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Learn about how computers operate based on instructions stored in memory, the functions of CPU components, memory unit components, and the interconnection between hardware parts.
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Chapter 20 Computer Operations
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.
20.1 Functions of and Interconnection between Computer Hardware Components
Central Processing Unit (CPU) • It controls all the other components in the computer.
It contains THREE main components: • Control unit (CU) • Arithmetic and logic unit (ALU) • Registers
Control Unit • It sends and receives control signals to and from peripheral devices. • It interprets the instructions stored in main memory.
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.
Three Main Components of the Control Unit • Instruction Register (IR) • It holds the current instruction fetched from main memory.
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.
Control unit Main memory +1 PC 01 02 03 READ A 04 READ B 05 06 Instruction decoder IR 03 READ A
Arithmetic and Logic Unit • It performs addition, subtraction, multiplication, division and decision making.
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.
Status Register (SR) • It describe the status of the current contents of the accumulator and holds a collection of bits.
Main Memory • It contains RAM, which is composed of a large number of memory locations.
Four Main Components of Memory Unit • Main Memory • Memory Address Register (MAR) • It holds the addresses of the data or instructions to be accessed.
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.
Control unit Memory unit +1 Main memory PC MAR 01 02 03 READ A 04 READ B 05 Instruction decoder IR Address decoder MDR 03 03 READ A READ A
Buses • It is a group of wires that is interconnected throughout the computer. • It is used to communicate among all components of the computer.
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.
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.
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.
The interconnection of buses within components of a computer