1.1 - Systems Architecture

What is a Computer?

A computer is an electronic device which takes input, processes data, and delivers output.

What does CPU stand for?

CPU stands for Central Processing Unit.

How does a computer carry out its functions?

A computer carries out its functions through the Fetch-Decode-Execute Cycle.

What is the Fetch stage of the FDE cycle?

In the Fetch stage of the FDE cycle -

• The CPU fetches the next instruction from the main memory.

What is the Decode stage of the FDE cycle?

In the Decode stage of the FDE cycle -

• The CPU inspects the instruction and works out what it is that needs doing.

What is the Execute stage of the FDE cycle?

In the Execute stage of the FDE cycle -

• The CPU carries out the instruction.

What is Clock Speed? What is it measured in?

Clock Speed is the speed at which the FDE cycle is done. It is measured in hertz.

The CPU is often referred to as the … of the computer system?

The CPU is often referred to as the brain of the computer system.

What does the ALU stand for and what does it do?

The Arithmetic Logic Unit is responsible for carrying out arithmetic calculations and making logical decisions.

What does the CU stand for and what does it do?

The Control Unit sends signals to control how data moves around the CPU.

What does the Cache do?

The cache provides fast access to frequently used instructions and data. It is a small amount of very fast memory.

What are the Registers?

The Registers are tiny, super fast pieces of onboard memory inside the CPU. They each have a very specific purpose -

• Accumulator

• Program Counter (PC)

• Memory Address Register (MAR)

• Memory Data Register (MDR)

What is a fixed program computer?

A fixed program computer doesn’t have changeable programs.

• Calculators - only for mathematics, can’t do anything else.

What is a stored program computer?

Stored program computers have changeable programs.

Who designed the first stored program computer? What name was given to this way of programming computers?

John Von Neumann designed what became known as the Von Neumann Architecture.

What are the key characteristics of the Von Neumann Architecture?

The VNA:

• CPU

• CU

• ALU

• Onboard Cache

• Internal Clock - provides a pulse at a constant rate to synchronise components.

What does the Program Counter (PC) do?

The PC holds the address of the next instruction to be executed in memory.

What does the Memory Address Register (MAR) do?

The MAR holds the address of where data is to be fetched from or stored into.

What does the Memory Data Register (MDR) do?

The MDR holds any data which has been fetched from memory or is about to be written to memory.

What does the Accumulator do?

The accumulator holds the results of calculations performed by the ALU.

What is the Fetch stage of the cycle? Use the registers in your answer.

Fetch -

• The PC is checked as it holds the address of the next instruction to be executed.

• This address is used by the MAR to fetch the instruction needed from the main memory and bring it to the MDR.

• The address in the PC is incremented to point at the next instruction.

What is the Decode stage of the cycle? Use the registers in your answer.

Decode -

• The CU decodes the instruction to see what has to be done.

What is the Execute stage of the cycle?

Execute -

• The instruction has now been executed.

• What is done to execute it depends on the instruction itself.

What does the processing power of a CPU depend on?

The processing power of a CPU depends on -

• Clock Speed

• Number of Cores

• Cache Size

What is clock speed measured in?

Clock speed is measured in hertz.

(number of cycles per second)

What clock speed do modern processors operate at?

Modern processors operate at Gigahertz (GHz)

What is the cache?

The cache is a temporary storage of data and instructions being read to and written from the main memory.

Every time this happens, a copy of the data is kept in the cache.

Where is the cache located?

The cache is located on the board or very near to the CPU.

Why is the cache important?

It is much quicker to get items from the cache than from RAM, therefore CPUs try to avoid getting instructions from RAM as much as possible as it costs time.

What is a core in a CPU in simple terms?

A core is a complete copy of the CPU.

How many processing units would a quad core processor have?

A quad core processor would have 4 separate processing units - each with own:

• Registers

• ALU

• CU

• Etc.

More cores = …?

More cores = More power

Why do more cores cost time and efficiency?

Because CPU cores must communicate with each other.

Does doubling the number of cores directly double the power of the CPU?

No. Doubling the number of cores doesn’t directly double the power of the CPU.

What effect would increasing:

• The number of cores

• The cache size

• The clock speed

have on a CPU?

The CPU would be faster, more efficient, and more powerful. However, it would likely be more expensive.

What are embedded systems?

Embedded systems are systems which are dedicated to a specific task.

Since embedded systems are dedicated to a specific task, what does that mean the design engineers can do?

Design Engineers can optimise the systems to -

• Reduce the size of the product

• Reduce the cost of the product

• Increase the reliability for the given task.

Embedded systems are usually mass produced, therefore…?

Embedded systems are usually mass produced, therefore we can get further benefits from economics of scale.

What are the properties of embedded systems compared to general purpose systems?

Properties of embedded systems compared to general purpose systems -

• Low power consumption

• Small size

• Rugged Operating System

• Low cost per unit

• More difficult to program and interact with (limited processing resources)

What are some examples of embedded systems?

Examples of embedded systems -

• Apollo Guidance Computer for moon missions (one of the earliest examples)

• Hospital instruments

• Car engine management systems

• Digital clocks

What percentage of all processors are manufactured as components of embedded sytems?

98% of all processors are manufactured as components of embedded systems.