Instruction CyclePhases of Instruction Cycle
Unit 3 - CONTROL UNIT DESIGN
Abhineet Anand
Computer Science and Engg. DepartmentUniversity of Petroleum and Energy Studies, Dehradun
November 26, 2012
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN
Instruction CyclePhases of Instruction Cycle
IntroductionDiagramCircuit Used
Instruction Cycle
A program residing in the memory unit of the computerconsist of a sequence of Instructions.The program is executed in the computer by going througha cycle for each instruction.Each instruction cycle in turn is subdivided into a sequenceof sub cycle or phase.
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN
Instruction CyclePhases of Instruction Cycle
IntroductionDiagramCircuit Used
Instruction Cycle
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN
Instruction CyclePhases of Instruction Cycle
IntroductionDiagramCircuit Used
Circuits Used
The circuits used in the CPU during the cycle are:Program counter (PC) - an incrementing counter thatkeeps track of the memory address of the instruction that isto be executed next.Memory address register (MAR) - holds the address of amemory block to be read from or written to.Memory data register (MDR) - a two-way register thatholds data fetched from memory (and ready for the CPU toprocess) or data waiting to be stored in memory.
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN
Instruction CyclePhases of Instruction Cycle
IntroductionDiagramCircuit Used
Circuits Used Contd..
Instruction register (IR) - a temporary holding ground forthe instruction that has just been fetched from memoryControl unit (CU) - decodes the program instruction in theIR, selecting machine resources such as a data sourceregister and a particular arithmetic operation, andcoordinates activation of those resourcesArithmetic logic unit (ALU) - performs mathematical andlogical operations
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN
Instruction CyclePhases of Instruction Cycle
Fetching the instructionDecode the instructionIn case of a memory instructionExecute the instruction
Phases of Instruction Cycle
In basic computer, each instruction cycle consists of thefollowing phases:
Fetch an instruction from Memory.Decode the instruction.Read the effective address from memory if the instructionhas an indirect address.Execute the instruction.
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN
Instruction CyclePhases of Instruction Cycle
Fetching the instructionDecode the instructionIn case of a memory instructionExecute the instruction
Fetching the instruction
The next instruction is fetched from the memory addressthat is currently stored in the program counter (PC), andstored in the instruction register (IR).Now, the PC points to the next instruction that will be readat the next cycle.
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN
Instruction CyclePhases of Instruction Cycle
Fetching the instructionDecode the instructionIn case of a memory instructionExecute the instruction
Decode the instruction
The decoder interprets the instruction.During this phases the instruction inside the IR (instructionregister) gets decoded.
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN
Instruction CyclePhases of Instruction Cycle
Fetching the instructionDecode the instructionIn case of a memory instructionExecute the instruction
In case of a memory instruction
In case of a memory instruction (direct or indirect) theexecution phase will be in the next clock pulse.Required data is fetched from main memory to beprocessed and then placed into data registers.During this phases the instruction inside the IR (instructionregister) gets decoded.If the instruction is direct, nothing is done at this clockpulse.If this is an I/O instruction or a Register instruction, theoperation is performed (executed) at clock Pulse.
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN
Instruction CyclePhases of Instruction Cycle
Fetching the instructionDecode the instructionIn case of a memory instructionExecute the instruction
Execute the instruction
The control unit of CPU passes the decoded informationas a sequence of control signals.Signals are passed to the relevant function units of theCPU to perform the actions required.These action may be :
Reading values from registers,Passing them to the ALU to perform mathematical or logicfunctions on them, orWriting the result back to a register
The result generated by the operation is stored in the mainmemory, or sent to an output device.
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN
Instruction CyclePhases of Instruction Cycle
Fetching the instructionDecode the instructionIn case of a memory instructionExecute the instruction
The Fetch Execute cycle in Transfer Notation
Expressed in register transfer notation:MAR <- PCMDR<- Memory {MAR address}; PC <- [PC]+1 (Incrementthe PC for next cycle at the same time)IR<- MDR
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN
Instruction CyclePhases of Instruction Cycle
Fetching the instructionDecode the instructionIn case of a memory instructionExecute the instruction
THANK YOU
Abhineet Anand Unit 3 - CONTROL UNIT DESIGN