Component 1
Primary storage includes RAM, ROM,
cache and the registers inside the CPU.
Primary storage can be directly accessed by the CPU. This is important is it either contains the instructions that
for programs that are currently running or the instructions needed when the computer first turns on.
Often there isn’t enough space in RAM for all the programs that are currently being run.
The operating system can use a special section of the hard drive to deal with the overflow if necessary. This is
called a page file and the process is known as virtual memory.
Data and instructions must be in RAM to be processed by the CPU so they need to be swapped in and out from
the hard drive before they can be processed.
This means that although there is more overall space, using virtual memory is much slower than using RAM.
Advances in ROM technology now mean that ROM can be made to be fully
rewritable. This is called flash memory.
Flash memory is useful as it is very fast compared to a hard disk drive and can be
used as secondary storage in USB memory sticks, SD cards and solid state drives.
(see the next topic).
Should ROM still be called Read Only Memory? You decide….
Secondary storage uses magnetic, optical or flash technology. It cannot be directly accessed by the CPU and data must be copied into RAM first. Secondary storage stores data permanently.
| Unit | Abbreviation | Number of Bytes | Notes |
|---|---|---|---|
| Bit | 1/8 | either a 0 or 1 | |
| Nibble | 1/2 | e.g. 1010 | |
| Byte | 1 | e.g. 11001100 | |
| Kilobyte | KB | 1,000 | written as 1KB |
| Megabyte | MB | 1,000,000 | or 1000KB |
| Gigabyte | GB | 1,000,000,000 | or 1000MB |
| Terabyte | TB | 1,000,000,000,000 | or 1000GB |
| Petabyte | PB | 1,000,000,000,000,000 | or 1000TB |