1.1 Systems Architecture
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Computer
An electronic device which takes an input, decodes and outputs a set of instructions given by the user.
Purpose of the CPU
To fetch, decode and execute instructions.
What is the FDE Cycle
A repetitive cycle where the CPU fetches, decodes and executes instructions given by the user.
Events that take place during the FDE Cycle
• The CPU receives an instruction from memory. • The CPU decodes the instruction. • The decoded instruction is executed. • The PC is incremented. • The process is repeated.
CPU Components
• Arithmetic Logic Unit (ALU) • Control Unit (CU) • Cache • Registers
ALU
Responsible for carrying out all arithmetic calculations and making logical decisions.
CU
Sends signals to control how data moves around the CPU.
Cache
Provides fast access to data and instructions that are used frequently.
CPU Registers
• Program Counter (PC) • Memory Address Register (MAR) • Memory Data Register (MDR) • Accumulator (ACC)
What are registers?
Tiny, superfast pieces of onboard memory inside the CPU.
PC
Holds the address of the next instruction in memory.
MAR
Holds the address of where data is to be fetched or stored.
MDR
Holds the data fetched from, or to be written to memory
ACC
Holds the results of calculations performed in the CPU
Von Neumann Architecture
Processor architecture that stores both instruction data and program data in the same memory in binary form. There is no way to know if the pure binary represents instructions or data.
Characteristic of Von Neumann Architecture
• CPU • Single Control Unit • ALU • Onboard cache • Internal clock • Memory unit (typically RAM)
Factors Affecting CPU Performance
• Clock Speed • Cache size • Number of cores
What is Clock speed?
Number of FDE cycles per second that the CPU can perform, measured in Hz. 3.2Ghz = 3.2bil instructions fetched per second.
What is Cache?
Small amount of memory that is closer to the CPU than RAM. Used to temporarily hold instructions and data that the CPU is likely to reuse.
What is a core?
A complete copy of a CPU.
Effect of Clock Speed on CPU Performance
The higher the clock speed, the more instructions that the CPU can execute per second, increasing the overall speed of the computer.
Effect of Cache Size on CPU Performance
The greater the cache size, more data and instructions can be fetched more quickly, as it is closer than RAM.
Effect of Number of Cores on CPU Performance
The more cores a computer has, the faster it can process instructions as it has multiple CPUs.
Disadvantages of multiple cores
• Doubling the number of cores doesn’t necessarily double overall speed as the cores have to communicate with each other, which takes time. • Many programs aren’t designed for multiple cores.