- [Craig] In this video, we take a look at the need for primary storage.
(uplifting piano jingle)
Storage devices fall into three broad categories.
Primary storage:
we consider these to be RAM, ROM, registers and cache memory.
Secondary storage:
this is any device which typically is able to hold the operating system, programs and data,
which are not currently in use.
When we talk about secondary storage devices,
we most commonly think about devices such as hard drives, but the category is much broader than this.
And tertiary storage:
any device used for the backing up and archiving of large amounts of data.
Tertiary devices are not covered in the GCSE specification, so we can ignore those.
We've just included them for completeness.
So, why do we actually need primary storage?
Well, to answer this question properly, it is important to do a quick comparison
of the main differences between primary and secondary storage devices.
All types of primary storage, with the exception of ROM, are volatile,
meaning that when we turn the power off to a computer, they lose any data being held.
Primary storage devices also have a relatively small storage capacity when compared to secondary devices.
Today, RAM is typically measure in gigabytes,
whereas secondary hard disks are often measured in terabytes.
All types of secondary storage are non-volatile,
meaning that when we turn the power off, they are able to hold the data stored on them.
As already mentioned, they also generally have a much greater storage capacity than primary devices.
So, given that RAM is volatile and has a relatively small capacity,
whereas secondary storage is non-volatile and has a much larger capacity,
you might ask, "What is the point in primary storage at all?"
Well, it all comes down to access speeds.
In other words, how quickly can we read from and write to a device?
Access times for primary storage is much faster than secondary storage.
It's hard to do a direct comparison as there are so many different forms of primary and secondary storage,
and technology is always evolving.
But typically speaking,
primary storage access speeds can be tens or hundreds of times faster than secondary storage.
This speed of access is the main need for primary storage.
Consider a three-gigahertz processor.
This is able to perform three billion operations per second.
Your CPU will often want to fetch and execute instructions related to your operating system.
When your computer is turned off,
your operating system will reside on a secondary storage device such as a magnetic hard disk.
Accessing all of your operating system instructions from your hard disk
would seriously hamper and slow down the overall operating speed of your system.
By loading the operating system from your secondary storage hard disk
into primary storage (e.g., RAM),
its instructions can now be accessed many times faster.
So, let's just recap.
Primary storage consists of RAM, ROM,
and to a lesser extent, registers and cache.
It holds the data and instructions which the CPU needs access to while a computer is running.
And the CPU can much more quickly access data from primary storage
than from secondary storage.
(uplifting piano jingle)