Looking to ace your exams with easy-to-access, free 1:1 advice from exam survivors? Join the Baguette Brigaders Discord server, home to a friendly community of students, teachers and professionals who can answer any of your questions and give you valuable exam tips, tricks and insights!

Latest update: 23/09/2023 18:45

1.1 - Systems architecture

1.1.1 Architecture of the CPU

The purpose of the CPU - the F-D-E cycle

The Fetch-Decode-Execute cycle is the main job of the CPU. To execute a program, the program code is copied from secondary storage into the main memory. The CPU’s program counter is set to the memory location where the first instruction in the program has been stored, and execution begins.
In a program, each machine code instruction takes up a slot in the main memory. These memory locations each have a unique memory address. The program counter stores the address of each instruction and tells the CPU in what order they should be executed.

Step by step:

The CPU checks the program counter to see the next instruction to execute.
The program counter gives a memory address to where the next instruction is
The CPU fetches the instruction from this memory address
The instruction is decoded and executed. For example the value decoded could be a number, which is entered to the ALU (arithmetic logic unit) and added to another fetched from cache and multiplied.
The program counter increments by 1
The cycle is repeated indefinitely

CPU components and their function(s)

The ALU (Arithmetic Logic Unit)

Performs calculations and logical operations
Where decisions are made (e.g if x > 10)

The CU (Control Unit)

Fetches, decodes and executes instructions
Controls hardware

Cache

A small amount of high speed memory physically inside the CPU.
Temporarily holds small amounts of data which the CPU will reuse often.
Speeds up the system - does not have to wait for some data in memory to be fetched
Level 1, 2, 3 cache - 1 is the fastest, most expensive, has the lowest amount of storage, likewise 3 is the slowest, least expensive but contains the most amount of potential storage

Registers are:

Small amounts of high speed memory in the CPU used to store small amounts of data needed for processing
Includes the address of the current instruction, the next instruction to be executed, and the results of calculations

In case you find the difference between Cache and Registers difficult, here’s something useful from Stack Overflow:

CPU register is just a small amount of data storage, that facilitates
some CPU operations.
CPU cache, is a high speed volatile memory which
is bigger in size, that helps the processor to reduce the memory
operations.
It is not very inaccurate to think of the processor’s
register as the level 0 cache, smaller and faster than the other
layers of cache in-between the processor and memory. The difference
is only that from the point of view of the instruction set, cache
access is transparent (the cache is accessed through a memory address
that happens to be a cached address at the moment) whereas registers
are explicitly referenced in each instruction.

There will probably be a table to tick what things do, or a 2-4 marker asking what something does and to explain it. I’d recommend learning 2 registers in detail, and what 4 just do.

Von Neumann architecture

This is basically a summary of the above.

Von Neumann architecture is the design of which most every computer now is comprised of. It states:

Data and instructions are stored as binary
Data and instructions are both stored in primary storage
Instructions are fetched from memory one at a time and in order
The processor decodes and executes an instruction, and then does the same for the next instruction
This will continue until there are no more instructions

A processor based on von Neumann architecture has five registers which it uses for processing:

the program counter (PC) holds the memory address of the next instruction to be fetched from primary storage
the memory address register (MAR) holds the address of the current instruction that is to be fetched from memory, or the address in memory to which data is to be transferred
the memory data register (MDR) holds the contents found at the address held in the MAR, or data which is to be transferred to primary storage
the current instruction register (CIR) holds the instruction that is currently being decoded and executed
the accumulator (ACC) is a special purpose register and is used by the arithmetic logic unit (ALU) to hold the data being processed and the results of its calculations

BBC Bitesize reference: Bitesize

Personally I find this the easiest way to remember them:

program counter counts the programs by 1 every cycle
memory address register has the current memory address
memory data register has the memory data at the address
don’t worry about the CIR because it’s self-explanatory
accumulator (ACC) holds data being processed and the results of it

1.1.2 - CPU performance

Clock speed

The clock speed is measured in gigahertz (GHz) and represents how many fetch-decode-execute cycles happen per second. 1 GHz = 1 billion cycles.

Task manager showing the clock speed of a computer with 6 cores.
My computer running at 4.27 GHz, overclocked from 3.7 GHz. This means that 4,270,000,000 fetch-decode-execute cycles are happening per second. Speedy, right?

A clock speed of 4.27 GHz means that 1 clock happens 274,000,000 times faster than a reflex action. In that time, light itself can only travel about 8cm. In other words, they’re really fast.

However, a CPU which cannot keep up with its clock will corrupt its data. A very fast clock speed will cause the CPU to overheat and thermal throttle, reducing its performance to stop it melting…

Cache size

Transferring data in and out of memory takes much, much longer than from cache. Therefore, by placing frequently accessed data on cache, it results in everything using that function (such as square roots) being executed much faster. The more cache there is, the more data can be stored closer to the CPU.
Cache is ‘graded’ at different levels depending on its speed. L1 is usually part of the CPU chip and is both the smallest and the fastest to access. Its size is often restricted to between 8 KB and 64 KB. L2 and L3 caches are bigger than L1. They are extra caches built between the CPU and the RAM. Sometimes L2 is built into the CPU with L1.
L2 and L3 caches take slightly longer to access than L1. Each CPU core has its own set of L1 cache, but they can share higher levels.

However, cache is very expensive (L1 costs ~£1 per kilobyte) and is limited by the space of the CPU die, and is very small, so cannot be a full replacement of memory.

Core count

A CPU has multiple cores on it. CPUs with multiple cores have more power to run multiple programs at the same time.

However, doubling cores does not double clock speed. Some headroom is needed to communicate between each core.

1.1.3 - Embedded systems

An embedded system is a a small computer inside of a larger system. PCs would be categorized as general purpose systems, as they can do pretty much anything. Embedded systems on the other hand have one specific function which they run.

General process computers are designed to access the Internet, play games, play videos, write programs. These all require applications to run. Tablets, phones and consoles are now more classed as general process as they now can run several applications.

Examples of embedded systems include GPS systems, digital watches and fitness trackers.

They are not reprogrammable either - all the programming is done in manufacturing.

Advantages of these systems include they require less power to run and are cheaper to make as their processors are not as powerful.

This is an snippet from our complete GCSE Computer Science Cheat Sheet.

For the full version, including comprehensive coverage of both Paper 1 and Paper 2, along with coding practice examples, visit https://ibaguette.com/cheatsheets/gcse and select “Computer Science: Paper 1 and 2.”

Feel free to share these revision resources with friends, family, and teachers!

Created with 💕 by Draggie