2. CPU & ITS EXECUTION OF
INSTRUCTION
Rajishma T
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3. Disclaimer: This presentation is prepared by trainees of
baabtra as a part of mentoring program. This is not official
document of baabtra –Mentoring Partner
Baabtra-Mentoring Partner is the mentoring division of baabte System Technologies Pvt .
Ltd

4. CPU
• Cpu or central processing unit is the brain of
every computer.
• Structure of a cpu:

5. • ALU
– The arithmatic/logic unit contains electronic circuitry that
executes all arithmatic and logical operations.
• CONTROL UNIT
– The control unit of the CPU contains cicuitry that uses
electrical signals to direct the entire computer system to carry
out,or execute,stored program instructions.
– The control unit must communicate with both the
arithmetic/logic unit and memory.

6. System bus
• Control bus
– The control bus is used by the CPU to direct and monitor the
actions of the other functional areas of the computer.
• Address bus
– The address bus consists of all the signals necessary to define any
of the possible memory address locations within the computer.
– The control bus is used by the CPU to direct and monitor the
actions of the other functional areas of the computer.
• Data Bus
– The bidirectional data bus, sometimes called the memory
bus, handles the transfer of all data and instructions between
functional areas of the computer.

7. Registers
• General purpose registers:
– A general purpose register is a memory cell.
– Each general purpose register has a unique name
– It is used to store intermediate result of complex computation
• Accumulator
– This Register is used for storing the Results those are produced by the
System..

8. • Instruction register:
– Instruction register Contains the current instruction being executed by
the CPU.
– The CPU will perform the operation indicated by the instruction code
contained in the instruction register.
• MAR(memory address register):
– This register holds the memory addresses of data and instructions.
– This regster is used to access data and instructions from memory during
the execution of an instruction.

9. • MDR(Memory data register)
– It is the register that contains the data to be stored in the
computer storage or data after a fetch from the computer
storage.
– It acts like a buffer and holds anything that is copied from the
memory ready for the processor to use it,
• Index register:
– A hardware element which holds a number that can be added
to (or, in some cases, subtracted from) the address portion of
a computer instruction to form an effective address.
– An index register in a computer's CPU is a processor register
used for modifying operand addresses during the run of a
program.

10. • Data register:
– A register used in microcomputers to temporarily store data
being transmitted to or from a peripheral device.
• Program counter:
– This register contains the address of the next instruction
– The value in the program counter will be increased after the
cpu finishes executing one instruction.

11. • Process status register:
– This register contains the various information about the cpu.
– For ex: the result of a comparison operation.
– When the CPU compares 2 numbers a and b the outcome of
the comparison is stored in the PSR The outcome of a
compare operation will allow the CPU to determine the
following fact between a and b
• Equal
• Not equal
• Greater than
• Greater than or equal
• Less than
• Less than or equal

12. Types of instruction
• Cpu executes following 3 types of instructions
• Arithmatic and logic operation
• Eg: +,-,and,or
• The result of an arithmatic operation is often stored
in general purpose registers.
• Memory transfer instruction
• Transfer the content from some specific memory
location to a specific register(memory cell) in the cpu
and vice versa

13. • Branch instructions
– A branch instruction will cause the cpu to branch or jump to
the specified location in memory.
– after the jump has occurred the cpu will continue to execute
the instructions in sequence until another branch or jump
instruction is encountered
– There are two types of branch instructions:
• A unconditional branch instruction always cause
the cpu to jump to the target location.
• A conditional branch instruction will only cause
the cpu to jump to the target location when the
specified condition is met.

14. Instruction execution cycle
• Fetch the instruction
• Decode the instruction
• Execute the instruction
• Store the instruction

15. fetch
1. Load the address of next instruction in the PC into the MAR.
– So that the control unit can fetch the instruction from the right part
of the memory.
2. Copy the instruction/data that is in the memory address
given by the MAR into the MDR.
– MDR is used whenever anything is to go from the CPU to main
memory, or vice versa.
3. Increment the PC by 1.
– So that it contains the address of the next instruction, assuming that
the instructions are in consecutive locations.
4. Load the instruction/data that is now in the MDR into the IR.
– Thus the next instruction is copied from memory -> MDR -> IR.

16. decode
1. Contents of IR split into operation code and
address if present e.g. store, add or jump
instructions.
2. Decode the instruction that is in the IR

17. execute
• If the instruction is a jump instruction then
– Load the address part of the instruction in the IR into the PC.
• If the instruction is an input / load (directly) instruction then
take data input and place in accumulator.
• If the instruction is a load (from memory) instruction.
– Copy address part of the instruction (to load from) in the IR into MAR.
– Copy data from memory address held in MAR to MDR.
– Copy data in MDR into accumulator

18. • Execute contd
• If the instruction is a store instruction then:
– Copy address part of the instruction (to store in) in the IR into MAR.
– Copy data in accumulator to MDR.
– Copy data in MDR into memory address held in MAR.
• If the instruction is an add instruction then:
– Copy address part of the instruction (of number to add) in the IR
into MAR.
– Copy number from memory address held in MAR into MDR.
– Add number in MDR to number in accumulator (accumulator will
now hold the result).

19. • If the instruction is an output (from memory) instruction
then:
– Copy address part of part of the instruction (of data to output) in IR
into MAR.
– Output contents of MDR.

20. Store the output
• Result of calculations in cpu stored in main
memory or sent to output devices.
• program Counter could be updated to a new
address

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