1. Chapter 20
Computer Operations
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2. Computers operate according to a set of instructions stored in the main memory.
The steps involved in the processing of an instruction is called the instruction cycle.
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3. The coding system used is the machine code.
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4. Interconnection between Computer Hardware Components
Functions of and
Interconnection between
Computer Hardware
Components
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5. Central Processing Unit (CPU)
It controls all the other components in the computer.
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6. It contains THREE main components:
Control unit (CU)
Arithmetic and logic unit (ALU)
Registers
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7. It sends and receives control signals to and from peripheral devices.
Control Unit
It sends and receives control signals to and from peripheral devices.
It interprets the instructions stored in main memory.
Computer Studies Today Chapter 20

8. It regulates the timing of all operations performed within the CPU.
It sequences the instructions to be executed.
It controls the flow of data between the CPU, main memory and peripheral devices.
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9. Three Main Components of the Control Unit
Instruction Register (IR)
It holds the current instruction fetched from main memory.
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10. It holds the address of the next instruction to be accessed.
Instruction Decoder
A logic circuit which interprets the current instructions stored in the instruction register.
Program Counter (PC)
It holds the address of the next instruction to be accessed.
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11. Control unit Main memory +1 PC 01 03 02 03 READ A 04 READ B 05IR
Instruction decoder
Main memory 01 02
03 READ A
04 READ B 05 06 03
READ A
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12. Arithmetic and Logic Unit
It performs addition, subtraction, multiplication, division and decision making.
Computer Studies Today Chapter 20

13. Two Main Components of the Arithmetic and Logic Unit
Accumulator (ACC)
A register which temporarily holds the intermediate or final result of arithmetic and logic operations.
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14. Status Register (SR)
It describe the status of the current contents of the accumulator and holds a collection of bits.
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15. Main Memory
It contains RAM, which is composed of a large number of memory locations.
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16. Four Main Components of Memory Unit
Main Memory
Memory Address Register (MAR)
It holds the addresses of the data or instructions to be accessed.
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17. Memory Data Register (MDR)
It holds the data or instructions read from or written to the main memory.
Address Decoder
It interprets the address held in the MAR.
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18. Control unit +1 PC IR Instruction decoder Memory unit 01 02 03 READ A
04 READ B 05 
MAR
MDR
Address decoder
Main memory 03 03
READ A
READ A
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19. Buses
It is a group of wires that is interconnected throughout the computer.
It is used to communicate among all components of the computer.
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20. Three Main Types of the Buses
Data Buses
They are lines which send data signals.
The number of lines represents the number of bits of the data.
Computer Studies Today Chapter 20

21. They are lines which send address signals of the data to be processed.
Address Buses
They are lines which send address signals of the data to be processed.
The more the address lines, the more the memory locations that can be addressed.
Computer Studies Today Chapter 20

22. The number of lines varies in different computer system.
Control Buses
They are lines which send control signals from the control unit to other parts of the computer.
The number of lines varies in different computer system.
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23. The interconnection of buses within components of a computer
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