Secondary Memory
Secondary memory, also called auxiliary memory, is a mass storage device. It is non-volatile which means that the device retains its content even if the power goes off. It is an external storage device. It stores data and instruction which are not being used by CPU currently. CPU cannot process this device directly.
Secondary memory consists of two parts namely device drive and device. For example, magnetic disk and drive. Hard disks are usually used as default secondary memory. Its access time is 5 to 10 milliseconds.
When the main memory is overflowed, the operating system stores remaining content in secondary memory. And when the content in secondary memory is required by CPU, it is transferred to main memory. But before this sufficient space is made in main memory by transferring some content to secondary memory.
Commonly used secondary memory devices are magnetic disks, optical disks and flash memory. Magnetic tapes are the oldest secondary storage device and not being used much today as they support only sequential access. Similarly floppy disks were used widely before two decades. Now they are successfully replaced by pen drives.
Characteristics
- Secondary memory is slow.
- It is a permanent storage device.
- It store program and data.
- It is smaller than main memory.
- It is cheap.