1.1.1. Structure and function of the
processor
What does CPU Stand for? - answer Central Processing Unit
What is the purpose of the CPU? - answer It fetches, decodes and executes
instructions; in other words - it processes data
What does it mean to "decode" an instruction? - answer Decoding an instruction means
that the opcode and operand of an instruction are analyzed to determine what needs to
be done to execute the instruction
i.e. specify the instruction type and the address mode used, fetch any required data, co-
ordinate the sequence of micro-operations.
What is opcode? - answerOpcode is used to determine the type of instruction and what
hardware is used to execute it.
What is the operand? - answerThe operand is the address of where the operation is
performed.
What is the purpose of the control unit? - answerThe Control Unit controls and
coordinates the activities of the CPU by organising the sequence in which program
instructions are executed, followed by decoding the instructions.
This means that the control unit is responsible for directing the operations of all the
other components of the processor.
What are some of the tasks that the control unit is responsible for? (4) - answer• It uses
control signals to enable data to be read (i.e. loaded) from and written (i.e. stored) to the
main memory
• It decodes every instruction that the processor will execute to determine what needs to
be done (for example, perform an arithmetic calculation, a logic operation, a branching
to another instruction, loading or storing data to the main memory)
• It organises the sequence of micro-operations that need to be performed in order to
carry out an instruction
• It uses control signals to determine the operation the ALU will carry out at each
instance
, What is the purpose of the Arithmetic and logic unit? - answerThe ALU is responsible
for performing arithmetic calculations and logical operations that include:
• addition, subtraction, multiplication, division
• logical bitwise operations, such as AND, OR, NOT, and XOR
• comparisons between values, such as greater than, less than, equal to
• shifting binary patterns to the left or right
What are registers? - answerRegisters are locations of computer memory within the
processor that provide a means of extremely fast access to sets of information.
How many registers are there in a computer? - answerA system has many general-
purpose registers, but there are 6 primary, special-purpose registers we need to know
about
What's the deal with general-purpose registers? - answerHaving to access the main
memory can slow down the execution of instructions.
Therefore, the processor uses registers to temporarily store and access the results of
operations.
The ALU is connected to a set of general-purpose registers that are used to store the
results of the intermediate calculations that are produced as part of a larger
computation.
What's the deal with special-purpose registers? - answerDedicated or special-purpose
registers are processor-based are register which have a specific function.
The registers used in the fetch-decode-execute cycle are special purpose registers
because each one of them has a specific role to play during each phase.
What are the 6 special-purpose registers? - answer• Program Counter (PC)
• Memory Address Register (MAR)
• Status Register (SR)
• Memory Data Register
• Current Instruction Register (CIR)
• Accumulator
What is the role of a Program Counter? - answerIt is an incrementing counter that holds
the memory address of the next instruction to be executed
processor
What does CPU Stand for? - answer Central Processing Unit
What is the purpose of the CPU? - answer It fetches, decodes and executes
instructions; in other words - it processes data
What does it mean to "decode" an instruction? - answer Decoding an instruction means
that the opcode and operand of an instruction are analyzed to determine what needs to
be done to execute the instruction
i.e. specify the instruction type and the address mode used, fetch any required data, co-
ordinate the sequence of micro-operations.
What is opcode? - answerOpcode is used to determine the type of instruction and what
hardware is used to execute it.
What is the operand? - answerThe operand is the address of where the operation is
performed.
What is the purpose of the control unit? - answerThe Control Unit controls and
coordinates the activities of the CPU by organising the sequence in which program
instructions are executed, followed by decoding the instructions.
This means that the control unit is responsible for directing the operations of all the
other components of the processor.
What are some of the tasks that the control unit is responsible for? (4) - answer• It uses
control signals to enable data to be read (i.e. loaded) from and written (i.e. stored) to the
main memory
• It decodes every instruction that the processor will execute to determine what needs to
be done (for example, perform an arithmetic calculation, a logic operation, a branching
to another instruction, loading or storing data to the main memory)
• It organises the sequence of micro-operations that need to be performed in order to
carry out an instruction
• It uses control signals to determine the operation the ALU will carry out at each
instance
, What is the purpose of the Arithmetic and logic unit? - answerThe ALU is responsible
for performing arithmetic calculations and logical operations that include:
• addition, subtraction, multiplication, division
• logical bitwise operations, such as AND, OR, NOT, and XOR
• comparisons between values, such as greater than, less than, equal to
• shifting binary patterns to the left or right
What are registers? - answerRegisters are locations of computer memory within the
processor that provide a means of extremely fast access to sets of information.
How many registers are there in a computer? - answerA system has many general-
purpose registers, but there are 6 primary, special-purpose registers we need to know
about
What's the deal with general-purpose registers? - answerHaving to access the main
memory can slow down the execution of instructions.
Therefore, the processor uses registers to temporarily store and access the results of
operations.
The ALU is connected to a set of general-purpose registers that are used to store the
results of the intermediate calculations that are produced as part of a larger
computation.
What's the deal with special-purpose registers? - answerDedicated or special-purpose
registers are processor-based are register which have a specific function.
The registers used in the fetch-decode-execute cycle are special purpose registers
because each one of them has a specific role to play during each phase.
What are the 6 special-purpose registers? - answer• Program Counter (PC)
• Memory Address Register (MAR)
• Status Register (SR)
• Memory Data Register
• Current Instruction Register (CIR)
• Accumulator
What is the role of a Program Counter? - answerIt is an incrementing counter that holds
the memory address of the next instruction to be executed
