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Semiconductor [Main] Memory
All computers except very small computers contain both semiconductors as well as magnetic memory. All modern computers use semiconductor memory as its main memory [Primary Memory]. Semiconductor memory is known as Random Access Memory [RAM] because any part of the memory can be accessed for reading and writing. It stores programs and data which is currently [currently under execution] needed by the CPU. RAM is used as temporary storage only because it is volatile. Data in RAM is lost in case of power interruption. Another part of main memory is Read Only Memory [ROM]. ROMs are those memories on which it is not possible to write the data. They can only be read.
Main memory holds the programs and data required by the CPU for carrying out its operations. The primary [main] storage is a semiconductor device that is built using integrated circuits. The data is stored in binary form by main memory. Numeric as well as nonnumeric data can be represented in binary form. With two binary digits, we can represent four different characters. With three binary digits, we can represent eight different characters. Computers internally use eight binary digits to represent characters and digits [A binary digit is referred to as bit and eight bits are called a byte]. 256 different characters can be represented by eight bits [one byte].
The capacity of a computer’s memory is usually expressed in terms of bytes. Coding system such as ASCII [American Standard Code for Information Interchange] uses various arrangements of bits to form byte that represents numbers 0 to 9, the letters of the alphabets, and many other characters [symbols] used in the computer system.
Storage capacities are frequently measured in Kilobytes [KB], Megabytes [MB], Gigabytes [GB], or Terabytes [TB].
Commonly Used Names, Abbreviations, And Storage Capacity In Bytes
Name [Abbreviations] – Number Of Bytes
Byte [B] – 1 Byte
Kilobytes [KB] – 1024 Bytes
Megabytes [MB] – 1024*1024 Bytes
Gigabytes [GB] – 1024*1024*1024 Bytes
Terabytes [TB] – 1024*1024*1024*1024 Bytes
Types Of Main Memory
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RAM [Random Access Memory]
1- The Read and write memory [R/W memory] of a computer is called RAM. The user can write information into RAM and read information from it. It is known as random access since any memory location can be accessed in a random manner for reading and writing. The access time is same for each memory location.
2- Random Access Memory [RAM] is really the main store and is the place where the program and software we load get stored. When CPU runs a program, it fetches the program instructions from the RAM and carries them out. Similarly, if the CPU needs to store the final results of calculations, it stores them in RAM. Thus, you [CPU] can both READ data from RAM and WRITE data into the RAM.
3- RAM is a volatile memory, that is, the stored data are lost, if the power goes off.
There Are Two Important Types Of RAMs.
Dynamic RAM [DRAM]
Dynamic RAM stores binary information in the form of an electric charge on the capacitor. Binary 1 is represented as the presence of change whereas binary 0 is represented as absence of electric charge. Stored charge has a natural tendency to discharge, therefore it requires frequent refreshing to maintain data. In static RAM binary values [1, 0] are stored using traditional flip fly logic gates. Dynamic RAMs are cheaper and consume less power. Dynamic RAM is cheaper so used for main memory.
Static RAM [SRAM]
Static RAMs retain stored information only as long as the power supply is on whereas a Dynamic RAM loses its stored information in a very short time [a few milliseconds] even though the power supply is on. Static RAMs are costlier and consume more power. Static RAMs have higher speed than dynamic RAMs. Static RAM is faster so used in cache memory.
Both static and dynamic RAMs use CMOS technology. CMOS devices consume less power. Static RAMs hold information in a flip-flop circuit consisting of two cross coupled inverters. In a RAM, the memory cell must be associated with read and write facility. The following are commonly used RAM chips.
EDO [Extended Data Output DRAM]: In an EDO RAM, any memory access stores 256 bytes of data into latches. The latches hold the next 256 bytes of information, so that in most programs which are sequentially executed, the data are available without wait states.
SDRAM [Synchronous DRAM] And SGRAM [Synchronous Graphics RAM]: These RAM chips use the same clock rate as the CPU uses. As a result, the memory chips remain ready to transfer data when the CPU expects them to be ready. SDRAM is often used as mass storage whereas SGRAM used as a high end graphics memory.
Dual-Ported DRAM: These types of RAM allow to access two memory locations simultaneously. Sometimes it is also called video RAM [or VRAM]. WRAM [Window RAM] is a special version of VRAM, which is commonly used in PCs running Windows Operating System and Windows Applications.
SIMM And DIMM: They stand for Single-Inline and Double-Inline Memory Modules. These are small printed circuit cards, on which several DRAM memory chips are placed. Such cards are plugged into the system board of the computer.
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ROM [Read Only Memory]
1- A Read Only Memory [ROM] is a nonvolatile memory, i.e., the information stored in it is not lost even if the power supply goes off. Thus, a Read Only Memory [ROM] is one in which information is permanently stored.
2- Unlike RAM, the information from ROM can only be READ and it is not possible to WRITE fresh information to it. That is the CPU can only fetch or READ instructions from ROM. This is the reason why it is called ROM. Computer almost always contains a small amount of Read Only Memory [ROM]. It is much cheaper compared to RAMs when produced in large volumes.
3- ROM is used for storing special set of instructions, which the computer needs when it startup [boot up].
4- Unlike RAM, the information from ROM can only be READ and it is not possible to WRITE fresh information to it by the users/programmers. That is the CPU can only fetch or READ instructions from ROM. This is the reason why it is called ROM. The contents of ROMs are decided by the manufacturers. The contents are permanently stored in a RAM at the time of manufacture.
5- From programming mode point of view, we have Masked-Programmed and User-Programmed.
6- ROMs in which contents are written at the time of IC manufacture are called mask-programmed ROMs. PROM, EPROM, and EEPROM or any other kind of PROM are user-programmable ROMs. If we simply write [or say] ROM it means masked programmed ROMs are called PROM, EROM, and EEROM, etc.
7- An example of ROM is the Toshiba mask ROM, TCS 534000.
PROM [Programmable ROM]
1- A variation of ROM chip is programmable read only memory [PROM]. A PROM is a memory chip on which data can be written only once.
2- ROM chips are supplied by computer manufacturer and it is not possible for a user to modify the programs stored inside the ROM chip. However, in case of PROM, it is possible for a user to customize a system by storing own program in a PROM chip.
3- Once a program has been written onto a PROM chip, the recorded information cannot be changed i.e., the PROM becomes a ROM and it is only possible to read the stored information.
4- PROM is also a non-volatile memory i.e., the stored information remains even if power is switched off.
5- The basic difference between PROM and a ROM is that a PROM is manufactured as blank memory, whereas a ROM is programmed during the manufacturing process. To write data on a PROM chip, you need a special device called a PROM programmer or a PROM burner. The process of programming a PROM is sometimes called burning the PROM. Earlier most of the electronic typewriters had a PROM chip.
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