Auxiliary Memory (Secondary Storage) in Computer Systems

Explore auxiliary memory (secondary storage) in computer systems, focusing on its role in long-term data storage. This guide explains different types of auxiliary memory, including magnetic disks (HDDs) and magnetic tape, comparing their characteristics, storage methods, and performance.



Auxiliary Memory in Computer Systems

What is Auxiliary Memory?

Auxiliary memory, also known as secondary storage, is a type of non-volatile memory (meaning it retains data even when the power is turned off) used for long-term storage of programs and data. It's typically slower than main memory (RAM) but offers much larger storage capacity at a lower cost per unit of storage.

Magnetic Disks

Magnetic disk drives (HDDs or hard disk drives) use circular platters coated with a magnetizable material to store data. Data is recorded as magnetic spots on the disk surface. Each platter is typically divided into concentric circles called tracks; each track is further divided into sectors. Multiple platters can be stacked on a single spindle, with read/write heads for each surface.

(An illustrative diagram of a magnetic disk, showing the platters, tracks, and sectors, would be beneficial here.)

Magnetic Tape

Magnetic tape storage uses a long, thin plastic strip coated with a magnetic material to store data. Data is recorded as magnetic spots along multiple tracks on the tape. Often, multiple bits are written simultaneously to form a character, typically including a parity bit for error detection.

Sequential Access

Unlike hard drives, which allow for random access to any data block, magnetic tapes are sequential-access devices. This means the tape must be advanced to the correct position before the data can be read or written. To improve efficiency, data is recorded in blocks (records) separated by inter-record gaps.

Tape Drive Operations

Tape drives support several operations: starting, stopping, moving forward, reversing, and rewinding. However, they cannot start and stop rapidly between individual characters, necessitating the use of records.

Magnetic Tape in the Memory Hierarchy

Magnetic tape generally represents the lowest-cost, highest-capacity, and slowest-access form of storage in a computer system’s memory hierarchy. It is often used for archiving large amounts of data due to its low cost and high storage density.

Conclusion

Auxiliary memory technologies like magnetic disks and tapes provide cost-effective solutions for long-term data storage. They differ significantly in access speed and data organization, with disk drives offering random access and tape drives providing sequential access.