Session Objectives#4
COULD explain the use of registers in the functioning of the processor
SHOULD describe the function and purpose of the control unit, memory unit and ALU
as individual parts of a computer
MUST describe the differences between the main types of primary memory
Create solutions to the Little Man Computer assignment
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Computer Processing
Try to list 5 factors which affect how a computer performs.
Ext: For each explain what affect it has and why.
1.Size of processor
2.Amount of Cache Memory
3.Number of cores
4.Amount of RAM
5.Transfer speed of data buses.
Amount of free Hard Disk space...If a computer
is using all or it’s RAM it will use a section
of hard disk as a supplement. This is called
VIRTUAL MEMORY. However this is still slower
than pure RAM.
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Primary Storage
This is also known as Main Memory or Primary Storage.
INVESTIGATION (7 mins): Research what is known as the
‘bootstrap’ program of a computer. Take notes and prepare to
explain where and how is stored in the computer and why it is so
This is the first instructions the CPU
receives when a computer is turned on
and is commonly called the BIOS (Basic
Input Output System). It contains code
which controls the basic hardware
settings of a PC. It is mainly stored
in ROM (Read Only Memory) which is NOT
erased when power is turned off.
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MEMEMORY – Remember??
To recap...
There are 2 main categories of memory:
RAM – Random Access Memory
This is volatile memory as it’s
erasable (without power) but can
be accessed quickly by the CPU.
ROM – Read Only Memory
This is non volatile as the data
Is retained without power. Therefore
Knows how to start up (or boot) after
Being turned off.
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4 Min Investigation
What type of memory is cache memory?
Are there any sub-divisions of memory? Create a
table and note their characteristics.
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Summary of Memory Use
Speed of data transfer...
TASK: Go shopping...Create a table and compare costs and
quantities of secondary storage, RAM and CPU with cache memory.
Secondary Storage
CPU (with cache)
What are your conclusions?...
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The Fetch-Execute Cycle
The CPU receives data and instructions in binary form. An
instruction will have 2 parts – an instruction and possibly some
data, a number or a memory location.
The programs that the CPU needs to process are stored in main
memory. The CPU simply fetches the next instruction it needs to
process, decodes it and executes it before repeating the
The speed of this cycle is determined by an electronic Real Time
Clock (RTC) chip. The computer synchronises all processes to
this clock signal. The clock speed is measured in Hertz (Hz) or
cycles per second.
TASK: 500 Hz would be 500 cycles per second, how many could a
3GHz processor be capable of?
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Machine Code with
The Little Man Computer
Visit the following site and follow the simulations of the
Fetch-Execute cycle using the Little Man Computer (LMC)
This is an interpretation of how a processor handles machine
code. The simulation shows instructions being excecuted.
The Op Code (or Operation Code Field) is part of the binary code
giving the instruction to be carried out i.e add or jump
The Operand (Operand Field or address field) gives the address
(memory location) where the data to be used in the operation can
be found.
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Flow Chart
Flow chart of solution to LMC Assignment 3
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Registers keep a check on the progress of the instructions and
data as it moves around the processor. Think of them as part of
a logical operation rather than individual registers.
Program Counter (PC) – In the CU. Counts the instructions as
they are carried out and increments by (+1). Contains the
address of the next instruction to be executed(Instructions are
always stored in order)
Memory Address Register (MAR) –stores the address of the memory
location currently in use, sent by the PC.
Memory Data Register (MDR) – A copy of the instruction held in
the MAR is stored here so that the memory unit and processor and
work on it at the same time.
Current Instruction Register (CIR) – the instruction now in the
MDR is copied into the CIR. In here it can be spilt into 2
parts; 1 part is sent to be decoded so that the processor knows
what the instruction is (and signals can be sent to other parts
of the processor which may be required to carry out the
instruction). The other part is an address stating whereabouts
in the memory the required data is.
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CIR continued...
E.G if an instruction of ADD 20 is given it will be split into:
ADD – ALU works out how to do an add
20 – is where the data to be added will be found (the address)
• The address will be sent back to the MAR
• Memmory will be searched and whatever is address ‘20’ will be
copied into the MDR.
• The value in the MDR can then be used as per instructed by the
CIR i.e Adding
• As the instruction is arithmetic it will be sent to the
Accumulator (a special storage register with the ALU) to be
carried out
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More on Buses
The signals being sent around the processor are again called
3 buses you need to remember are:
The Control Bus – The CU uses this to send commands to different
parts of the processor.
The Data Bus – carries data from one register to another.
The Address Bus – carries the location address to which the data
is going.
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