Main Memory

Introduction

• Main memory, is often referred to simply as “memory” or “RAM” (Random Access Memory) because data can be accessed in any order, regardless of its physical location in memory. This allows the CPU to quickly retrieve and manipulate data without the need to sequentially read through the entire memory.
• It plays a critical role in the operation of a computer system by providing fast, temporary storage for data and instructions needed by the CPU during program execution. Its speed and capacity significantly impact system performance and responsiveness.

Definition

• Main Memory is a type of volatile/temporary computer memory that stores data and instructions that are actively being used by the CPU (Central Processing Unit) or other hardware components currently. 

Features

• It serves as the primary workspace for the computer’s operating system, applications, and currently executing programs hence main memory is considered as Primary Memory.
• It is considered as the busiest memory in the computer.
• It is a fast and large memory but is slower than processor and cache memory.
• This memory is accessed directly by the processor to retrieve data.
• It stores programs and data which are currently required by the CPU for carrying out its operations.
• The CPU communicates directly with the main memory for data & instructions.
• The size of the main memory is kept small because of its high cost.
• It is a volatile type of memory that its contents are lost when the power is turned off. In other words, main memory cannot retain data when the computer is powered down.
• The access time of main memory is about 50-70 ns.
• The main memory is a semiconductor type of memory or device that is built using integrated circuits.
• The data is stored in binary form in the main memory. Numeric as well as non-numeric data can be represented in binary form.
• The capacity of main memory is measured in bytes (e.g., megabytes or gigabytes) and determines the amount of data and instructions that can be stored and accessed at any given time. Modern computers typically have several gigabytes or even terabytes of RAM.
• Main memory is part of the memory hierarchy in a computer system. The memory hierarchy is designed to optimize data access by storing frequently accessed data closer to the CPU at faster memory levels.
• Main memory is commonly implemented using either Dynamic Random Access Memory (DRAM) or Static Random Access Memory (SRAM).

Types of Main Memory

• Memory/Main memory can be broadly classified into two major parts –

(A) RAM(Random Access Memory) and

(B) ROM (Read Only Memory)

(A) RAM(Random Access Memory) :

• • 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.
  • The information in RAM can be written/erased electrically.
  • It is called random access since any memory location can be accessed randomly for reading and writing.
  • The access time in this mode is the same for each memory location.
  • It is also called temporary memory, i.e. when the system is shut down, the memory data is lost.
  • RAM is the main storage place where the program and software get loaded/stored.
  • When the 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, the CPU can both READ data from RAM and WRITE data into the RAM.
  • In a RAM, the memory cell must be associated with a read-and-write facility. There are six (6) transistors are needed per memory cell in a static RAM. Dynamic RAMs require fewer transistors per memory cell.

Types of RAM

• • There are two important types of RAMs:
    (i) Static RAM (SRAM)
    (ii) Dynamic RAM (DRAM)
  • Both static and dynamic RAMs use CMOS(Complementary metal–oxide–semiconductor)technology. CMOS devices consume less power. CMOS technology is used for constructing integrated circuit (IC) chips, including microprocessors, microcontrollers, memory chips (including CMOS BIOS), and other digital logic circuits.

(i) Static RAM (SRAM)

• • • DRAM is cheaper and more dense but requires periodic refreshing to maintain data integrity, while SRAM is faster and consumes less power but is more expensive and less dense.
    • Static RAMs retain stored information only as long as the power supply is ON.
    • Static RAMs are costlier and consume more power.
    • Static RAMs have a higher speed than dynamic RAMs.
    • SRAM is faster and consumes less power but is more expensive and less dense so used in cache memory.
    • SRAM does not need to be refreshed periodically like DRAM.
    • Static RAMs hold information in a flip-flop circuit consisting of two cross-coupled inverters.

(ii) Dynamic RAM (DRAM)

• • • Dynamic RAM loses its stored information in a very short time (a few milliseconds) even though the power supply is on.
    • Dynamic RAMs are cheaper and denser and consume less power and hence are mostly used in main memory.
    • Dynamic RAM requires the data to be refreshed periodically to retain or maintain data integrity.

