1.2 Memory & Storage

• RAM


  Random Access Memory
• What is RAM?


  It is where programs are loaded when the user opens them
• Is RAM volatile or non-volatile?

  Volatile
• Virtual Memory


  When the RAM is full a proportion of the secondary memory storage (usually the hard disk) is used as an overflow allowing programs to function but a bit slower
• Types of primary storage


  - RAM

  - ROM

  - Registers

  - Cache
• RAM summary


  - RAM is needed to store programs that are currently being used

  - RAM helps to enable 'multi-tasking', which means having several programs open and using them all at the same time

  - It does this by copying the data needed by programs at that time so that it can be passed to the CPU where it needs to process that data

  - It also stores all the instructions/modules from the open programs that the CPU will require for processing

  - This is mainly done because RAM is much faster than the hard disk

  - Temporarily stores data as small charges of electricity in tiny transistors etched into a circuit in the device

  - The charge needs to be refreshed every few milliseconds otherwise the charge leaks away

  - RAM is therefore volatile memory - it loses data when there is no power

  - Holds information being executed by the CPU

  - Much faster the a hard disk
• Small amount of RAM


  Limited multi tasking and greater need to use slower virtual memory
• Large amount of RAM


  Greater storage of programs / data leading to faster performance and effective 'multi-tasking
• How does RAM store data?

  Ram holds data using capacitors and transistors.
• How does a capacitor store electricity?


  When electricity is being held a 'bit' is represented as a 1 and when it's not the 'bit' is presented as a 0
• How does a transistor work?


  Acts as a switch that lets the computer read the capacitor as see if it's empty or not
• ROM


  Read Only Memory
• ROM summary


  -ROM is a special memory chip that can store data even without power - it's non-volatile. It is used by a computer to store the boot sequence instructions- everything the computer needs to boot up and load the operating system

  - When you switch on your computer, data in the ROM is accessed by the CPU: All hardware is checked and then the operating system is loaded into the CPU ready for the user to interact

  - The data on ROM cannot easily be over written by the user (READ ONLY)

  - ROM has fast data access rates
• Why do we need ROM?


  - When you switch off your computer off that data that the CPU can access (RAM) loses its data

  - This is a problem because, when you restart you computer, data must be in the RAM for the computer to work
• Why do we need virtual memory?


  - RAM is great for multitasking. You can have multiple programs open and use them simultaneously

  - But what happens if you've got so many programs open that the RAM becomes filled up with programs (instructions) and data? Does the computer stop working?

  - No, a typical modern computer system has another option up its sleeve...
• What is virtual memory?


  - Virtual memory is actually hard-drive (hard-disk) space which acts as RAM (although it operates a lot more slowly than RAM)

  - So if you are browsing the internet and listening to music, the RAM should cope fine

  - •If you then decide to play a game with good graphics then this may overload the RAM - too many programs (and their data) need to be copied to RAM.

  - What the computer will do is relocate programs that have not been recently used to the hard-disk.

  - This process will take time as hard-disk data speeds are much slower - but when it is done the system should be nice and fast again.

  - Often the operating system will use the hard disk as virtual memory. A lot of programs need to be loaded as part of the operating system but many can sit on the hard-disk as they are used infrequently.
• Cache Memory


  - CPU processes data faster than the RAM can supply it with data, so system performance can suffer.

  - Cache is simply a few Mb of memory that sits inside the CPU.

  - Being located in the CPU means that data access speed is = to the CPU speed. CPU performance is therefore maintained

  - Well written programs will make sure that 'sets' of regularly used instrucfions are copied to the cache so thạt the program can run more efficiently and much faster.

  - Poorly written programs will tend to need to fetch the instructions they require from the RAM one at a time and so access to the RAM is slow, the program performs less efficiently
• Optical Storage Technology


  Using reflective material to store data by burning 'pits' and leaving 'land'. Data is read by shining a laser and seeing which elements reflect (0) and which do not (1).
• Magnetic Storage Technology


  Using materials with magnetic properties (ability to change polarity (north/south)) to store data (0/1)
• Solid State Technology


  Using circuitry, which traps electrons into specific orders in order to store data
• What do we need secondary storage?

  Because RAM is volatile and ROM is read only
• What are the uses of secondary storage?


  - Storage of programs and data when power is turned off

  - Semi-permanent storage of data that can be changed

  - Back-up data files

  - Archive of data files
• Drive


  The device that reads and writes data from secondary storage
• Media


  What the data is actually stored on
• Common types of storage devices

  - Optical

  - Magnetic

  - Solid State
• Examples of optical storage


  - CD-R

  - CD-RW

  - DVD-R

  - DVD-RW

  - Blue-Ray
• Positives of optical storage


  - Cheap

  - Light weight

  - Portable
• Negatives of optical storage


  - Slow access times

  - Prone to scratches
• How does magnetic storage work?


  Magnetic storage uses different patterns of magnetisation in a magnetisable material to store data (tape/hard disk). Magnetised points on a pattern represents '1' and demagnetised points represent '0
• Positives of magnetic storage


  - Cheap

  - Large capacity
• Negatives of magnetic storage


  - Fragile

  - Slow access time
• How does solid state storage work?


  They work by sending a large electric current to force electrons through a barrier and trapping them in that position. The flash of the electric current used to achieve this gives us the name for this type of memory. The different arrangement of electrons gives us data
• Positives of solid state storage?


  - Durable (no moving parts)

  - Fast access time

  - Cooler then HDD
• Negatives of solid state storage

  - Cost

  - Limited read/write
• Examples of magnetic storage

  Hard Disk Drive
• Examples of solid state storage


  - SSD

  - USB Stick

  - SD Card
• When is optical storage used?


  - Suitable for read only data distribution on a large scale.

  - Suitable for small capacity situations.
• When is magnetic storage used?


  - Suitable when very high data capacity is required.

  - Suitable for fast access to data.

  - Suitable for low cost situations.

  - Suitable for cloud storage on server farms.
• When is solid state storage used?


  - Suitable for low power, small embedded systems.

  - Suitable for rugged applications: portable devices.

  - Suitable for small to medium data capacity requirements.

  - Suitable for silent operation.

  - Suitable for very fast access to data.

  - Suitable for situations where the devices need to be small and lightweight.
• Bit


  A single binary digit: 1 or 0