OCR Computing A-Level Revision

Memory & Storage

Primary Memory (1.4.e)

There are two types of primary memory, ROM and RAM. Any other type of storage (such as magnetic, optical, or solid-state media) is secondary storage. Data stored in secondary storage cannot be accessed by the processor until it is moved to RAM.

Read Only Memory (ROM)

Read Only Memory, or ROM, is where data that is vital to the operation of the computer, such as BIOS and UEFI, because it cannot be deleted, altered, or tampered with. The main disadvantage of this is that the software stored in ROM cannot be upgraded or updated.

Random Access Memory (RAM)

RAM stores the programs that are being used by the computer (including the operating system). RAM is volatile memory, as any data stored in it is lost when the computer is switched off, or loses power.

Secondary Storage (1.4.f)

Secondary storage is where data is stored that does not need to be accessed by the processor. It is generally far larger than primary storage - a powerful computer might have 16GB of primary memory, but 2,000GB of secondary storage. Secondary storage is also non-volatile, which is good, otherwise every time there is a power cut, or the computer gets turned off, all the data stored on the computer would be lost (except the ROM).

Optical

Optical media is popular because it is relatively cheap, and it is very reliable, and can be dropped and mistreated with no ill effects, as long as the surface of the disc is protected. Data is stored on optical media using "pits" (binary 0) and "land" (binary 1), and is read using a laser. CDs, DVDs, and Blu-Ray discs are all examples of optical media. Most optical media is read-only, however it can sometimes be rewritten, for example CD-RWs.

A disc refers to optical media, but a disk refers to magnetic media!

Magnetic

Broadly speaking, there are two types of magnetic media - tape and disks. Tape is mainly used for archiving, as it takes a long time to "seek" between two sections of tape. Hard disks are mainly used for long term data storage, where speed is also important, for example the storage of hundreds of high-quality movies on a computer. Hard disks are read using a "head", which skims the surface of several "platters", or disks. These are all encased in one sealed unit. Hard drives are faster than tape and optical media, but slower than a solid state drive.

Solid-state

Solid state storage is generally divided into two categories - "slow" storage (such as a flash drive), and "fast" storage (such as an SSD). All solid state storage is very good at dealing with small files, as no movement is required to "find" (seek) files, like with magnetic or optical storage. However, SSD's are fast enough to replace a traditional hard drive, as they can be 4 or 5 times faster than a spinning hard drive. (A flash drive can also be used to replace a hard drive, however flash drives can be 20 times slower than hard drives, even though they can seek faster).

Seagate Technology Paper on Solid State Storage (SSS)

Data Transfer (1.4.g)

Data can either be stored in primary memory or secondary storage. However, sometimes data needs to be moved, for example, because the computer is about to be switched off, and anything in volatile memory (such as RAM) will be lost, or if the processor wants to work with some data, which is currently stored in secondary storage (but needs to be stored in primary storage for the processor to use it). Because the memory controller (which controls primary memory - RAM in this case) is part of the processor, the data has to be copied from secondary storage to the ALU (or vice versa).

However, we know that the ALU is extremely fast, compared with primary storage which is (relatively speaking) slow. This means that the processor will be waiting time waiting for secondary storage, when it should be processing something else. The solution is to have a buffer, which fills up. When it is full, an "interrupt" is sent to the processor, and the data in the buffer is transferred to the ALU, and then to primary memory. The same system works in the opposite way when transferring data from primary memory to secondary storage.