1. Processor Components Flashcards
What are the components of the CPU?
- Control Unit (CU)
- Buses
- Arithmetic Logic Unit (ALU)
- Dedicated registers
What are the roles of the Control Unit?
- Controls and co-ordinates the activities of the CPU
- Directs flow of data between CPU and other devices
- Accepts next instruction
- Decodes instruction into several sequential steps
- Manages the execution
- Stores resulting data in memory or registers
What is a bus?
A set of parallel wires connecting two or more components of a computer
Name the 3 buses.
- Address bus
- Data bus
- Control bus
How is the processor connected to the main memory?
Via the 3 buses
What happens when the CPU wishes to access a particular main memory location?
- Sends address to memory on address bus
- Data in address location returned to CPU on data bus
- Control signals sent along control bus
What are the 3 buses called collectively?
System bus
What is the role of the address bus?
To transmit memory addresses specifying where data is to be sent to or retrieved from
What is the role of the data bus?
To transport data and instructions between components
What are the roles of the control bus?
To transmit control signals between internal and external components
Co-ordinates use of address and data buses
Provides status information between system components
Is the address bus bi-directional?
No
Is the data bus bi-directional?
Yes
Is the control bus bi-directional?
Yes
Name the 6 control lines.
- Bus Request
- Bus Grant
- Memory Write
- Memory Read
- Interrupt Request
- Clock
What does the Bus Request control line do?
Indicates that a device is requesting the use of the data bus
What does the Bus Grant control line do?
Indicates that the CPU has granted access to the data bus
What does the Memory Write control line do?
Causes data on the data bus to be written into the addressed location
What does the Memory Read control line do?
Causes data from the addressed location to be placed on the data bus
What does the Interrupt Request control line do?
Indicates that a device is requesting access to the CPU
What does the Clock control line do?
Synchronises operations
How is memory divided internally?
Into units called words
What is a word?
A fixed size group of digits
What is the ALU used for?
Performing arithmetic (+, -, x, /) and logical (e.g. AND, OR, NOT, XOR, binary shifts) operations on data
What are registers?
Small memory cells that operate at very high speeds
What are registers used for?
Temporarily storing data
Site of arithmetic, logical and shift operations
Name the 5 registers.
- Program Counter (PC)
- Accumulator (ACC)
- Memory Address Register (MAR)
- Memory Data Register (MDR)
- Current Instruction Register (CIR)
What is the purpose of the PC register?
Holds address of next instruction to be executed
What is the purpose of the ACC?
Stores results from calculations
What is the purpose of the MAR?
Holds address of the location to be read from or written to
What is the purpose of the MDR?
Temporarily stores data that has been read or needs to be written
What is the purpose of the CIR?
Holds the current instruction being executed, divided into operand and opcode
What does the operand contain?
The data or address of the data which the operation is to be performed on
What does the opcode contain?
The type of operation that needs to be executed
What is the Fetch-Decode-Execute cycle?
Sequence of operations completed to execute an instruction
What happens during the Fetch phase?
- Address from PC is copied to MAR
- Instruction held at that address is copied to MDR via data bus
- Simultaneously, contents of PC incremented
- Value held in MDR is copied to CIR
What happens during the Decode phase?
- Contents of CIR divided into operand and opcode
- Operand may contain the following:
a. address of data to be used (copied to MAR)
b. actual data to be operated on (copied to MDR)
c. data to be operated on may be passed to ALU or
ACC
What happens during the Execute phase?
- Decoded instruction is executed
