0001110010110010
……
00001000
0101000000110000
……
00001001
0110101100001101
……
• The system above has a
word length of 16 bits.
Higher Computing
Computer Systems
S. McCrossan
Higher Grade Computing Studies
2. Computer Structure
Main Memory
• RAM has the following features:
• The CPU can write to, or read from, RAM at high speed.
RAM can be changed by the CPU at any time - it is volatile.
• All information stored in RAM is lost when the computer is
switched off.
• Static RAM holds its contents as long as there is a power
supply.
• Dynamic RAM, has to be refreshed by reading and rewriting its
contents frequently.
• DRAM is more common because it needs less power and its
circuitry is simpler.
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Computer Systems
S. McCrossan
Higher Grade Computing Studies
2. Computer Structure
Main Memory
• ROM has the following features:
• Data in ROM is permanently written into a microchip.
• Data in ROM can be read by the CPU from the moment
the computer is switched on.
• The bootstrap loader is stored in ROM.
• Data stored in ROM is not lost when the computer
is turned off.
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Computer Systems
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Higher Grade Computing Studies
2. Computer Structure
Main Memory
• PROM (Programmable Read Only Memory)
• Empty of data when the chip is manufactured, can be programmed by
the user. Once programmed the data cannot be erased.
• EPROM (Erasable PROM)
• Like PROM only the chip can be removed from the computer and the program
erased and another stored in its place using ultraviolet light.
• EEPROM (Erasable PROM)
• Like EPROM but electricity is used to erase and reprogram selected contents.
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Higher Computing
Computer Systems
S. McCrossan
Higher Grade Computing Studies
2. Computer Structure
Storage Outside Main Memory
• Reading /writing data to and from memory can slow
down the system performance. To combat this problem, the
processor can store data in cache memory or its internal
registers instead of the RAM.
• Cache memory
• small amount of memory e.g. 512 KB.
• stores the next instructions to be read.
• physically closer to the processor than RAM.
• uses static memory (SRAM).
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Higher Computing
Computer Systems
S. McCrossan
Higher Grade Computing Studies
2. Computer Structure
Storage in the CPU - Registers
• Registers are temporary storage locations in the processor.
• A register can be used for tasks like ;
• holding data for a calculation.
• storing the address of the next instruction to be executed.
• holding the instruction as it is being decoded and executed.
• We will look at registers in more detail in the Fetch-Execute
cycle.
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Higher Computing
Computer Systems
S. McCrossan
Higher Grade Computing Studies
2. Computer Structure
The Processor and Main Memory
Main
Memory
Processor
Other
registers
Memory
Address
Register
Memory
Data
Register
ALU
11
Control
Unit
Address
00001101
00001100
00001011
00001010
Address bus
32 lines
Data bus
16 lines
Control bus
6 lines
Higher Computing
Computer Systems
S. McCrossan
00001001
00001000
00000111
00000110
00000101
00000100
00000011
00000010
00000001
00000000
Higher Grade Computing Studies
2. Computer Structure
Buses
• A bus is an electronic highway or a collection of cables where
each cable can transmit one bit.
• There are three buses:
• The address bus, which is used by the processor to pinpoint the
memory location, needed. This is a one-way bus.
• The data bus, which is used to transfer the data. Its size will
usually match the word size i.e. the size of the memory locations.
This is a two-way bus.
• The control bus, which is used to initiate and control what is
happening.
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Higher Computing
Computer Systems
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Higher Grade Computing Studies
2. Computer Structure
The Address Bus
Memory
Address
Register
0001000
Processor
Main
Memory
Address bus
• information is carried from the processor to the main memory
• this informs the main memory which memory location
will be read or used to store data.
• each wire on the bus carries one bit of information at a time.
• the number of wires in this bus determines the number of memory
locations.
• 8 lines will allow 256 memory locations.
• 32 lines will allow 68,719,476,736 memory locations.
• increasing the width of this bus, increases the number
of memory locations that it is possible to address.
