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The document summarizes the memory hierarchy in computers including main memory and auxiliary memory. It discusses how memory is organized in a pyramid structure from fastest and smallest cache memory to larger but slower main memory, and largest but slowest auxiliary memory. It describes the different types of main memory which include RAM that is volatile and loses data when powered off, and ROM that is non-volatile and permanent. RAM includes SRAM and DRAM that differ in how they store bits. Auxiliary memory includes hard disks, floppy disks, CDs, USBs and provides bulk storage when the computer is powered off.

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MEMORY HIERARCHY, MAIN MEMORY & AUXILLARY MEMORY Presented By: Mehta Aesha, Patel Krina , Nandasana Mansi Enrollment number: 12102080601009, 12102080601065,
• A memory unit is a collection of storage units or devices together.The memory unit stores the binary information in the form of bits.Generally, memory is categorized into two types:(1)Volatile memory • (2) Non-volatile memory. • (1) Volatile memory:This loses its data when power is switched-off. • (2) Non-volatile memory:This is a permanent storage and doesnot lose its data when power is switched-off.
Memory Hierarchy • The memory unit is an essential component in any digital computer since it is needed for storing programs and data • Not all accumulated information is needed by the CPU at the same time • Therefore, it is more economical to use low-cost storage devices to serve as a backup for storing the information that is not currently used by CPU
Memory Hierarchy • Computer Memory Hierarchy is a pyramid structure that is commonly used to illustrate the significant differences among memory types. • The memory unit that directly communicate with CPU is called the main memory • Devices that provide backup storage are called auxiliary memory • The memory hierarchy system consists of all storage devices employed in a computer system from the slow by high-capacity auxiliary memory to a relatively faster main memory, to an even smaller and faster cache memory
Figure
Main memory • Most of the main memory in a general purpose computer is made up of RAM integrated circuits chips, but a portion of the memory may be constructed with ROM chips • RAM– RandomAccess memory – Integrated RAM are available in two possible operating modes, Static and Dynamic • ROM– Read Only memory
RAM  ARAM chip is better suited for communication with the CPU if it has one or more control inputs that select the chip when needed  Read/write memory, that initially doesn’t contain any data  The computing system that it is used in usually stores data at various locations to retrieve it latter from these locations  Its data pins are bidirectional (data can flow into or out of the chip via these pins), as opposite to those of ROM that are output only  It loses its data once the power is removed, so it is a volatile memory  It has a directional select signal R/W’; When R/W’=1, the chip outputs data to the rest of the circuit; when R/W’ = 0 it inputs data from the rest of the circuit
FIG: RAM
Random-Access Memory Types Static RAM (SRAM) Each cell stores bit with a six-transistor (Diode) circuit. Retains value indefinitely, as long as it is kept powered. Relatively insensitive to disturbances such as electrical noise. Faster and more expensive than DRAM. Dynamic RAM (DRAM) Each cell stores bit with a capacitor and transistor. Value must be refreshed every 10-100 ms. Sensitive to disturbances. Slower and cheaper than SRAM.
FIG :SRAM FIG:DRAM
ROM ROM is used for storing programs that are PERMENTLYresident in the computer and for tables of constants that do not change in value once the production of the computer is completed. The ROM portion of main memory is needed for storing an initial program called bootstrap loader, witch is to start the computer software operating when power is turned off
FIG: ROM
ROM Types  Masked ROM – programmed with its data when the chip is fabricated  PROM – programmable ROM, by the user using a standard PROM programmer, by burning some special type of fuses. Once programmed will not be possible to program it again  EPROM – erasable ROM; the chip can be erased and chip reprogrammed; programming process consists in charging some internal capacitors; the UV light (method of erase) makes those capacitors to leak their charge, thus resetting the chip  EEPROM – Electrically Erasable PROM; it is possible to modify individual locations of the memory, leaving others unchanged; one common use of the EEPROM is in BIOS of personal computers.
Auxiliary Memory The main memory construction is costly. Therefore, it has to be limited in size. The main memory is used to store only those instructions and data which are to be used immediately. However, a computer has to store a large amount of information. The bulk of information is stored in the auxiliary memory. This is also called backing storage or secondary storage. They include hard disk, floppy disks, CD-ROM, USB flash drives, etc. When the electricity supply to the computer is off, all data stored in the primary storage is destroyed. On the other hand, this is not true for secondary storage. The data stored in secondary storage can be stored for the desired time
FIG:AUXILLARYMEMORY DEVICES
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