Introduction
Auxiliary memory is the lowest-cost, highest-space, and slowest approach to storage in computer systems. Data, programs, and information are preserved for long-term storage or not in direct use. The most usual auxiliary memory devices used in computer systems are magnetic disks and magnetic tapes.
What is Auxiliary Memory in Computer Architecture?
Typical auxiliary memory devices in computer systems are magnetic tapes and magnetic disks. Other components like magnetic drums, magnetic bubble memory, and optical disks are used but not as frequently.
To fully understand the physical mechanism of auxiliary memory devices, we must know magnetics, electronics, and electromechanical systems.
The physical properties of the storage devices can be pretty complex, but we can compare and characterize their logical properties by a few parameters. The critical characteristics of any instrument are its access mode, transfer rate, access time, cost, and capacity. The average time needed to reach a storage location in memory and fetch its contents is access time.
In electromechanical devices such as disks and tapes, the access time consists of a seek time required to position the read-write head and a transfer time required to transfer data to and from the device. Since the seek time is usually much longer than the transfer time, we organize auxiliary storage in records or blocks.
A record or block is a specified number of characters or words. The reading or writing is always done on the entire record.
The transfer rate is the number of characters or terms that the device can transfer per second after being positioned at the beginning of the record.
Magnetic drums and disks are pretty similar in operations. Both consist of high-speed rotating surfaces coated with some magnetic recording medium. The rotating surface of the disk is a round flat plate and that of the drum, a cylinder. The recording surface rotates at a uniform speed and is not started or stopped during access operations. Bits are recorded as magnetic spots on the surface as it passes a stationary mechanism called a write head.
It detects stored bits by changing the magnetic field produced by a recorded spot on the surface passing through a read head. The surface of a disk recording is more significant than a drum of equal physical size. Thus, we can store more information on a disk than on a drum of similar size. For this reason, we no longer use drums in the latest computers.
Types of Auxiliary Memory
Magnetic Disks
A magnetic disk is a round plate forged of metal or plastic coated with magnetized material. We use both sides of the disk and stack multiple disks on one spindle with read/write heads available on each surface.
All disks spin together at high speed and are not stopped or initiated for access. Memory Bits are saved in the magnetized surface in marks along concentric circles known as tracks. We divide the concentric circles (tracks) into regions known as sectors.
Some disk units use a single read/write head for each disk surface. In this unit type, the mechanical assembly uses track address bits to move the head into a specified track position before reading or writing.
In some disk systems, we provide separate read/write heads for each track in each surface. The address bits can then select a certain way electronically through a decoder circuit. This unit is more costly and is found only in extensive computers. We use permanent timing tracks in disks to synchronize the bits and identify the sectors.
Address bits address disk systems that specify the disk number, the disk surface, the sector number, and the track within the sector. When the read/write heads are in place in the specified track, the system must wait until the rotating disk reaches the specified sector under the read/write head.
Data transfer is high-speed once we reach the beginning of a sector. Disks may have multiple heads to transfer bits from several tracks simultaneously.
A track near the circumference is longer than a track near the disk's center. If we record bits with equal density, some tracks will contain more recorded bits than others.
Disks use variable recording density with higher density on tracks near the center than on the circumference to make all the records in a sector of equal length. It equalizes the number of bits on all the tracks of a given sector.
Disks that are permanently conjoined to the unit assembly and cannot remove occasionally are called hard disks. Floppy disks are disk drives with removable disks. The floppy disk drive uses small removable disks made of plastic-coated with magnetic recording material.
There are two sizes generally used, with diameters 5.25 and 3.5 inches. The 3.5 inches disks are smaller and can store more data than the 5.25-inch disks.
Floppy disks are vastly used in personal computers to distribute software to computer users.
Magnetic Tape
Magnetic tape is a storage medium that permits data collection, archiving, and backup for different kinds of data. We make magnetic tape using a plastic strip coated with a magnetic recording medium.
We list Bits as a magnetic stain on the tape along various tracks. We record seven or nine bits together to form a character with a parity bit. Read/write heads are mounted in each track; therefore, that information can be recorded and read as a series of symbols.
We can stop Magnetic tape units initiated to move forward, opposite, or reversed. However, they cannot be started or stopped quickly between characters. For this reason, we refer to recorded data in blocks as records. Gaps of unrecorded tape are added between records to stop the tape.
The tape begins affecting while it is in a gap and achieves its permanent speed when it arrives at the next record. Each record on tape has a recognition bit design at the starting and end. The tape control recognizes the data number by reading the bit design at the starting.
The tape is a strip of plastic coated with a magnetic recording medium. It records Bits as magnetic spots on the tape along several tracks.
It mainly records seven or nine bits simultaneously to form a character and a parity bit. Read/write heads are mounted in every track to record and read as a sequence of characters.
Magnetic tape units can be stopped, moved forward or in reverse, or rewound.
However, they cannot be started or stopped quickly between individual characters.
For this reason, it records data in blocks referred to as records. Gaps of unrecorded tapes are inserted between records to stop the tape.
The tape starts moving in a gap and attains its steady speed when it reaches the next record. Every record on tape has an identification bit pattern at the start and end.
The tape control determines the record number by reading the bit pattern at the beginning.
By reading the bit pattern at the end of the record, the control recognizes the start of a gap.
We address the tape unit by specifying the record number and the number of characters in a record. Records may have fixed or variable lengths.
Flash Memory
Flash memory is a non-volatile memory used to store and transfer data between a personal computer (PC) and digital devices. We can electronically reprogram and erase it. We can often see it in USB flash drives, MP3 players, digital cameras, and solid-state drives.
Flash memory is an electronically programmable, erasable, and read-only memory (EEPROM), but it can also act as standalone storage devices such as USB drives.
Optical Disk
An optical disk is any computer disk that uses optical storage techniques and technology to read and write data. It is a storage device using optical (light) energy. It is a computer storage disk that stores data digitally and uses laser beams to read and write data. It uses optical technology in which laser light is central to the spinning disks.
Memory hierarchy
The Memory Hierarchy is a system wherein the memories are divided into multiple layers in an organized way for faster data processing. Each layer in this system has different properties like speed, cost and capacity. The data is stored in that memory level which provides the right balance of speed, cost and capacity. The frequently accessed data is stored in faster and smaller memory levels as it is closer to the CPU and it is easy to access the memory level while the less accessed data is stored in larger and slower memory levels.