CS17Memory and Storage

A computer has four types of memory on the AQA course. Each has a different job, so a computer needs all four.

• RAM (random access memory): holds the programs that are currently running and the data they use. Needed because the CPU must be able to read and write these quickly while they run.
• ROM (read only memory): holds the instructions the computer needs to start up. Needed because RAM is empty when the power comes on.
• Cache: a small area of fast memory built into the processor, holding frequently used instructions and data. Needed so the CPU doesn’t have to wait for main memory as often.
• Registers: memory locations on the processor that can be accessed quickly, holding the data and results for the instruction being executed. Needed because the CPU works directly on these values.
• How cache size and registers affect performance is in The CPU and the fetch-execute cycle.

From registers to secondary storage: the faster the memory, the smaller and more expensive per byte it is.

Cache

A small area of fast memory built into a modern processor, holding frequently used instructions and data that the processor is likely to reuse.

Register

A memory location on a processor that can be accessed quickly.

• Main memory is any form of memory that is directly accessible by the CPU (except for cache and registers). RAM and ROM are main memory.
• Secondary storage is any non-volatile storage mechanism not directly accessible by the CPU. Solid state drives and magnetic hard disks are secondary storage.
• To run a program kept in secondary storage, it is first copied into main memory (RAM), where the CPU can reach it.

Main memory

Any form of memory that is directly accessible by the CPU (except for cache and registers).

Secondary storage

Any non-volatile storage mechanism not directly accessible by the CPU.

• A volatile storage device does not retain data contents when power is removed. A non-volatile storage device will retain data contents even when power is removed.
• RAM is volatile main memory. It is used to hold currently running programs and associated data. It can be read from and written to. Its contents are lost when power is removed.
• ROM is non-volatile main memory. It is used to hold instructions that are needed for the computer to run, especially when the computer is turned on. It can be read from but not written to. Its contents are kept when power is removed.
• A desktop computer has far more RAM (many gigabytes) than ROM, and its applications are stored in secondary storage, not ROM.

Volatile

A volatile storage device does not retain data contents when power is removed.

Non-volatile

A non-volatile storage device will retain data contents even when power is removed.

RAM (random access memory)

Volatile main memory, used to hold currently running programs and associated data. It can be read from and written to.

ROM (read only memory)

Non-volatile main memory, used to hold instructions needed for the computer to run, especially when it is turned on. It can be read from but not written to.

• RAM is volatile: everything in it is lost when the power is switched off. Programs, documents and photos must be kept somewhere that keeps data without power.
• Main memory is limited: RAM is too small (and too expensive per GB) to hold every program and file a user owns.
• Secondary storage is non-volatile and has a much larger capacity for its cost, so it keeps the operating system, applications and files between uses.
• Some secondary storage is portable, so data can be moved between computers or kept as a backup.

Magnetic devices such as hard-disk drives (HDDs) have a spinning disk and use magnetic fields to represent 0s and 1s.

• Inside are one or more platters coated in magnetic material, spinning at high speed.
• A read/write head on a moving arm moves across the platter to reach the right track.
• Writing: the head magnetises tiny regions of the surface in one of two directions. One direction represents 0, the other represents 1.
• Reading: the head detects the direction of magnetisation of each region as it passes underneath.
• Data stays magnetised without power, so magnetic storage is non-volatile.
• Strengths: very large capacity and low cost per GB.
• Weaknesses: moving parts, so it can be damaged if dropped while running, makes some noise, uses more power, and is slower to access data because the head must move and the disk must spin to the right place.

A hard disk drive: a spinning platter and a read/write head on a moving arm.

Magnetic storage

Devices such as hard-disk drives that have a spinning disk and use magnetic fields to represent 0s and 1s.

Read/write head

The part of a hard disk drive that magnetises regions of the platter to write data and senses them to read it.

Solid state devices use non-volatile memory to store data. They have fast data access and write speeds and are durable as they do not have any moving parts.

• SSDs use electrical circuits to store data persistently: the data stays when the power is off. (You don’t need to know how the circuits work.)
• Examples: the solid state drive (SSD) in most laptops and phones, USB memory sticks and SD cards.
• No moving parts, so they are durable (they survive knocks and vibration), silent, light, small and use little power.
• Fast: there is no head to move or disk to spin, so data can be read and written very quickly, and computers start up and open files more quickly.
• Weaknesses: higher cost per GB than magnetic storage, and each part of the memory can only be written a limited number of times before it wears out.

Solid state storage

Devices that use non-volatile memory (electrical circuits) to store data, with fast access and write speeds and no moving parts.

• Choose solid state for laptops, phones, tablets and anything carried or moved, and where speed matters (starting up, loading programs).
• Choose magnetic for storing large amounts of data cheaply where speed matters less: backups, archives and servers holding many large files.
• Many desktops have both: a smaller SSD for the operating system and programs (fast start-up), and a large HDD for files (cheap capacity).
• Older books and videos also cover optical storage (CDs and DVDs). It is no longer on the AQA course.

Storing data at a remote location, using magnetic and/or solid state storage, and allowing online access is known as cloud storage. Access to cloud storage is made via the Internet.

• The data is still on physical drives (magnetic and/or solid state). They are just owned and run by a provider in a data centre somewhere else.
• Local storage means drives in or attached to your own device.

Cloud storage: your data on someone else’s drives, reached over the Internet.

Cloud storage

Storing data at a remote location, using magnetic and/or solid state storage, and allowing online access via the Internet.

Local storage

Storage in, or attached to, the user’s own device.

An embedded system is a computer system built into a larger device. An example is the control system for a microwave or a washing machine.

• Examples of embedded systems: the controller in a microwave, washing machine, dishwasher, central-heating thermostat, car engine management system, traffic lights or a digital camera.
• Examples of non-embedded systems: a desktop PC, a laptop, a tablet. These are general-purpose computers that can run many different programs chosen by the user.
• An embedded system usually has one dedicated job and runs one program, which rarely changes.
• It usually has less processing power and memory than a general-purpose computer, which makes it smaller, cheaper and lower-powered.
• Its program is usually stored in ROM, so it often has more ROM than RAM, and it may have no secondary storage.
• Embedded systems are often programmed in assembly language, to control their specific hardware directly.

The embedded system inside a washing machine.

Embedded system

A computer system built into a larger device, such as the control system for a microwave or a washing machine.