Secondary memory is where programs and data are kept on a long-term basis. Common secondary storage devices are the hard disk and optical disks.
- The hard disk has enormous storage capacity compared to main memory.
- The hard disk is usually contained inside the case of a computer.
- The hard disk is used for long-term storage of programs and data.
- Data and programs on the hard disk are organized into files.
- A file is a collection of data on the disk that has a name.
A hard disk might have a storage capacity of 500 gigabytes (room for about 500 x 109 characters). This is about 100 times the capacity of main memory. A hard disk is slow compared to main memory. If the disk were the only type of memory the computer system would slow down to a crawl. The reason for having two types of storage is this difference in speed and capacity.
Large blocks of data are copied from disk into main memory. The operation is slow, but lots of data is copied. Then the processor can quickly read and write small sections of that data in main memory. When it is done, a large block of data is written to disk.
Often, while the processor is computing with one block of data in main memory, the next block of data from disk is read into another section of main memory and made ready for the processor. One of the jobs of an operating system is to manage main storage and disks this way.
