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Computer Architecture and the Fetch-Execute Cycle FetchThe Fetch-Decode-ExecuteFetch-Decode-ExecuteReset Cycle
Learning ObjectivesDescribe in simple terms the fetch / decode / execute / reset cycle and the effects of the stages of the cycle on specific registers.
The Fetch-Decode-ExecuteFetch-Decode-ExecuteReset CycleThe following is an algorithm in diagrammatic form that shows the steps in the cycle. It is the control unit which controls and synchronises this cycle.
Loads / copies, decodes and executes.
At the end the cycle is reset and the algorithm repeated.
Key for following slides:PC
Program Counter Memory Address Register Memory Data Register Current Instruction Register
MAR
MDR
CIR
Idle RegistersNote that during the Fetch - Decode Execute - Reset cycle demonstrated on the following slides, some registers are idle (not being used). This will be discussed and resolved in the next lesson.
Fetch
CPUMAR
MemoryCopy of instruction in memory address held in MAR
PC incremented by 1 Copy of address of PC next instruction
MDRInstruction
CIR
Decode
CPUMAR
PC
MDR
CIRSplit instruction into operation code & address if present. Then decode operation code.
Execute
CPUMAR
PC
Execute instruction (What is involved in thisdepends on the instruction being executed - demonstrated on the following slides).
MDR
CIR
Click an instruction or move on to see each instruction in turn.
Jump Input / Load (number directly) Input / Load (from memory) Store Add (a number directly) Add (a number from memory) Output (directly from accumulator) Output (from memory)
Jump instructionExecute Diagram
Execute Jump
CPUMAR
PC
MDR
CIRBack to list of instructions
Input / Load (number directly) into accumulator instructionExecute Diagram
Execute Input / Load (number directly) into accumulator
CPUPC MAR MDR AccumulatorNumber inputted / to be loaded.
CIRBack to list of instructions
Reason for the CIR & MDRAs you can see the MDR is used to store the number inputted / to be loaded during the execution of this Input / Load instruction. Therefore, if there was no CIR register to hold the Input / Load instruction and as no register can hold more than one thing at a time the control unit would lose the Input / Load instruction.
i.e. It would no longer know what it was supposed to do.
You will find that the contents of the MDR may be modified for similar reasons during other later instructions.Back to list of instructions
Load (from memory) instructionExecute Diagram
Execute Load (from memory)
CPUMAR
MemoryCopy of data in address held in MAR
PC
MDR Accumulator CIRBack to list of instructions
Reason for the PC & MARAs you can see the MAR is now used to store the address part of instruction during the execution of this Load (from memory) instruction. Therefore if there was no MAR register the PC would used to hold this address so the control unit would no longer know the correct address of the next instruction.
You will find that the contents of the MAR may be modified for similar reasons during other later instructions.Back to list of instructions
Store instructionExecute Diagram Assume data has either been inputted, loaded (directly or from memory) or a calculation has been performed. Any of the above will mean there is data in the accumulator and it is this data that will be stored.
Execute Store
CPUMAR
MemoryCopy of data in MDR stored in memory address held in MAR
PC
MDR Accumulator CIRBack to list of instructions
Add (a number directly) instructionExecute Diagram Assume a number has already been inputted or loaded (directly or from memory) into the accumulator.
Execute Add (a number directly)
CPUPC MAR
MDR
ALUAccumulatorNB. The ALU now does the arithmetic. Accumulator value is now the result of the addition. i.e. Accumulator = Accumulator + contents of MDR
Number to be added.
CIRBack to list of instructions
Add (a number from memory) instructionExecute Diagram (Assume a number has already been inputted or loaded into the accumulator.)
Execute Add (from memory)
CPUMAR
MemoryCopy of number in memory address held in MAR
PC
MDR
ALUAccumulatorNB. The ALU now does the arithmetic. Accumulator value is now the result of the addition. i.e. Accumulator = Accumulator + contents of MDR
CIRBack to list of instructions
Output (directly from accumulator) instructionExecute Diagram
Execute Output (directly from accumulator)
CPUPCOutput data in accumulator
MAR MDR
Accumulator
CIRBack to list of instructions
Output (from memory) instructionExecute Diagram
Execute Output (from memory)
Memory
CPUMARCopy of data in memory address held in MAR
PC
Output data in accumulator
MDR
Accumulator
CIRBack to list of instructions
Reset
CPUPCCycle is reset (restarted) by passing control back to the PC.
Idle RegistersNote that during the Fetch - Decode Execute - Reset cycle demonstrated on the previous slides, some registers were idle (not being used). This will be discussed and resolved in the next lesson.
Fetch Decode - Execute Reset Cycle in writingThe following slides describe the cycle in writing.
i.e.What you will have to do when answering an exam question.
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. Fetch
MDR is used whenever anything is to go from the CPU to main memory, or vice versa. So that it contains the address of the next instruction, assuming that the instructions are in consecutive locations.
3. Increment the PC by 1.
4. Load the instruction/data that is now in the MDR into the CIR.
Thus the next instruction is copied from memory -> MDR > CIR.
5. Contents of CIR split into operation code and address if present e.g. store, add or jump Decode instructions. 6. Decode the instruction that is in the CIR.
6. Execute the instruction but what is involved in this depends on the instruction being executed (there are several different instructions you need to know about). If the instruction is a jump instruction then
Load the address part of the instruction in the CIR into the PC.
Execute
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 CIR into MAR. Copy data from memory address held in MAR to MDR. Copy data in MDR into accumulator.
If the instruction is a store instruction then:
Copy address part of the instruction (to store in) in the CIR 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:
Execute
Copy address part of the instruction (of number to add) in the CIR 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).
If the instruction is an output (directly from accumulator) then output number in accumulator.
Execute
If the instruction is an output (from memory) instruction then:Copy address part of part of the instruction (of data to output) in CIR into MAR. Output contents of MDR.
Reset
7.
Cycle is reset (restarted) by passing control back to the PC (step 1).
PlenaryDescribe the fetch / decode part of the fetch / decode / execute / reset cycle, explaining the purpose of any special registers that you have mentioned.
PlenaryContents of PC loaded into MAR PC is incremented Contents of address stored in MAR loaded into MDR Contents of MDR loaded into CIR Instruction in CIR is decoded. PC (program counter) stores the address of the next instruction to be executed. MAR (memory address register) holds the address in memory that is currently being used MDR (memory data register) holds the data (or instruction) that is being stored in the address accessed by the MAR. CIR (current instruction register) holds the instruction which is currently being executed.