Lecture 6 on general aspects of computer organization.

1. General Aspects of Computer Organization
(Lecture-6)
R S Ananda Murthy
Associate Professor and Head
Department of Electrical & Electronics Engineering,
Sri Jayachamarajendra College of Engineering,
Mysore 570 006
R S Ananda Murthy General Aspects of Computer Organization

2. Memory Access Time, RAM and SAM
The time interval between the application of MEMREAD
signal to the memory and appearance of data on the data
bus of the memory is called Memory Access Time.
A memory unit is called Random Access Memory (RAM) if
the access time is same for all the locations in the memory
unit.
All semiconductor memories are RAM.
A memory unit is called Sequential Access Memory (SAM)
if the access time varies depending upon the address or
the location of data in the unit. Eg. Magnetic disks and
tapes.
In microcontroller-based systems we may not need
external memory if the internal memory in MCU is
sufficient.
R S Ananda Murthy General Aspects of Computer Organization

3. Types of Semiconductor Memory
ROM A memory chip pre-programmed by the
manufacturer. User cannot alter its contents.
Non-volatile. Also known as masked ROM.
PROM Programmable Read Only Memory can be
programmed only once by the user. Cannot be
reprogrammed. Non-volatile.
EPROM Erasable PROM can be programmed, contents of
all locations are erased by exposing to ultraviolet
light for several minutes (typically in 15-60
minutes), and can be reprogrammed by the user.
Non-volatile.
R S Ananda Murthy General Aspects of Computer Organization

4. Types of Semiconductor Memory
EEPROM Electrically Erasable PROM can be programmed,
contents of selected locations can be erased
electrically (typically in 1-10 s), and reprogrammed
by the user. Non-volatile.
Flash A type of EEPROM having faster erase cycles as
compared to non-flash EEPROM (< 1s).
Non-volatile. Can be electrically erased and
reprogrammed. Used as program memory in
MCUs.
SR/WM Static Read/Write Memory. Volatile. Each bit is
stored in a flip-flop. Commonly called SRAM.
Cache memory is of this type. Very fast and costly
memory as compared to DRAM. Used as data
memory in MCUs.
R S Ananda Murthy General Aspects of Computer Organization

5. Types of Semiconductor Memory
NV-R/WM Non-volatile Read/Write Memory. Each bit is
stored in a flip-flop as in SRAM. Commonly called
NV-RAM. Has a built-in lithium-ion battery. When
there is no power supply, because of the battery
backup, contents are retained typically for upto
5-10 years.
DR/WM Dynamic Read/Write Memory. Volatile. In this
each bit is stored as a charge on a capacitor.
Frequent refreshing is required, typically every 6-8
ms, to retain data. Commonly called DRAM.
Cheaper than SRAM. Used as primary memory in
desktop computers and laptops.
R S Ananda Murthy General Aspects of Computer Organization

6. Memory Images
UV Erasable EPROM DRAM Module
SRAM
R S Ananda Murthy General Aspects of Computer Organization

7. Memory Related Technical Terms
Width Number of bits per location. Bit organized memory
has 1 bit in each location. Nibble organized
memory has 4 bits in each location. Byte
organized memory has 8-bits in each location.
Most computers have byte organized memory.
Cell Smallest addressable unit in a memory. Also
called 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 the
address, M = width of the memory, i.e., number of
bits per location.
Capacity Total number of bits stored in a memory unit
= N ×M
R S Ananda Murthy General Aspects of Computer Organization

8. Big Endian and Little Endian Byte Ordering
0x90AB12CD900x1000
AB0x1001
120x1002
CD0x1003
Hex Address
CD 0x1000
12 0x1001
AB 0x1002
90 0x1003
Hex Address32-bit Number
Big Endian Little Endian
In big endian computers, the higher significant byte is
stored at lower address. Eg. Motorola processors and Sun
SPARC processors follow big endian storage.
In little endian computers, the lower significant byte is
stored at lower address. Eg. Intel processors and IBM
mainframes follow little endian storage.
In MIPS processors user can select big or little endian
storage.
R S Ananda Murthy General Aspects of Computer Organization

9. License
This work is licensed under a
Creative Commons Attribution 4.0 International License.
R S Ananda Murthy General Aspects of Computer Organization