- [Craig] The central processing unit fetches instructions from memory.
There are two main types of memory in a computer system, also known as primary storage.
In this video, we explore RAM and ROM.
(uplifting piano jingle)
The ROM is a small piece of memory soldered to the motherboard.
It contains the very first instructions for the computer,
to check the hardware is installed correctly and to load the basic input-output system.
These very first instructions are also known as the bootstrap.
Software stored on the ROM is referred to as firmware.
In some computer systems, ROM chips can be changed in order to change the program being executed.
An example of this are game cartridges in portable entertainment systems.
The RAM is a temporary storage of instructions and data for programs that are being executed by the processor.
Although the long-term storage is usually on the hard disk, it would be too slow to execute instructions from there.
Therefore, they are transferred from the disk to the memory first.
The RAM also holds the operating system when the computer is running.
Here, we see four important internal components of a computer system.
ROM, the CPU, RAM and a hard drive of some description.
When your computer very first turns on and receives power,
there are no instructions in the CPU.
What it needs to do is load the operating system so you can start using it.
Of course, the problem with this is that the operating system is stored on the hard drive
and at the moment your computer turns on, it doesn't even know the hard drive exists,
let alone that it should look there to find the operating system and load it into RAM.
The solution, as explained earlier, is ROM.
It contains the bootstrap, a set of initial startup instructions placed on it during manufacturing.
Now, this is slightly beyond the GCSE specification,
but these first initial instructions perform, amongst other tasks, a power-on self-test,
known as the POST,
which sends signals to all the other connected components such as the hard drive.
The POST, in very simple terms, wakes up all the connected components
and makes your CPU aware of their existence.
Now our computer is aware of the hard drive and the software stored on it,
it can go ahead and load these instructions into RAM and start the process of booting up your operating system.
So, let's just recap what we've learned.
Although both ROM and RAM contain instructions and data for the computer,
that is where the similarity ends.
The key difference is that RAM holds the operating system, programs and data
in use by the CPU when the computer is running.
It is volatile; the contents are lost when the power is turned off.
It's read and writeable, and it's large in comparison to ROM.
ROM holds the first instructions for when the computer is first turned on, known as the bootstrap.
Programs may be stored in ROM in embedded systems.
ROM is non-volatile; the contents remain when the power is turned off.
It is read-only and it is small in comparison to RAM.
(uplifting piano jingle)