How A Computer Works
1 Learning Objectives
By the end of the lesson…
- All Should Know that computers can store data in memory.
- All Should Be able to explain the concept of primary storage (RAM).
- Most Should Should Be able to explain the concept of Secondary Storage in terms of Magnetic or Optical mediums.
2 Storing Bits
Learn It
- You've seen how logic gates can be used to perform some simple calculations such as adding bits together and maybe even subtracting bits.
- In order to do anything useful, computers need to be able to store the bits though.
- You may have seen in an earlier assessment how NAND gates can be used to store bits using an arrangement known as a D-Latch.
- For longer term storage of data, computers use volatile and non-volatile storage.
3 A Nibble or a Byte of a Word
Learn It
- Storage is measured in bits. This is the number of 1s and 0s that the storage can potentially hold.
- A single bit is not a very useful measure, as there's not much a computer can do with just a 1 or a 0
- Sometimes we talk in terms of a nibble of data. A nibble is 4 bits -
1001
for instance. - More usefully we use the term byte, which is 8 bits -
10101010
. - Sometimes we talk of a word which can be 8, 16, 32 or 64 bits. You'll see why later.
- Even then, a few bytes of data is far too small, when we are talking about the capacity of modern storage devices.
- The webpage for last lesson is stored on a server and takes up 25,000 bytes. We use the standard prefix kilo to represent 1000, so this becomes 25kB.
- We can keep going up, in thousands, to describe the quantities of data. Below is a table with some examples.
bytes | value | prefix | symbol | example |
---|---|---|---|---|
1000 | thousand | kilo | kB | Words in this document up to this point - 10kB |
1000000 | million | mega | MB | A song in MP3 format - 8MB |
1000000000 | billion | giga | GB | An HD movie on Blu-ray - 27GB |
1000000000000 | trillion | tera | TB | The back up of Wikipedia is around 4TB of data |
1000000000000000 | quadrillion | peta | PB | Facebook stores around 400PB of photos |
1000000000000000000 | quintillion | exa | EB | The annual global exchange of data on the internet is around 950EB |
4 Non-volatile storage
Learn It
- Non-volatile memory stores data without needing to be permenantly powered.
- Examples of non-volatile memory are
- Solid State Hard drives and Flash memory
- Magnetic Hard disks
- Optical discs such as Blu-ray, DVDs and CDs
- Solid state hard drives use NAND gates with special transistors that hold their charge to store bits of information.
- Magnetic disks and Optical discs are different though.
5 Assessment
Badge It - Silver
- Conduct some research and learn how Magnetic disks and Optical discs are able to store bits.
- Right click to save the template to your homedirve. You will need to use this template for the Silver, Gold and Platinum badges for this lesson.
- Make sure you include the capacity (maximum amount of data they can hold) of the disks or discs.
- Present your findings in a suitable format for assessment.
6 Volatile storage
Learn It
- Volatile storage is only able to store bits while it is being powered.
- Volatile storage is often called Primary Storage or Random Access Memory (RAM).
- There are two main types of RAM. Static RAM (SRAM) and Dynamic RAM (DRAM
- SRAM is fast and expensive and is used to hold bits for short periods of time before operations are carried out on them like addition and subtraction. The D-Latch arrangement of NAND gates is a typical memory cell capable of holding a single bit of data inside SRAM. We normally call this type of memory Cache.
- DRAM is slower and cheaper. DRAM is often simply called RAM and is used to store instructions for the computer and for storing bits of data for slightly longer periods.
- If you were to run the following script…
x = 4 y = 5 z = x + y
- the values of x and y would be stored in DRAM, as well as the instruction to add the numbers together, followed by instructions to store the value of z in DRAM as well.
- A single memory cell in RAM (that can hold 1 bit) is made up of some transistors and a capacitor. The capacitor can hold charge for a short amount of time, but quickly drains and needs to be constantly recharged.
7 RAM
Learn It
- Inside DRAM there are rows and columns of memory cells, each capable of holding 1 bit of data.
- A memory cell consists of some transistors and a capacitor. Capacitors can hold charge, but drain quickly.
- Below you can see a simplification for how RAM works
- The diagram show RAM with a word size of 16. That means that each row contains 16 cells.
- If we want to add some data into the RAM, we need to choose where to put it.
- Computers use addresses to indicate where in the RAM the data will be stored. In this example each address consists of 8 memory cells, so it can hold 1 byte of data.
- Modern computers usually have 32 or 64 bit address spaces, but such an example wouldn't fit on the webpage.
- Memory addresses are numbered. The first memory address is 0, the second is 1, the third is 10 and the fourth is 11.
Try It
- Have a go at typing an address in binary into the Address input box and hit submit.
- You should see the numbers in the top and left bar change colour, indicating where the address is.
- To write data to that address we need to open the correct row and then send current down the correct columns to charge the memory cells.
- Type in an 8-bit binary number into the data box and then hit submit.
- You should see the rows and columns turn red to indicate that current is flowing through them. The cells should also turn red to indicate that they are holding charge.
- Now select a new address and write some data to it.
- The memory cells from the previous write should still be red, as they'll hold the charge for a short peiod of time. However, they'll need to be frequently recharged to keep the data.
- RAM can be read by detecting whether the tiny capacitors are more than 50% charged. If they are then the cell is holding a 1, otherwise they're holding a 0.
8 Assessment
Badge It - Gold
- Use screen shots of the the RAM simulation to describe the process by which data can be stored in RAM.
- Ensure that you use the following key words: memory cell, word, address, capacitor.
Badge It - Platinum
- Conduct some research and learn how data can be read from the memory cells in RAM.
- Present your findings in a suitable form.