Fetch cycle (phase):
o The CPU puts the address of the instruction to be executed on the address bus. The
address information comes from the program counter (PC) maintained by the control
o The control bus holds the information for reading the memory location and the data
bus holds the instruction from the memory which is stored into the instruction register
o PC is updated to point to the next instruction.
Execute cycle (phase):
o Instruction in the IR is decoded
o The required data transfer and the required logical and arithmetic operation are
o The result is written back either to one of the registers or memory or I/O device
Common operations performed in ALU are:
o addition, subtraction,
o logical AND, OR, XOR, NOT
o increment, decrement, shift, clear, etc.
CPU on a single IC
The terms CPU, µP and MPU (microprocessor unit) are synonymous
CPU is the combination of the ALU and control unit of any computer.
When the CPU is a single IC; it is called a µP and is also referred to as the MPU.
When the MPU is connected to memory and I/O, the arrangement becomes a
Many different MPUs are produced by many different IC vendors (e.g., Motorola, Intel,
Texas Instruments, Zilog, National Semiconductors, etc.).
Major differences are in
o word size,
o the number and types of instructions that can be performed,
o the types of external control signals available,
o the amount of memory that can be addressed.