Primary Memory - RAM and ROM
All modern computers use semiconductor memory as primary memory. One of the important semiconductor memories used in desktop computers is Random Access Memory (RAM). Here “random access” means that any storage location can be accessed (both read and write) directly. This memory is faster, cheaper, and provides more storage space in lesser physical area. These very large-scale integrated semiconductor memory chips are mounted on pluggable printed circuit boards (PCBs). Enhancement or replacement of memory with such PCB memory modules is easy. These characteristics have made semiconductor memory more popular and attractive.
The only drawback of semiconductor memory is that it is volatile, i.e., it loses its contents whenever power is switched off.
Random Access Memory (RAM)
RAM holds the data and instructions waiting to be processed by the processor. In addition to data and program’s instructions, RAM also holds operating system instructions that control the basic functions of a computer system. These instructions are loaded into RAM every time the computer is turned on, and they remain there until the computer is turned off.
There are two types of RAM used in computer systems– dynamic and static.
- Dynamic RAM (DRAM) is a type of RAM that employs refresh circuits to retain its content in its logic circuits. Each memory cell in DRAM consists of a single transistor. The junction capacitor of the transistor is responsible for holding the electrical charge that designates a single bit as logical 1. The absence of a charge designates a bit as logical 0. Capacitors lose their charge over time and therefore need to be recharged or refreshed at pre-determined intervals by a refreshing circuitry.
- A more expensive and faster type of RAM, Static RAM (SRAM), does not require such type of refreshing circuitry. It uses between four to six transistors in a special ‘flip- flop’ circuit that holds a 1 or 0 while the computer system is in operation. SRAM in computer systems is usually used as processor caches and as I/O buffers. Printers and liquid crystal displays (LCDs) often use SRAM to buffer images.
SRAM is also widely used in networking devices, such as routers, switches, and cable modems, to buffer transmission information. Both dynamic and static RAM are volatile in nature and can be read or written to. The basic differences between SRAM and DRAM are listed in Table.
Static RAM | Dynamic RAM |
It does not require refreshing. |
It requires extra electronic circuitry that ‘‘refreshes’’ memory periodically; otherwise its content will be lost. |
It is more expensive than dynamic RAM. |
It is less expensive than static RAM. |
It is lower in bit density. | It holds more bits of storage in a single integrated circuit. |
It is faster than dynamic RAM. |
It is slower than SRAM, due to refreshing. |
There are several popular types of dynamic RAM used in computers. They are :-
- SDRAM (Synchronous Dynamic RAM),
- RDRAM (Rambus Dynamic RAM) and
- DDR RAM (Double Data Rate RAM).
The SDRAM used to be the most common type of RAM for personal computers. It was reasonably fast and inexpensive. It is no more used in the present day for personal computers as much improved RAMs are available now.
The RDRAM was developed by Rambus Corporation and is its proprietary technology. It is also the most expensive RAM and is used mostly in video interface cards and high- end computers that require fast computation speed and data transfer. RDRAMs are preferred for high-performance personal computers.
The DDR RAM is a refinement of SDRAM. DDR stands for Double Data Rate. It gives faster performance by transmitting data on both the rising and the falling edges of each clock pulse. DDR 2, DDR3 are other higher-speed versions of DDR RAM. Another type of RAM, termed Video RAM (VRAM), is used to store image data for the visual display monitor. All types of video RAM are special arrangements of dynamic RAM (DRAM). Its purpose is to act as a data storage buffer between the processor and the visual display unit.
There is a persistent mismatch between processor and main memory speeds. The processor executes an instruction faster than the time it takes to read from or write to memory. In order to improve the average memory access speed or rather to optimize the fetching of instructions or data so that these can be accessed faster when the CPU needs it, cache memory is logically positioned between the internal processor memory (registers) and main memory. The cache memory holds a subset of instructions and data values that were recently accessed by the CPU. Whenever the processer tries to access a location of memory, it first checks with the cache to determine if it is already present in it. If so, the byte or word is delivered to the processor. In such a case, the processor does not need to access the main memory. If the data is not there in the cache, then the processer has to access the main memory. The block of main memory containing the data or instruction is read into the cache and then the byte or word is delivered to the processor. There are two levels of cache.
Level 1 (Primary) cache This type of cache memory is embedded into the processor itself. This cache is very fast and its size varies generally from 8 KB to 64 KB.
Level 2 (Secondary) cache Level 2 cache is slightly slower than L1 cache. It is usually 64 KB to 2 MB in size. Level 2 cache is also sometimes called external cache because it was external to the processor chip when it first appeared.
Read Only Memory (ROM)
Read Only Memory (ROM) It is another type of memory that retains data and instructions stored in it even when the power is turned off. ROM is used in personal computers for storing start-up instructions provided by the manufacturer for carrying out basic operations such as bootstrapping in a PC, and is programmed for specific purposes during their fabrication. ROMs can be written only at the time of manufacture.
Another similar memory, Programmable ROM (PROM), is also non-volatile and can be programmed only once by a special device. But there are instances where the read operation is performed several times and the write operation is performed more than once though less than the number of read operations and the stored data must be retained even when power is switched off. This led to the development of EPROMs (Erasable Programmable Read Only Memories).
In the EPROM or Erasable Programmable Read Only Memory, data can be written electrically. The write operation, however, is not simple. It requires the storage cells to be erased by exposing the chip to ultraviolet light, thus bringing each cell to the same initial state. This process of erasing is time consuming. Once all the cells have been brought to the same initial state, the write operation on the EPROM can be performed electrically.
There is another type of Erasable PROM known as Electrically Erasable Programmable Read Only Memory (EEPROM). Like the EPROM, data can be written onto the EEPROM by electrical signals and retained even when power is switched off. The data stored can be erased by electrical signals. However, in EEPROMs the writing time is considerably higher than reading time. The biggest advantage of EEPROM is that it is non-volatile memory and can be updated easily, while the disadvantages are the high cost and the write operation takes considerable time.
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