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Fetch execute cycle
Transcript of Fetch execute cycle
The Fetch Execute Cycle describes the steps needed for processing to take place in a computer. It is made
up of four parts. Have a read through the description below, but don't worry if it doesn't make any sense.
There is a more friendly description below.
Fetch data from main memory
It reads the effective address from main memory if the instruction has an indirect address. Fetch required
data from main memory to be processed and placed into registers.
Execute the instruction
From the instruction register, the data forming the instruction is decoded by the control unit. It then
passes the decoded information as a sequence of control signals to the relevant function units of the CPU
to perform the actions required by the instruction such as reading values from registers, passing them to
the Arithmetic logic unit to add them together and writing the result back to a register. A condition
signal is sent back to the control unit by the ALU if it is involved.
The result generated by the operation is stored in the main memory, or
sent to an output device. Based on the condition feedback from the ALU, the PC is either incremented to
address the next instruction or updated to a different address where the next instruction will be fetched.
The cycle is then repeated.
repeat the process again and again and again until you computer dies.
Fetch the instruction from main memory
The CPU presents the value of the PC (program counter) on the address bus. The CPU then fetches the
instruction from main memory via the data bus into the MDR
(memory data register)
. The value from the
MDR is then placed into the current instruction register CIR
(current instruction register)
, a circuit that holds the instruction
temporarily so that it can be decoded and executed.
Fetch execute cycle
Imagine you're helping someone to bake a cake. Let's say it's a Carrot cake (because they are delicious).
You have fetched the instruction to bake a cake from the cake shop (step 1 from above) and so you know
a cake is required. Think of the cake shop as the main memory and the street to your bakery as the bus as
it's the link between the different parts. You place that instruction on a table in the bakery so that
everyone important knows where it is. The bakery is the MDR from step 1. There is
a table in the bakery where the next cake is prepared. This table is the CIR.