Type of RAM’s Chip

There are the following types of chips commonly used in the making of RAM –

(i) EDO (Extended Data Output) RAM:

• • 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 that are sequentially executed, the data are available without wait states.

(ii) SDRAM (Synchronous Dynamic) 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 busy.
  • SDRAM is often used as mass storage whereas SGRAM is used as a high-end graphics memory.

(iii) Dual-Ported DRAM :

• • These types of RAM allow one 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 and WINDOWS applications.

(iv) SIMM and DIMM:

• • These 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.

(B) ROM(Read Only Memory) :

• • A Read-Only memory (ROM) is a non-volatile 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 stored permanently.
  • A ROM is a memory chip on which data is written/programmed only once during the time of its manufacture. Information is written into it through a process of Masking(not electrically).
  • ROM’s data can’t be erased(not erasable/not possible to erase).
  • Unlike RAM, the information from ROM can only be READ and it is not possible to WRITE fresh information to it i.e. the CPU can only fetch or READ instructions from ROM. This is the reason why it is called ROM.
  • ROM chips are supplied by computer manufacturers and a user can’t modify the programs stored inside the ROM chip.
  • Computers almost always contain a small amount of Read-Only Memory (ROM).
  • It is much cheaper compared to RAMs when produced in large volumes.
  • ROM is used for storing a special set of instructions, which the computer needs when it starts up (boots up).
  • The contents of ROMs are decided by the manufacturers and the contents are permanently stored in a ROM at the time of manufacture.

Types of ROM

(A) From the programming mode point of view, ROMs are of two types –

(a) Masked-Programmed ROM

• • • ROMs in which contents are written at the time of IC manufacture are called mask-programmed ROMs.
    • If we write (or say) ROM it means masked programmed ROM.
    • An example of a ROM is the Toshiba mask ROM, TCS 534000.

(b) User-Programmed ROM

• • • PROM, EPROM, and EEPROM or any other kind of PROM are user-programmable ROMs in which user requirement type of modification can be done anytime after manufacture.
      • PROM(Programmable ROM)
        • PROM is a Read Only Memory.
        • A PROM is a memory chip on which data can be written only once but any time after its manufacture.
        • In PROM, a user can customize a system by storing their own program in a PROM chip only once.
        • 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.
        • PROM is also a non-volatile memory i.e. the stored information remains even if power is switched off.
        • The basic difference between a PROM and a ROM is that a PROM is manufactured as blank memory, whereas a ROM is programmed/written during the manufacturing process.
        • To write data on a PROM chip, we need a special device called a PROM programmer or a PROM burner. The process of programming in a PROM is sometimes called burning the PROM.
        • A PROM is a non-volatile memory.
        • Information is written into it electrically.
        • PROM’s data can’t be erased(not erasable/not possible to erase).

• • • • EPROM(Erasable PROM)
        • It is a Read-only  memory.
        • It is a non-volatile memory.
        • Information is written into it electrically.
        • Data can be erased many times using UV light.

• • • • EEPROM(Electorally EPROM)
        • It is a Read-only memory.
        • It is a non-volatile memory.
        • Information is written into it electrically.
        • Data can be erased many times electrically.

Flash Memory/Pen Drive

• Pen Drive is a very convenient, flexible/removable data storage medium that can store up to 1TB/more data.
• Pen Drives are a smaller, faster, durable, and more reliable/removable storage medium.
• It is a portable USB flash memory device.
• It is integrated with a USB (Universal Serial Bus) interface.
• It can be used to quickly transfer data from one system to another.
• The name pen drives because these devices resemble a small pen or pencil in shape and size.
• Flash drives implement the USB mass storage device class so modern operating systems can read and write from them without installing the device driver software.
• Most computers can even boot up from flash drives.
• Advantages :
  • It has a large portable storage device that is smaller, faster, and more reliable in data processing.
• Disadvantages :
  • Most USB flash drives do not have a write-protect mechanism.
  • Due to their small size, they can easily be misplaced or lost.