1.2 Memory and Storage

Primary Storage

Primary storage is low capacity, internal storage that can be directly accessed by the CPU. Program instructions and data must be copied from the hard drive into the RAM to be processed by the CPU because primary storage access speeds are much faster than secondary storage devices like the hard drive. The types of primary storage are RAM and ROM. RAM is volatile (temporary) storage that stores all programs that are currently running. RAM also stores parts of the OS to be accessed by the CPU. RAM is made up of a large number of storage locations, each can be identified by a unique address. ROM is non-volatile storage that cannot be changed. ROM stores the boot program/BIOS for when the computer is switched on. The BIOS then loads up the OS to take over managing the computer.

Secondary Storage

Secondary storage is used for the long-term storage of data and files because it is non-volatile. It is typically larger in storage size but it is slower because it is not directly accessed by the CPU.

Secondary storage includes magnetic, optical, and solid state storage.

Volatile and Non Volatile Storage

Storage is also split into two types, volatile and non-volatile. Volatile storage is temporary so data is lost when the computer is turned off. An example of this is RAM. Non-volatile storage saves the data even when not being powered. Data can be stored long term and accessed when the computer is switched on. An example of this is ROM.

How virtual memory works - transfer of storage between RAM and secondary storage when RAM is full

Programs stored in RAM tend to be processed by the CPU. Even if there is insufficient space in RAM for all programs the computer can use the Hard Disk Drive (HDD) as an extension of RAM, this is called virtual memory. If new data is needed to be stored in the RAM, unused data is moved to the secondary storage to make space for it. If the original data is required again it can be moved back from virtual memory into RAM. Using virtual memory is beneficial because it allows more programs to be run at the same time with less system slow down.

Magnetic Storage

A magnetic HDD is the most common form of secondary storage within desktop computers. A read/write head moves nanometres above the disk platter and uses the magnetic field of the platter to read or edit data. The characteristics of magnetic storage are; a large capacity and cheaper cost per gigabyte than SSD, modern external HDDs are small and well protected so they are durable and portable but because of the moving parts they should not move when powered on as this can damage the device. Slower access speeds than SSDs but faster than optical storage. Their fragility means that they are semi-reliable.

Optical storage

Optical storage uses a laser to project beams of light onto a spinning disk allowing it to read data from a CD, DVD, or Blu-Ray. This makes optical storage the slowest of the four types of secondary storage. Disc drives are traditionally internal but external disc drives can be bought for devices like laptops. The characteristics of optical storage are; a low capacity (700MB for CD, 4.7GB for DVD, 25GB for Blu-Ray), not durable as discs are fragile and can snap or break very easily, discs are thin and very portable and also very cheap in bulk, optical discs have the slowest access speeds. As they have a plastic case that holds the disc well they are reliable.

Solid State Storage

SSDs have no moving parts. SSDs are replacing HDDs in modern computers as they are generally quieter, faster and use less power. A USB flash drive is another type of SSD storage that is used to transport files easily because of its small size. Memory cards such as SD and micro SD cards are also examples of SSDs. The characteristics of SSDs are; a high capacity and the fastest access speeds because they contain no moving parts, a greater cost per gigabyte than magnetic, usually more durable but cheap USB sticks can snap or break, their small size and no moving parts makes them portable and they can fit easily into a bag or pocket.

Why data must be stored in binary format

Computer systems consist of billions of tiny transistors which are switches that only have two values, on and off or 1 and 0, so all data must be represented and processed in binary. Everything that a computer needs to process must be converted into binary format including text, images, videos, and audio.

Data units and what they can store

All data in a computer system is made of bits. A single bit is a 0 or a 1.
4 bits (such as 0101 or 1101) is called a nibble.

8 bits is called a byte. A byte can store a single character.

1000 bytes is called a kilobyte. A kilobyte can store a short email.

1000 kilobytes is called a megabyte. A megabyte can store about 1 minute of music.

1000 megabytes is called a gigabyte. A gigabyte can store about 500 photos.

1000 gigabytes is called a terabyte. A terabyte can store about 500 hours of films.

1000 terabytes is called a petabyte. A petabyte can store about 1.5 million CDs.

Equation for sound file size

sound file size = sample rate x duration (s) x bit depth

Equation for image file size

image file size = colour depth x image height (px) x image width (px)

Equation for text file size

text file size = bits per character x number of characters

Most Significant Bit and Least Significant Bit

In a binary number, the Most Significant Bit (MSB) is the leftmost digit, representing the highest value, while the Least Significant Bit (LSB) is the rightmost digit, representing the lowest value

Common scenarios where compression may be needed

When large files require storage on devices with limited space or need to be transmitted quickly.

Advantages and Disadvantages of both types of compression

Advantages of compression include:

Files take up less storage space so more files can be stored

Files can be transferred quicker because they are smaller

Files can be read from/ written to quicker

Disadvantages of compression include:

Loss of data

Reduction in quality

Lossy compression

With lossy compression data that cannot be seen or heard by humans is removed, for example with audio humans can only hear between 20Hz and 20,000 Hz so any frequencies outside of that are removed. This reduces the file size. Sometimes further data that can be seen or heard by humans is also removed and here it is clear that there is a reduction of quality. Lossy compression removes data permanently.

Lossless compression

Lossless compression reduces the file size without permanently removing any data. Because of this, the file is returned to its original form when decompressed so no quality is lost. Lossless Compression doesn’t sacrifice the accuracy of the content.