TYPES OF COMPUTER MEMORY
- Secondary Memory
Computer memory is responsible for storing data and applications on a temporary or a permanent basis. It enables a person to retain the information stored on the computer. Without a memory device/arrangement in place, the processor would not be able to find a place which is needed to store the calculations and processes. There are various types of computer memory that can be installed, depending upon the actual need for functioning and specifications of the system.
Computer memory can be primarily classified into two types: Primary Memory and Secondary Memory.
Primary Memory (also called main memory), is used for immediate access of data by the processor. While primary memory storage demonstrates faster processing ability, it is costly and hence is not largely used for data storage. Most computer systems around the world use primary memory only for bootstrapping and related purposes, and use secondary memory devices for personal data storage purpose.
Primary Memory can be divided into two types - Random Access Memory (RAM) and Read Only Memory (ROM). RAM retains its contents as long as the power supply is on. A RAM chip is used as primary memory in most computers today. However, older computers (in the '80s) used ROM devices (floppy disks, magnetic tapes, paper clips or punches; but more prominently floppy disks) as primary memory mechanism.
RAM is a memory scheme within the computer system responsible for storing data on a temporary basis, so that it can be promptly accessed by the processor as and when needed. It is volatile in nature, which means that data will be erased once supply to the storage device is turned off. RAM stores data randomly and the processor accesses these data randomly from the RAM storage. The information stored in the RAM is typically loaded from the computer's hard disk, and includes data related to the operating system and certain applications. When the system is switched off, the RAM loses all stored information. The data remains stored on secondary storage though, and can be retained when the system is running again. Some of the earliest computers used delay-line format of computer storage. Most modern computers use an embedded RAM circuitry on the motherboard which reads data in bursts. So, modern RAM devices are not random memory devices as such; they are burst memory access devices, but the term RAM has stuck in everyday usage.
There are primarily two forms of RAM: Static RAM (SRAM) and Dynamic RAM (DRAM).
Static RAM: The most expensive of the lot, SRAM uses bistable latching circuitry to store one bit each, and hence is faster than its counterpart. Its high price prevents it from being widely used in everyday computing machines, but many modern machines use SRAM as the processor's cache register.
Dynamic RAM: Widely used in modern computers as primary memory, DRAM is slower than SRAM, but is inexpensive due to its one transistor-one capacitor paired assembly of memory storage.
Unlike RAM, ROM is a permanent form of storage. ROM stays active regardless of whether power supply to it is turned on or off. In spite of this, ROM was used (in rare cases is still used) as the primary device for most computers back in the '80s. This was because ROM devices do not allow data stored on them to be modified. As the name itself suggests, data can only be accessed and read by the user, not overwritten, upgraded, or modified. This made it an ideal choice as bootable devices for old computers, programmable interpreters, and portable OS files carrier. The system programs stored on a ROM device could never be altered and hence, stayed secure for use.
The ROM memory used in modern computers is pre-programmed by the circuit manufacturer and cannot be altered by the user. The main reason why ROMs are not widely used in modern computer systems is because of the masking and error-retrieval costs. These processes are very expensive, and virtually negate the inexpensive manufacturing involved.
Secondary memory is available on mass storage devices for permanent data storage. Data stored on a secondary device is retained even when it is not supplied any power. This data can be transported in most cases, and looks and appears the same on any machine, irrespective of where the data was first copied onto the secondary storage device.
Unlike primary memory, secondary memory is not directly accessible by the computer. When a computer needs to run or execute an application stored in secondary memory, it first brings it to primary memory storage for a while, to control and carry out its execution. Once execution of the application is done, the processor releases the application and restores its control and memory data with the secondary memory device.
Popular secondary memory devices include hard disk drives, flash drives (pen drives, memory cards etc.), and zip drives. A couple of decades ago, as the 'personal computer' (PC) revolution was gathering storm, especially in America, floppy disks had acquired almost a cult status amongst PC users. Eventually, floppy disks were phased out by a better technology - a contemporary form of the optical drive called the Compact Disc or CD. CDs came with better speed and larger storage alternatives as compared to floppies. DVDs eventually took over the mantle from CDs, courtesy their ability to store almost 4 times more data. Although DVDs are still widely used, the preferred devices of secondary storage nowadays are portable hard disk drives or flash drives.
The essential data storage techniques of the '50s and '60s, punch tapes and punch cards have become passé since the advent of newer data storage formats.
Punch Tapes: A 0.1 mm thick paper strip was used to store data in the form of punched holes. A keyboard was used to punch the desired alphabet onto the tape. This alphabet was represented on the tape by a certain number and a select pattern of holes. A separate tape machine was used to send and receive these tapes for distance communication purposes. For computing purposes, stored data on the tapes would be read by the processing unit's inbuilt decoding machine.
Punch Cards: Primarily used in textile and handloom industries, punch cards stored instructions of operation for machines. Early digital computers made punch cards popular as data storage assemblies. Their working is pretty much similar to that of punch tapes, except for the fact that instead of paper strips, this technique uses cards about 3¼ inches × 7⅜ inches in size. Around the 1920s and 1930s, IBM hit upon a series of card innovations which enabled pre-punched data verification cards and cards with the ability to read alphabets, numbers, and signs (symbols) on a single multipurpose card.
