in this video we discuss primary storage with a focus on ram and rom storage devices fall into three board 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 gcc 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's 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 measured in gigabytes whereas the 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're 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 is 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 3 billion operations per second your cpu will often want to fetch and execute instructions relating 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 axing 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 eg ram its instructions can now be accessed many times faster 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's 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 again cartridges and 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're 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 vape 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 the rom essentially contains a fixed set of instructions which are read-only and have been burnt onto it during the manufacturing stage now the important thing to note here is that rom is what is known as non-volatile and that means when the computer uses power the instructions in it remain now this is very different from ram which is a volatile storage medium when the computer is turned off all the instructions are lost this means when your computer boots up it can go to wrong find the set of initial startup instructions and begin the process of booting up your computer one of the first set of instructions passed from rom to the cpu is what's known as the power on self test or post this program is responsible for sending signals to all the various connected components it wakes them up it checks their working and essentially tells the cpu that these other devices exist 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 that's everything you need to know for the exam pause the video and take some notes [Music] [Applause] [Music] you