L6 primary-memory

General Aspects of Computer Organization(Lecture-6)

R S Ananda Murthy

Associate Professor and HeadDepartment of Electrical & Electronics Engineering,

Sri Jayachamarajendra College of Engineering,Mysore 570 006

R S Ananda Murthy General Aspects of Computer Organization

Memory Access Time, RAM and SAM

The time interval between the application of MEMREADsignal to the memory and appearance of data on the databus of the memory is called Memory Access Time.A memory unit is called Random Access Memory (RAM) ifthe access time is same for all the locations in the memoryunit.All semiconductor memories are RAM.A memory unit is called Sequential Access Memory (SAM)if the access time varies depending upon the address orthe location of data in the unit. Eg. Magnetic disks andtapes.In microcontroller-based systems we may not needexternal memory if the internal memory in MCU issufficient.


Types of Semiconductor Memory

ROM A memory chip pre-programmed by themanufacturer. User cannot alter its contents.Non-volatile. Also known as masked ROM.

PROM Programmable Read Only Memory can beprogrammed only once by the user. Cannot bereprogrammed. Non-volatile.

EPROM Erasable PROM can be programmed, contents ofall locations are erased by exposing to ultravioletlight for several minutes (typically in 15-60minutes), and can be reprogrammed by the user.Non-volatile.



EEPROM Electrically Erasable PROM can be programmed,contents of selected locations can be erasedelectrically (typically in 1-10 s), and reprogrammedby the user. Non-volatile.

Flash A type of EEPROM having faster erase cycles ascompared to non-flash EEPROM (< 1s).Non-volatile. Can be electrically erased andreprogrammed. Used as program memory inMCUs.

SR/WM Static Read/Write Memory. Volatile. Each bit isstored in a flip-flop. Commonly called SRAM.Cache memory is of this type. Very fast and costlymemory as compared to DRAM. Used as datamemory in MCUs.



NV-R/WM Non-volatile Read/Write Memory. Each bit isstored in a flip-flop as in SRAM. Commonly calledNV-RAM. Has a built-in lithium-ion battery. Whenthere is no power supply, because of the batterybackup, contents are retained typically for upto5-10 years.

DR/WM Dynamic Read/Write Memory. Volatile. In thiseach bit is stored as a charge on a capacitor.Frequent refreshing is required, typically every 6-8ms, to retain data. Commonly called DRAM.Cheaper than SRAM. Used as primary memory indesktop computers and laptops.


Memory Images

UV Erasable EPROM DRAM Module

SRAM


Memory Related Technical Terms

Width Number of bits per location. Bit organized memoryhas 1 bit in each location. Nibble organizedmemory has 4 bits in each location. Byteorganized memory has 8-bits in each location.Most computers have byte organized memory.

Cell Smallest addressable unit in a memory. Alsocalled a location.

Address Bit pattern used to identify a cell.Size Indicated as N ×M where N = number of

locations = 2n, where n =number of bits in theaddress, M = width of the memory, i.e., number ofbits per location.

Capacity Total number of bits stored in a memory unit= N ×M


Big Endian and Little Endian Byte Ordering

0x90AB12CD900x1000AB0x1001120x1002CD0x1003

Hex Address

CD 0x100012 0x1001AB 0x100290 0x1003

Hex Address32-bit Number

Big Endian Little Endian

In big endian computers, the higher significant byte isstored at lower address. Eg. Motorola processors and SunSPARC processors follow big endian storage.In little endian computers, the lower significant byte isstored at lower address. Eg. Intel processors and IBMmainframes follow little endian storage.In MIPS processors user can select big or little endianstorage.


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L6 primary-memory

Engineering

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