Monday, September 23, 2002		Feature

In the memory of the PC
Sukpreet Singh Giani

FOR human beings, memory plays a critical role in the retention and retrieval of experiences and this experiences influences subsequent behaviour. The same is true for the computer memory in order to have a "good behaviour" from the system. There are numerous ways through which the computer memory could be classified:

• By location in the system

• By principles of operation

• And by function

However the easiest method to classify the computer memory is to make the distinction as primary (or main) and secondary (or auxiliary) storage devices.

The secondary storage includes the computer’s permanent storage devices, such as hard disk; floppy disk and CD ROMs are among the recent ones in use. The primary storage device is the computer’s random access memory (RAM)—which is impermanent functionally but a storage device nevertheless. However its contents are temporary.

Among secondary storage devices is the most used one — hard disk drive. It is interesting to note that the hard disk drives get their name from the material on which the data is saved. Hard drives save the data on metal or ceramic (hard) disks coated with a thin magnetic film. When the "save" icon is pressed the disk tends to spin at 3600 revolutions per minute and the read/write heads ride over the surface of the disk on a cushion of air, 10 to 25 millionths of an inch deep. Data is placed or read by small magnetic head that floats over the spinning platters. Hard disks are manufactured in a total dust free environment and sealed to prevent contaminants from interfering with the close tolerances between the head and the disk. Over the years, maximum efforts have been put in to enhance the storage capacity of the hard disk and from 10 MB to 100 GB is a great evolution indeed as in present day scenario the hard disk can also store large amounts of data for a small amount of money.

Floppy disk what we see from outside is just the protective plastic jacket containing within itself an all important round and flat piece of Mylar coated with ferric oxide capable of holding a magnetic field. Data is stored on the floppy disk by read write head of the floppy disk drive, which alters magnetic orientation of the particles. Orientation in one-direction represents binary 1; orientation in the other, binary 0. Depending on its capacity, such a disk can hold from a few hundred thousand to over one million bytes of data.

CD-ROM, acronym for compact disk read-only memory, is the most recent form of storage characterised by high capacity (roughly 600 megabytes) and the use of laser optics rather than magnetic means for reading data. Although CD-ROM devices are strictly read-only, they are similar to optical WORM (write once, read many).

The primary memory is mainly linked with Random Access Memory or RAM, a semiconductor-based memory that can be read and written to by the microprocessor or other hardware devices. The Disk Cache is a portion of a computer’s random access memory set aside for temporarily holding information read from the hard disk. A disk cache does not hold entire file as does a RAM disk (a portion of memory that acts as if it were a disk drive). Instead, a disk cache is used to hold information that either has recently been requested from disk or has previously been written on to disk. For example, a program might need to be accessed repeatedly from hard disk, if the required information remains in a disk cache; access time is considerably faster than if the program must wait for the disk-drive mechanism to fetch the information from disk. As the CPU can handle only small amount of data at a time, the CPU utilises cache as a temporary holding spot while it performs an operation. Cache assists the CPU in the manipulation process.

When Random Access Memory (RAM) and cache both are used for temporary storage of information where is the basic difference in them is the most obvious question that comes to the mind. The difference between cache and RAM is the amount of information stored. RAM stores part or complete portions of programs and the manipulated data whereas cache holds only small parts of the manipulated data. The linkage of the RAM with the permanent storage devices can be understood by stating that without RAM, the CPU would have to access the computer’s hard disk or floppy disk thus slowing down the CPU because the computer has to convert the information from impulses stored on magnetic media to electrical impulses. Cache speeds up the CPUs ability to manipulate the data while RAM speeds up the CPUs access to the program and the remaining data. RAM and cache both store information electronically which means the information disappears when the computer is turned off or restarted, hence rightly classified as primary or auxiliary memory while the hard disk, floppy disk and CD ROM are capable of storing the memory even when the computer is switched off and hence classified as main or secondary storage devices.