Machine instruction cycle
1. Fetch the instruction
The next instruction is fetched from the memory address that
is currently stored in the Program Counter (PC), and stored in the Instruction
register (IR). At the end of the fetch operation, the PC points to the next
instruction that will be read at the next cycle.
2. Decode the instruction
The decoder interprets the instruction. During this cycle the
instruction inside the IR (instruction register) gets decoded.
3. Execute
The Control Unit of CPU passes the decoded information as a
sequence of control signals to the relevant function units of the CPU to
perform the actions required by the instruction such as reading values from
registers, passing them to the ALU to perform mathematical or logic functions
on them, and writing the result back to a register. If the ALU is involved, it
sends a condition signal back to the CU.
4. Store result
The result generated by the operation is stored in the main
memory, or sent to an output device. Based on the condition of any feedback
from the ALU, Program Counter may be updated to a different address from which
the next instruction will be fetched.
Data Bus
A data bus is a connection between the different parts of a
computer that information is sent on.
Address Bus
The address bus is a data bus that is used to specify a
physical address. A CPU will specify the memory location.