Exam Board:
OCR A-Level
1.1 - Structure and Function
of the Processor
Specification:
Computer Science H446
Watch on YouTube:
CPU components
Registers
Buses
The FDE cycle
CPU performance
Pipelining
Processor architecture
The Central Processing Unit (CPU) is the most important component in every computer system.
The purpose of the CPU is to process data and instructions by constantly repeating the fetch-decode-execute cycle. In this cycle, instructions are fetched from RAM and transferred into the registers of the CPU to be decoded and executed.
CPU Components
The CPU has three key components:
​
-
The control unit directs the flow of data and instructions inside the CPU and manages the FDE cycle, especially decoding instructions.
​​
-
The arithmetic logic unit (ALU) performs all arithmetic calculations and logical operations inside the CPU.
​​
-
Registers are small, ultra-fast storage locations that temporarily hold data, instructions or addresses during processing.
​
The CPU also contains cache memory, which is temporary storage space for frequently accessed data.
Registers
A register is a small storage space for temporary data, instructions or addresses in the CPU. Each register has a specific role in the FDE cycle:​
​
-
The Program Counter (PC) ​stores the memory address of the next instruction to be fetched from RAM.​
​​
-
The Memory Address Register (MAR) stores the memory address currently being accessed, which may be an instruction or data.
​​
-
The Memory Data Register (MDR)​ stores the data that is transferred from RAM to the CPU.​
​​
-
The Current Instruction Register (CIR) stores the instruction that has been fetched from RAM.​​
​
-
The Accumulator (ACC) ​stores data currently being processed and the result of calculations or logical operations made by the ALU.
Buses
Data and signals are transmitted between components across internal connections called buses.
​
There are three types of computer bus:
​
-
The data bus transmits data and instructions between the CPU, memory and other components such as input/output devices. It is bidirectional (data is sent both ways).
​​
-
The address bus transmits the location in memory that the CPU is accessing. It is unidirectional (one-way) from the CPU to RAM.
​​
-
The control bus transmits control signals (e.g. 'read' or 'write') from the CPU to coordinate other components. It is bidirectional.
The FDE Cycle
In the Fetch Decode Execute (FDE) cycle, instructions are fetched from RAM, then decoded (understood) and executed (processed) in the CPU.
​
This cycle is performed by the CPU millions of times every second using the registers and buses explained above.
​
This cycle is how the CPU processes data and instructions for each program or service that requires its attention.
CPU Performance
The performance of the CPU is affected by three main factors:
​
-
Clock speed is the number of cycles per second, so a higher clock speed means more instructions can be executed per second.
​​
-
The number of cores is important as more cores allow a CPU to carry out multiple instructions simultaneously, improving multitasking and parallel processing.
​​
-
Cache memory is small and very fast memory inside the CPU that stores frequently used instructions, reducing the time needed to access RAM.
Pipelining
Pipelining is the concurrent processing of multiple instructions.
An instruction can be fetched while another is decoded and another is executed.
The output of one process is often the input of the next, especially for uses such as graphics processing.
Computer Architecture
Computer architecture refers to the design and organisation of a system’s components and how they interact. There are two types of architecture to know:
​
-
Von Neumann architecture uses a single main memory (RAM) that stores both program instructions and data.
​​
-
Harvard architecture separates the storage of program instructions and data into two different memory locations.
You also need to know about contemporary (modern) architecture features, which include onboard graphics, performance boosting mode and virtual cores.
Questo's Key Terms
​Components of the CPU: Control Unit - ALU - Registers
​​​
Registers: PC - MAR - MDR - CIR - ACC
​​​​​​
Buses: Data bus - Address bus - Control bus
​​​
FDE Cycle: Fetch stage - Decode stage - Execute stage
​​​​​​​
CPU Performance: Clock speed - Number of cores - Cache memory
​​​​
Pipelining: Pipelining
​​​​​​​
Computer architecture: Von Neumann - Harvard - Contemporary
Did You Know?
The Apollo Guidance Computer (AGC) for NASA's Apollo 11 mission, when humans first set foot on the moon, had a CPU clock speed of about 1 megahertz - slower than many GCSE-level calculators used today.
