Central Processing Unit (Instruction Circle)
- 1. THE CENTRAL PROCESSING UNIT INSTRUCTION CIRCLE Name: Afieq Khair Saswin Desmond Theva
- 2. INSTRUCTION CIRCLE • A program residing in the memory unit of the computer consist of a sequence of instruction • The program is executed in the computer by going through a cycle for each instruction • Each instruction cycle in turn is subvided into a sequence of sub cycle or phase
- 4. CIRCUITS USED • The circuits used in the CPU during the cycle are: • Program Counter(PC) – an incrementing counter that keeps track of the memory address of the instruction that is to be executed next • Memory address register(MAR) – holds the address of a memory block to be rad from or written to • Memory data register(MDR) – a two way register that holds data fetched from memory (and ready for the CPU to process) or data waiting to be stored in memory
- 5. CIRCUITS USED • Instruction register(IR) – a temporary holding ground for the instruction that has just been fetched from memory • Control unit(CU) – decodes the program instruction in the IR,selecting machine resources such as data source register and a particular arithmetic operation , and coordinates activation of those resources • Arithmetic logic unit(ALU) – performs mathematical and logical operations
- 6. PHASES OF INSTRUCTION CIRCLE • In basic computer,each instruction circle consists of the following phases: • Fetch an instruction from memory • Decode the instruction • Read the effective address from memory if the instruction has an indirect address • Execute the instruction
- 7. FETCHING THE INSTRUCTION • The next instruction is fetch from the memory address that is currently stored in the program counter(PC),and stored in the instruction register(IR) • Now,the PC points to the next instruction that will be read at the next circle
- 8. DECODE THE INSTRUTION • The decode interprets the instruction • During this phases the instruction inside the IR (instruction register) gets decoded
- 9. IN CASE OF A MEMORY INSTRUCTION • In case of a memory instruction(direct or indirect) the execution phase will be in the next clock pulse • Required data is fetched from main memory to be processed and them placed into data registers • During this phases the instruction inside the IR(instruction register)gets decoded • If the instruction is direct , nothing is done at this clock pulse • If this is an I/O instruction or a Register instruction,the operation is performed(executed) at clock Pulse
- 10. EXECUTE THE INSTRUCTION • The control unit of CPU passes the decode information as a sequence of control signals • Signals are passed to the relevant function units of the CPU to perform the action required • These action may be: • Reading values from registers • Passing them to the ALU to perform mathematical or logic functions on them • Writing the result back to a register • The result generated by the operation is stored in the main memory or sent to an output device
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