The CPU contains a control unit that decodes instructions and issues signals to other units to execute them. It obtains instructions from memory. The ALU performs arithmetic and logical operations. A microprocessor is a CPU contained on a single chip. The CPU contains registers like the MAR, MBR, PC, IR, accumulator, and I/O registers that help process instructions and transfer data.

1. CPU and Memory

3. CPU(Central processing unit)

4. Control Unit

5. • Decoder – It has necessary circuitry to decode and interpret the meaning of
every instruction supported by the CPU
• Each instruction is accompanied by microcode – very basic direction that
tells the CPU how to execute the instruction
• Control unit obtains instructions from programs stored in main memory,
interpret the instruction, and issues signals that cause other units of the
system to execute them.

6. ALU
• When the entire CPU is contained on a single tiny silicon chip, it is
called a microprocessor

7. Instruction set

8. Registers
• Although the number of registers varies from computer to computer,
there are some registers common to all
computers(MAR,MBR,PC,IR,A,I/OR)

10. 1. MAR – it holds the address of the active memory location.
2. MBR – it holds the contents of the accessed memory word. To store a word
in a memory, the system first transfers it to MBR and then writes it in
memory from MBR.
3. PC – it holds the address of the next instruction for execution.
4. AC(accumulator) – it hold the data on which the system has to operate,
intermediate results, and results of operations performed. In many
computers, there are more than one accumulator registers
5. IR- it holds the current instruction under execution.As soon as the
instruction is stored in this register, its operation and address part are
separated. The system sends the address part to MAR and while it sends
the operation part to control unit.
6. I/O Register – An input device transfers all input information's to this
register. The system transfers all output information to this register and an
output device picks up data for output from here.

11. Execution of an instruction by CPU