System's Architecture
Secondary Storage

• Any non-volatile, long-term storage mechanism not directly accessible by the CPU.

• Without secondary storage all programs and data would be lost the moment the computer is switched off.

• There are three main types of secondary storage in a computer system: solid state storage devices, optical storage devices, magnetic storage devices.

• Not all computers require secondary storage. Embedded computers, such as those found in a washing machine or central heating system, do not need to store data when the power is turned off.

RAM is volatile, so will not store data when the power is removed. Secondary storage, will contain the you data/files for when the computer is switched on again.

The operating system, data, images, programs, documents, etc...

• Uses magnetism to store data.

• Most common device is a Hard Disk Drive (HDD).

• Reliable and low cost per GB.

• Ideal for large amounts of storage.

• Currently in the multi-terrabyte capacities.

• Likely to be damaged by shock (dropped) and strong magnetic fields.

• Stack of disks, called platters, coated in a magnetic material.

• Effectively billions of tiny magnets, north = 1 and south = 0.

• The platters rotate and a set of sensors (heads) move across the platters sensing the change of north/south alignment of the magnets.

• Uses flash memory, electronic circuits to store data.

• Very fast access times, due to no moving parts.

• Relatively expensive compared to magnetic storage (changing rapidly).

• Many different form factors (shapes): SD card, USB drive, SATA, M2...

• Replacing magnetic drives for most systems, exception being very large storage requirements.

• Magnetic drives have a delay (latency) in accessing data because the head needs to move across the platter, SSD do not have moving parts, reducing the latency.

• The lack of moving parts also means the power consumption is lower and there is no heat or noise.

• SSD's do not experience slow speeds due to data fragmentation.

• SSD's are smaller and lighter, allowing more portable devices.

• SSD's are more robust as they are not as affected by shocks.

• CDs, DVDs, Blu-Ray

• Use light to read data

• CD — 700 MB

• DVD — 4.7 GB single layer or 8.5 GB dual layer

• Blu-Ray — 25 GB single layer or 50 GB dual layer

• Tend to be used to store data never to be changed.

• Lasers are used to write and read the data.

• The surface of the disk is covered in billions of indentations.

• When the light is shone on the disk the indentations will reflect differently depending on whether is hits an indentation or not.

• This difference in the reflected light is interpreted as either a 1 or a 0 to represent binary data.

• Data that is accessed via the internet rather than being stored locally on your computer, phone, etc.

• A network of servers that store data or run applications, that are housed in huge data centres around the world.

• Users generally do not know the geographical location of the data storage.

• Storage: Google Drive, One Drive, iCloud Drive, DropBox

• Applications: Google Docs, Office 365, GMail

• Files and applications can be accessed from anywhere in the world with an internet connection.

• Applications are kept up to date without the user having to update anything themselves.

• Reduces the workload for network managers and technical support staff.

• Storage space is flexible, users can buy more storage as they need it.

• Improved security, the hosting company is responsible for keeping users data backed up.

• Sharing files with others is easier.

• Can only access files whilst there is an internet connection.

• Users no longer control the security of their files. Hackers more likely to attack a hosting company with many users data than an individuals computer system.

• Not always clear who owns the data (read about Facebook's terms and conditions).

• Hosting companies may change their terms and conditions, prices and even cease to exist.

• Can become expensive in the long term. 

The use of storage, services and applications that are accessed via the internet rather than being accessed locally on a device.

File storage, e.g. Google Drive

Applications, e.g. Google Docs

• An internet connection is required to access the data.
• Not much control over the security of the data or where it is stored.
• Terms and conditions can be changed with little notice.
• Can be more expensive in the long term.

• A computer system that has a dedicated function as part of a larger device.

• Generally a single chip (microcontroller), with some circuitry.

• Size and cost can be reduced if a system only needs a fixed range of tasks.

• Includes some ROM to store the program.

• May include some RAM to store user inputs/outputs.

• Low power, so can operate with a small power source (mobile phone)

• Small in size, so can fit into portable devices (fitness watch)

• Rugged, so can operate in harsh environments (car engine management)

• Low cost, so suitable for mass production (microwave ovens)

ROM is non-volatile, so will store the data and instructions needed to operate the device.

RAM is volatile, so will store the inputs/outputs for the user.

User selection for time, power level,...

Display user selections, timer countdown, ping,...

Many options, discuss: power, size, rugged, cost