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Higher Computing
Computer Systems
S. McCrossan
Address
00001101
00001100
00001011
00001010
00001001
00001000
00000111
00000110
00000101
00000100
00000011
00000010
00000001
00000000
Higher Grade Computing Studies
2. Computer Structure
Main
Memory
The Data Bus
11100101
Processor
Memory
Data
Register
Data bus
11100101
• information is carried to and from the processor and main
memory.
• this stores data in a memory location and reads data from a
memory location.
• each wire on the bus carries one bit of information at a time.
• the description of the computer i.e 32 bit computer, informs the
user of the number of wires in this bus.
• increasing the width of this bus, increases the quantity of data that can be carried at one
time and so increases the performance of the computer system.
Higher Computing
Computer Systems
14
S. McCrossan
Address
00001101
00001100
00001011
00001010
00001001
00001000
00000111
00000110
00000101
00000100
00000011
00000010
00000001
00000000
Higher Grade Computing Studies
2. Computer Structure
Main
Memory
The Control Bus
Processor
Control
Unit
Control bus
Informs the memory that data is to be sent to the
processor from a particular memory location.
Write
Informs the memory that data is to be stored in a
particular memory location.
Clock
Generates a constant pulse which
regulates the flow of information.
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A clock of 600MHz (megahertz)
generates a pulse 600,000,000
times a second.
00001101
00001100
00001011
00001010
00001001
00001000
00000111
00000110
00000101
00000100
00000011
00000010
00000001
00000000
• each wire on the bus has its own separate function and is
activated independently of the others.
Read
Address
Higher Computing
Computer Systems
S. McCrossan
Higher Grade Computing Studies
2. Computer Structure
The Control Bus
Processor
Main
Memory
Control
Unit
Control bus
Interrupt This is a signal informs the processor that some
important external activity has taken place. The
current state of processing is saved, then the
processor deals with the interrupting device, it then
returns to the previous state when finished with the
interrupt request.
NMI
The Non-Maskable Interrupt operates like an
interrupt but it cannot be ignored. It must be dealt
with immediately.
Reset
Clears all internal processor registers
and returns the computer to its initial
switched on state.
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Higher Computing
Computer Systems
S. McCrossan
Address
00001101
00001100
00001011
00001010
00001001
00001000
00000111
00000110
00000101
00000100
00000011
00000010
00000001
00000000
Higher Grade Computing Studies
2. Computer Structure
The Links Between Processor and Memory
• The Stored Program concept:
• The program and data are stored in RAM until needed by
the processor.
• The FETCH-EXECUTE cycle:
• The first instruction is fetched from memory into the
processor where it is decoded and executed. The same happens
to the second instruction and so on until the program ends.
• The processor has to be able to pinpoint any memory location
in which instructions or data are stored. Each memory location
is assigned an unique address.
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Higher Computing
Computer Systems
S. McCrossan
Higher Grade Computing Studies
2. Computer Structure
Read From Memory
• A processor needs an instruction from memory or requires some
data to perform a calculation. Here are the steps and buses needed
to fetch that information from memory.
1. Processor sets up address bus with the
address to be read in memory in the MAR
This pinpoints the desired location
2. Processor activates the read line on the
control bus
This tells the memory a ‘read’ is to
occur and the memory finds the
requested location
3. Data from the memory is copied onto the
data bus and transferred into the MDR
Data is transferred to the processor
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Higher Computing
Computer Systems
S. McCrossan
Higher Grade Computing Studies
2. Computer Structure
Write To Memory
• Here are the steps and buses needed to write information to
memory.
1. Processor sets up address bus with the
address of the location to be written to in the
MAR
2. Processor sets up data bus with the data to
be written to memory into the MDR
3. Processor activates the write line on
the control bus
4. Data from the processor is copied
onto the data bus and into memory
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This pinpoints the desired location
This prepares the data
This tells the memory a ‘write’ is to
occur and the memory finds the
requested location
Data is transferred to the memory
Higher Computing
Computer Systems
S. McCrossan