3.1.1.5 FDE Cycle & Clock (B01)
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fetch
Retrieve the next instruction from memory (as indicated by the program counter) and then increment the program counter
decode
Decode the value in the instruction register, which is a binary sequence, to work out what the instruction requires.
execute
Perform the action required by the instruction in the instruction register.
which components are used at each stage of the FDE cycle
F: CPU, RAM, bus D: CPU E: CPU
opcode
an action, such as LOAD, STOP, STORE, ADD etc
operand
the data that an opcode acts upon
How does the CPU know whether a binary sequence is an instruction, or just an item of data?
instructions and data are fetched at different times in the FDE cycle. Instructions are received in the “Fetch” part, whilst data might be fetched during the “Execute” part
pipelining
is a method of keeping all the components busy so as to improve overall CPU performance. Pipelining attempts to keep every component of the CPU busy at all times. Instructions flow through the CPU in stages. This means that an instruction can be completed every clock cycle. This increases CPU throughput allowing overall performance to be optimised. A CPU is called ‘fully pipelined’ if an instruction can be fetched every clock cycle
FDE cycle in full (full)
Instructions are loaded into RAM from secondary memory. The instructions are stored in RAM in numbered memory locations. The CPU sends a signal along the address bus requesting an instruction from a specific numbered location in RAM. The instruction is transferred via the data bus to the CPU. The CPU decodes the instruction. The CPU may fetch data held in memory if referenced in the instruction. The CPU executes the instruction. The result may be stored back into RAM. The cycle repeats
where is clock located
outside of CPU
clock speed
The clock speed is the number of clock pulses, and hence number of instructions a single processor can carry out per second. It is measured in hertz (meaning ‘number per second’). The higher the clock speed, the more cycles a CPU can complete per second. Obviously the higher the clock speed the faster programs are executed.
clock function
When a CPU receives a clock signal, it will execute one instruction per cycle. The clock emits pulses at fixed intervals (tiny fractions of a second), and these are used to coordinate and synchronise the activities of the CPU.
What factors apart from the speed of a processor affect the speed at which a computer executes programs?
what the processor is made of how many ‘cores’ a processor has got how much work is performed in one clock cycle
benchmarking
it gets two computers to complete the same task (e.g. calculate a complex Excel spreadsheet or run a certain section of a game) and then compares their performance (e.g. time taken or frames dropped).