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This is what the memory address register is for. A signal is then sent down to the address bus, to the RAM. The control unit sends out a memory read signal and the contents of the address are copied through the data bus to the memory data register. As our first instruction has been fetched, the system is at the end of the fetch stage of the cycle. The program counter can be incremented by one so the system is ready to read the next instruction.
It is sent via the data bus to the control unit. Here, the data is split into two sections. There is the operation code, or op code, which in this example is the first four bits.
This is the command that the computer will carry out. The second four bits of the operand. This is the address in RAM where the data to be operated on is stored. The control unit can translate op code into instructions, so here, the control unit translates the op code into a load from RAM instruction. The operand is copied to the MAR as this is the address of the data that needs to be loaded.
As it is not an instruction but simply data, it is then passed to the accumulator. This is a complete fetch, decode, execute cycle. We'll run through the next cycle a little faster so that you can see the entire program being executed. The instruction op codes and address operand is placed in the IR and the PC increase by one again. To finish off this stage, the two values in the accumulator are passed in through the ALU, where they can be added together as was instructed by the op code.
The results are then placed back into the accumulator. So that's a second cycle complete. The last cycle is for the instruction at It uses the op code , which is store, and the operand , which is the last address in the RAM shown. So this cycle takes the results of the addition in the accumulator and stores it back into RAM. The Program Counter PC starts at This means that the first address in RAM where the computer will look for an instruction is at A signal is now sent down through the Address bus to the RAM.
The Control Unit sends out a memory read signal, and the contents of the address are copied through the data bus to the Memory Data Register. As the data fetched during the fetch stage is an instruction, it is copied into the Instruction Register IR.
As the first instruction has been fetched, the system is at the end of the Fetch stage of the cycle. The program counter can be incremented by 1, so the system is ready to read the next instruction when the next Fetch cycle starts. Now the instruction needs to be decoded.
It is sent via the data bus to the control unit, where it is split into two parts. The first part is the operation code or opcode , which in this example CPU is the first 4 bits. The second part, in this case the second 4 bits, is the operand.
This is an address in RAM where data will be read from or written to, depending on the operation. The Control Unit can translate opcodes into instructions.
Now the command will be executed. The operand is copied to the MAR, as this is the address of the data that needs to be loaded. As it is not an instruction but simply data, it is then passed to the Accumulator Acc. The PC now holds , so we next fetch, decode and execute the instruction at that address. So this cycle takes the results of the addition in the accumulator and stores it back into RAM at address , as requested.
There is a lot of information to take in to understand the Fetch, Decode, Execute cycle. Many people find it confusing when following it for the first time. You may find it helpful to run through this activity a 2nd time. Learn how data is represented through media; audio, visual and text. Supported by Google. Included in Unlimited. Improve your ability to teach maths and logic in computing while building elements of an escape room. Understand how components of a computer system interact with each other on this online course for teachers supported by Google.
Close transcript. Decode Now the instruction needs to be decoded. Execute Now the command will be executed. This is a complete Fetch Decode Execute Cycle. The PC is at , so this is the next instruction to be fetched. The instruction opcode and address operand are placed in the IR, and the PC increased by 1 again. Now the instruction is decoded, while the address of the data to be acted upon is placed in the MAR.
The instruction turns out to be ADD, which adds two pieces of data together. The new data is fetched from the address and eventually ends up in the accumulator, along with the results of the previous cycle.
To finish off this stage, the two values in the accumulator are passed into the ALU, where they can be added together, as was instructed by the opcode.
The result is then placed back into the accumulator. The 3rd cycle The last cycle is for the instruction at To recap, our program used three instructions to add two numbers and store the result in memory: The first instruction LOADed a piece of data from a specified address. The second ADDed this to the data found in another address.
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Fetch, decode, execute
The instruction cycle also known as the fetch—decode—execute cycle , or simply the fetch-execute cycle is the cycle that the central processing unit CPU follows from boot-up until the computer has shut down in order to process instructions. It is composed of three main stages: the fetch stage, the decode stage, and the execute stage. In simpler CPUs, the instruction cycle is executed sequentially, each instruction being processed before the next one is started. In most modern CPUs, the instruction cycles are instead executed concurrently , and often in parallel , through an instruction pipeline : the next instruction starts being processed before the previous instruction has finished, which is possible because the cycle is broken up into separate steps.
Machine Level Architecture: The Fetch–Execute cycle and the role of registers within it