Download Main memory

1.2) Main memory
Memory:
• Stores data and instructions in the
beginning, intermediate and final stages of
the processing.
Main memory
• For storing data, a computer contains a
large collection of circuits, each capable of
storing a single bit.
• Also known as RAM “Random Access
Memory”
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Memory organization
• A computer main memory is organized in
manageable units called cells.
• Cell: A unit of main memory (typically 8 bits
which is one byte)
• Each cell’s size equal to eight (8) bits.
• higher-order end: the left end of the row of
bits on the memory
• lower-order end: the right end
• most significant bit: the last bit at the highorder end.
• least significant bit: the bit at the right end.
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Memory organization
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Memory organization
• To identify individual cell in a computer’s
main memory, each cell is assigned a
unique “name”, called its address.
• Address: A “name” that uniquely identifies
one cell in the computer’s main memory.
As shown in Figure
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Measuring memory
capacity
• Kilobyte = 1024 byte
• Megabyte = 1024 kilobyte
• Gigabyte = 1024 megabyte
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Technology of memory building:
There are two significant types for
technologies used in memory building
( design ):
1. Magnetic core
2. Semiconductor memory
1- Magnetic core:
• Consists of million of rings, each ring in the size
of pinhead, these rings connected to each other
forming a mesh, electricity flows through these
rings, the rings are magnetized. If the electricity
flows in clock wise, the ring represents 1 but in
counter clock wise it represents 0. In this kind of
technology data are not lost even if the power
goes off.
• Note: magnetic core is non-volatile memory
Magnetic core:
1
0
2- Semiconductor memory:
• Is an integrated circuit, is put on a silicone
board, each cell contains transistor which
works as switch and a capacitor which
keeps the charge. If the capacitor is
charged, it means the value stored is 1,
other wise the value is 0.
Advantages of semiconductor:
•
•
•
•
•
less bulky than the magnetic core.
survive for a longer period (less damage)
data can be stored in a smaller size
less cost
faster in information retrieval
Disadvantages of semiconductor:
• it is volatile, it losses its contents as soon
as the power goes off. For this purpose
UPS is used
3- Bubble memory:
• It has come to take the advantages of
magnetic cores and semiconductor
technology
Memory contains:
1. Address register: contains the address of
the data to be brought or put in the
memory ( the address value is brought
from the CPU )
2. Word register: it contains the data
brought or to put from/in the memory
3. LCU (local control unit): supervises all the
parts of the memory
4. The memory cells ( matrix ) ( The cell
size is eight bits )
Memory contains:
Least significant bit ( Lower order
end )
Most significant bit ( High order
end )
CPU
ADDRES
S
REGISTE
R
LCU
MATRIX
WOR
D
REGIS
TER
Measuring Memory capacity:
•
•
•
•
•
Bit = 0 or 1
Byte = 8 bits
KByte = 1024 Byte = 210 byte
MByte = 1024 * 1024 Byte = 220 byte
GByte = 1024 * 1024 * 1024 Byte = 230
byte
Ex: given the address register of length
16 bits and the length of the word is 4
bits, computes the size of the memory
in bytes
Sol.:
No. of address locations = 216 = 65536
No. of bits in the memory = 65536*4 =
262144 bits
Size of the memory in bytes = 262144 /
8 = 32768 bytes
Types of memory:
•
RAM ( Random Access Memory )
–
–
–
It is a memory where data can be read or written.
It is a volatile memory
It is important in putting the computer specification
–
DRAM ( Dynamic RAM ) or SDRAM ( Synchronous DRAM ):
Is used in reference to DRAM that applies additional techniques to
decrease the time needed to retrieve the contents from its memory cells
ROM ( Read Only Memory )
•
•
•
–
It is only for reading, it comes from the manufacture on a chip. For example the
ASCII code is store in ROM
This memory is non-volatile
PROM ( Programmable Read Only Memory)
Can be programmed by the manufacture or by the user (can be
programmed for one time only )
–
EPROM (Erasable Programmable Read Only Memory )
Can be erased and reprogrammed