Systems Architecture

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1.1.1 The Fetch-Execute Cycle

A computer is an electronic device that takes input, processes data and delivers output. The purpose of the CPU is to fetch, decode and execute instructions. An instruction is copied from main memory into the CPU (FETCH). The Control Unit interprets the instruction (DECODE). The instruction is carried out (EXECUTE).

1.1.1 CPU Components And Their Functions

The CPU consists of an arithmetic logic unit (ALU), control unit (CU), cache and registers. The ALU performs calculations and logical decisions. The CU sends signals to control how data moves around the CPU. The cache provides fast access to frequently used instructions and data. The registers are tiny, superfast pieces of onboard memory in the CPU each with a specific purpose.

1.1.1 Von Neumann Architecture

It is based on the stored-program concept. Instruction data and program data are stored in the same memory in binary form. The Program Counter (PC) holds the address of the next instruction in memory. The Memory Address Register (MAR) holds the address of where data is to be fetched or stored. The Memory Data Register (MDR) holds data fetched from, or to be written to memory. The Accumulator holds the results of calculations.

1.1.2 Characteristics Of CPUs

A CPU's performance can be affected by clock speed, cache size and number of cores. Clock speed is the number of cycles per second and is measured in Hertz (Hz). 3.2GHz clock speed means 3.2 billion instructions can be fetched per second. A faster clock speed allows more instructions to be fetched, decoded and executed at the same time. The cache temporarily stores data and instructions being read to and written from RAM. It is much faster to find data from a smaller cache and it avoids going and getting instructions and data from memory as it costs time. A core is a complete copy of a CPU, so quad core CPU would have 4 separate processing units, each with its own ALU, CU, registers and accumulator. Doubling the number of cores might not double the overall speed because CPU cores take time to communicate with each other and many programs are not designed to make use of multiple cores.

1.1.3 Embedded Systems

An embedded system is a computer system with a dedicated function within a larger mechanical system. They have a low power consumption, small size, rugged operating ranges, and low costs per unit. Examples include traffic lights, factory equipment and hospital equipment.