Transcript Standard Grade Computing Studies Commercial Data Processing

Computer Structure
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Higher Computing
Computer Systems
Two State Machine
• We have seen that all types of data can be
represented by binary numbers
• ...and so all components used in a computer system
and all data storage devices have only two states.
• for example a switch is on or off, transistors
conduct or don’t conduct and a signal is a pulse of
electricity or no pulse.
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Stored Program Concept
• To execute a program you must first load the program and
any relevant data in to the computer’s memory (RAM) from
disk.
• The program and data is stored in memory until needed by
the processor – this is the stored program concept.
• By changing the program and data stored before execution, a
computer is termed a general purpose machine.
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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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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.
• This is known as the fetch execute cycle
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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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Higher Grade Computing Studies
2. Computer Structure
Main Memory
• The main
memory part
can be visualised
as a series of
individual boxes
with their own
unique address.
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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
• The system above has a
width of the data pathways (or
buses) within the computer, between word length of 16 bits.
the processor and the memory.
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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 (DRAM) 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
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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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 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 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 Grade Computing Studies
2. Computer Structure
The Processor and Main Memory
Main
Memory
Processor
Other
registers
Memory
Address
Register
Memory
Data
Register
ALU
Control
Unit
Address bus
32 lines
Data bus
16 lines
Control bus
6 lines
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Address
00001101
00001100
00001011
00001010
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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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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Address
00001101
00001100
00001011
00001010
00001001
00001000
00000111
00000110
00000101
00000100
00000011
00000010
00000001
00000000
Address Bus
Address Bus
•One-way.
•Carries address information from CPU to Main Memory and
any other devices attached to the bus.
•The width of the address bus (i.e. number of wires in it)
determines the number of memory locations which the CPU
can address.
•Every time a bit is added to the width of the address bus, the
address range doubles.
e.g. 2 bits = 4 addresses, 3 bits = 8 addresses etc
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Higher Grade Computing Studies
2. Computer Structure
The Data Bus
Memory
Data
Register
11100101
Processor
Main
Memory
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
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Address
00001101
00001100
00001011
00001010
00001001
00001000
00000111
00000110
00000101
00000100
00000011
00000010
00000001
00000000
Data Bus
Two way
•Carries data to and from the CPU, main memory and any other
devices attached to the data bus.
•The number of wires in the data bus determines the quantity of
data that the data bus can carry.
•More bits in the data bus means more data carried in a single
operation (increased computer performance).
•The size of the data bus determines the word length of a
computer.
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Higher Grade Computing Studies
2. Computer Structure
The Control Bus
Main
Memory
Control bus
Processor
Control
Unit
• each wire on the bus has its own separate function and is
activated independently of the others.
Read
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.
Higher Computing
Computer Systems
Address
00001101
00001100
00001011
00001010
00001001
00001000
00000111
00000110
00000101
00000100
00000011
00000010
00000001
00000000
Higher Grade Computing Studies
2. Computer Structure
The Control Bus
Main
Memory
Control bus
Processor
Control
Unit
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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Address
00001101
00001100
00001011
00001010
00001001
00001000
00000111
00000110
00000101
00000100
00000011
00000010
00000001
00000000
Control Bus
•Two way
•Consists of separate lines which each do a different control task
•all the lines work independently.
The Lines consist of:•read line
•write line
•clock line
•interrupt line
•non-maskable interrupt line
•reset line
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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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Computer Systems
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 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
Read From Memory
Steps
Effect
Processor sets up the address bus with the
appropriate address.
This pinpoints the desired
location.
Processor activates the read line on the
control bus.
This tells the location that it
is to be read from.
Data from the memory location is copied
onto the data bus.
Data is transferred to the
processor.
Processor deactivates all lines.
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Write to Memory
Steps
Effect
Processor sets up the address bus with the
appropriate address.
This pinpoints the desired
location.
Processor sets up the data bus with the
data to be stored.
Prepares the data for copying
into the memory location.
Processor activates the write line on the
control bus.
This tells the location that it
is to be written to.
Data is copied from the data bus into the
memory location.
Data is stored in memory.
Processor deactivates all lines.
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Task
• Questions on p24 of booklet
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