Memory chips are classified mainly based on their capability to retain stored data when power is turned off or interrupted, and below easily stored data can be altered (overwritten). shows various types of memory chips. They are described below.
A. When people talk about computers memory, they usually mean the volatile RAM memory. Physically, this memory consists of some integrated circuit (IC) chips either on motherboard or on a small circuit board attached to motherboard. A computer's motherboard usually has flexibility to esily add more memory chips for enhancing the memory capacity of the system. Hence, if a user decides to have more memory than his/her computer currently has, he/she can buy more memory chips and plug them in empty memory slots on the motherboard. Normally, service engineers do this job. Additional RAM chips, which plug into special sockets on motherboard, are known as single in-line memory modules (SIMMSs).
RAM chips are of two types-dynamic and static. Dynamic RAM (DRAM) uses an external circuitry to periodically "regenerate" or refresh storage charge to retain the stored data. On the other hand, static RAM (SRAM) does not need any special regenerator circuit to retain the stored data. Since it takes more transistors and other devices to store a bit in a static RAM, these chips are more complicated and take up more space for a given storage capacity than dynamic RAMs do. Moreover, a static RAM is faster, costlier, and consumes more power than dynamic RAM. Due to these reasons, large memories use dynamic RAM and static RAM is used mainly for specialized applications. The main memory of most computers uses dynamic RAM.
A. A special type of RAM, called red-only memory (ROM), is a non-volatile memory chip in which data is stored permanently. Usual programs cannot alter this data. In fact, storing data permanently into this kind of memory is called "burning in of data" because data in such memory is stored by using fuse-links. Once we burn fuse-links for some data, it is permanent. We can only read use data stored in a ROM chip (we cannot change them). This is the reason why it is called read-only memory (ROM). Since ROM chips are non-volatile, a ROM chip does not lose its stored data in-case of power off or interruption of power. This is unlike a volatile RAM chip. ROMSs are also known as field stores, permanent stores, or dead stores.
System designers use ROMs mainly to store programs and data, which do not change and are used frequently. A computer carries out most of its basic operations by wired electronic circuits. However, several higher-level operations used frequently, require complicated electronic circuits for their implementation. Hence, instead of building electronic circuits for these operations, designers use special programs to perform them. These programs are called micro-programs because they deal with low-level machine functions and are essentially substitutes for additional hardware. Computer manufactures store these micro-programs in ROMs so that users cannot modify them.
A good example of micro-program is the "system boot program", which contains a set of star-up instructions that are executed when a system is powered on to check if the system hardware (like memory, I/O devices, etc.) is functioning properly. It looks for an operating system and loads its core part in the volatile RAM of the system to produce the initial display-screen prompt. Note that a computer needs this micro-program every time it is switched on, and the computer must retain it even when it is switched off. Hence, the computer's ROM is an ideal storage for this micro-program.
There are two types of read-only memory (ROM)- manufacturer-programmed and user-programmed. A manufacturer-programmed ROM is one in which data is burnt in by the manufacturer of the electronic equipment in which it is used. For example, a personal computer manufacturer may store the system, boot program permanently in a ROM chip located on the motherboard of each PC manufactured by it. Similarly, a printer manufacturer may store the printer controller software in a ROM chip located on the circuit board of each printer manufactured by it. It is not possible for users of such equipment to modify the programs or data stored in the ROM chip. On the other hand, a user-programmed ROM is one in which a user can load and store "read-only" programs and data. That is, it is possible for a user to "customize" a system by converting his/her programs to micro-programs and storing them in a user-programmed ROM chip. Such a ROM is known as Programmable Read-Only Memory (PROM) because a user can program it. Once the user programs are stored in a PROM chip, the system can execute them usually in a function of the time previously required. A special device known as PROM-programmer is used to program a PROM chip (record information in it). However, once a user programs a PROM chip, it becomes a ROM. That is, the information recorded in it can only be read (it cannot be changed). PROM is also non-volatile storage, i.e., information stored in it remains intact even in case of power off or interruption of power.
Once information is stored in a ROM or PROM chip, a user cannot alter it. Erasable Programmable Read-Only Memory (EPROM) chip overcomes this problem by allowing users to reprogram it easily to store new information. R&D personnel (experimenters) often use EPROMs because they frequently change micro-programs to test a system's efficiency with new programs. EPROMs are also useful for those applications in which one may like a store a program in a ROM that would normally not change but under some unforeseen conditions, one may like to alter it. When an EPROM is in use, the user can only''read'' the information stored in it, and the information remains in the chip until the user erases it.
EPROM chips are of two types- Ultra Violet EPROM (UVEPROM), which requires exposing the chip for some time to ultraviolet light for erasing stored information stored in it, and Electrically EPROM (EPROM), which requires use of high voltage electric pulses for erasing information stored in it. It is easier to alter information stored in an EEPROM chip as compared to an UVEPROM chip. EEPROM is alo know as flash memory because of the ease with which a user can alter programs stored in it. May new I/O and storage devices like USB (Universal Serial Bus) pen drive MP3 music player use flash memory.
We saw that use of main memory helps in minimizing the disk-processor speed mismatch to a large extent because the rate of data fetching by CPU from main memory is about 100 times faster than that from high-speed secondary storage like disk. Still the rate at which CPU can fetch data from memory is about 10 times slower than the rate at which CPU can process data. Performance of processors often gets limited due to this memory. processor speed mismatch. Obviously, we can improve the overall system performance greatly by minimizing the memory-processor speed mismatch. Cache memory (pronounced "cash" memory) is commonly used for this purpose. It is an extremely fast, small memory between CPU and main memory whose access time is closer to the processing speed of CPU. It acts as a high-speed buffer between CPU and main memory and computer systems use it to store temporarily very active data and instructions during processing. Since cache memory and instructions available in cache that processes need during their present processing.