Magnetic tape as a recording technique was invented in 1928. This formed the basis for magnetic digital information storage. This form of data storage gained immense popularity in the '70s, when magnetic tapes were wound around 10.6-inch reels. The device used for the read-write operations on these tapes is called a tape drive. Until the early 1980s, magnetic tape drives were huge external devices. With the introduction of IBM's 3480 family of magnetic tape cartridges, most magnetic tape storage assembly went inside the central processing unit.
Transferring data at around 7,200 characters/second, magnetic tapes store data in sequential order, which also can be accessed only in a sequential order. The magnetic tape's storage density and feasibility offered it a ready-made advantage against punched storage techniques. Even through the '90s, as floppy disks and compact discs were taking over the market, magnetic tapes held a fan-like following among large corporations for large-scale data storage. By the turn of the century, as solid state data storage took over, magnetic tapes lost hold with them too.
The floppy disk memory technique uses a thin plastic-coated film covered with magnetic material. It is covered with a protective plastic cover. Initially developed by IBM as inexpensive microcode feeders in 1967, floppy disks were made commercially available in 1971 to the public.
Floppy disks began as giant 8-inch diskettes, and eventually evolved into 5¼-inch diskettes, and later 3½-inch diskettes. Floppy disks can easily be termed the most popular data storage forms ever, considering they were launched in 1971 and were broadly used right up to the late 2000s. The fact that their availability coincided with the rise of popularity of personal computers among the general public can be attributed as one of the main reasons behind its immense popularity, the others being portability, feasibility, cost-effectiveness, and the lack of better options.
As of today, no modern machines are integrated with a floppy disk drive, though their popularity amongst low cost data management companies remains intact.
Philips and Sony collaborated in the '70s on a project to create a new digital audio disc. This collaboration brought together the optical disc drive technologies both the companies were earlier separately working on. Launched in 1982-83, the Compact Disc (CD) eventually went on from being an audio disc to a data storage device.
The DVD, (originally Digital Video Disc, but later amended to Digital Versatile Disc) format was based on the CD format, and was developed together by Philips, Sony, Toshiba, and Panasonic around the early '90s. It was launched in 1995 and became an instant success by the virtue of being same size as a CD yet offering almost 4 times its memory space. While data storage isn't forfended, DVDs are mostly used for audio and video recording/storage/playback purposes.
In the late '90s, the popularity of CDs took a major three-way hit. While the launch of DVD had already put it out of favor with video enthusiasts, its audio and data storage purposes also waned in lieu of advancing technology. Affordable portable hard disk drives and flash drives drove CDs out as a preferred form of data storage. On the other hand, the easy availability of MP3 players and the legendary rise of Apple's iPod, practically drove audio CDs out of the market. The DVD too has found successors in the form of HD DVD and Blu-ray discs, and is in the gradual process of being phased out from regular use.
The dominant technique for storing data in current times, a hard disk consists of rapidly rotating discs with a magnetic head to read and write data. Data can be accessed randomly. HDDs were introduced by IBM (Yes, again IBM!) around the late 1950s for real-time transaction processing machines. A few years later, IBM commercially launched the IBM 1311 model, which was almost as big as a dishwasher, and had the capacity to store about 2 million characters.
Eventually, hard disk drives began to shrink in size and increase in storage capacity. Hard disk drives were sold to PC and Mac users in the '80s as an external device with a SCSI port on the back of the machines. A series of innovation on part of industry leaders through the '80s and early '90s led to the hard disk being integrated inside the CPU. A typical desktop hard disk is 3.5 inches in size, while for laptops it is 2.5 inches.
As portability and convenience became keywords in modern times, hard disks have again become portable. External hard disks typically use the USB plug-and-play mechanism and are very cost-effective. Modern external hard disks can hold up to 2 terabytes of data.
Hard disks seem to be in for the long haul. No better option is in sight for now, and one can easily predict that hard disks should continue to rule as the preferred form of data storage in the years to come.
A flash drive is a data storage device that uses flash memory for storage purposes. Typical in design, flash drives are light-weight and small in design; and are hence easily portable. Flash drives operate from the power supplied by a computer's USB port (the port in which they are plugged in). The data on it can be erased and re-programmed as per the user's requirements. It only has a specific number of erase and write cycles that it can withstand, after which it creates a tendency to lose out on the stored information. Memory cards and USB flash drives are some modes of this type of memory storage. Low cost, minimal power consumption, and portable features make flash drives extremely desirable and popular in modern times.
The concept of computer memory has evolved since the first electronic computer (ENIAC) was set up in 1946 with a primitive read only pre-stored programming mechanism. ENIAC used function tables for storing instructions. Its maximum storage capacity was 600 two-hundred digit decimal instructions. The way data is stored today and the volumes in which it can be stored today is like a million miles ahead of that.
Memory management has become an important concept in every computer programmer's textbook. Corporations and computer scientists keep researching for newer, simpler, easier, and cost-effective methods of memory storage that can hold larger and larger capacity of data than what is currently possible. Computer memory and its evolution is a constant process, much like the rest of technology. It has changed multifold over the last few decades; expect it to change multifold in the decades to come.