The document discusses computer memory and the memory hierarchy. It begins by describing the main types of memory - volatile primary memory and non-volatile secondary memory. It then discusses the memory hierarchy from fastest to slowest: registers, cache memory, main memory, magnetic disks, and removable media like magnetic tapes. The rest of the document provides more details about each level of the memory hierarchy, including descriptions of different types of main memory like RAM, DRAM, and SRAM. It also includes block diagrams to illustrate how different memory components work.

1. UNIT 4
1. Memory, Main Memory, DRAM,
2. Memory Organization Unit
3. Memory Organization, Cache Memory
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By M Indraja
Assistant Prof (C)
RGUKT-Srikakulam

2. Computer memory is of two basic type – Primary memory / Volatile memory and Secondary memory / non-volatile
memory.
Random Access Memory (RAM) is volatile memory and Read Only Memory (ROM) is non-volatile
memory.
Memory is used to store the information in digital form. The memory hierarchy is given by:
Register
Cache Memory
Main Memory
Magnetic Disk
Removable media (Magnetic tape)
2Introduction of Memory :

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4. Memory and Auxiliary Memory:
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1. The memory unit is an essential component in any digital computer since it is needed for storing programs and data.
The memory unit that communicates directly with the CPU is called the main memory.
2. Devices that provide backup storage are called auxiliary memory.
3. 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.

5. Memory hierarchy in a computer system.
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6. Memory Hierarchy:
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1. The main memory occupies a central position by being able to communicate directly with the CPU and
with auxiliary memory devices through an I/O processor.
2. A special very high speed memory called cache is used to increase the speed of processing by making
current programs and data available to the CPU a rapid rate.
3. CPU logic is usually faster than main memory access time, with the result that processing speed is limited
primarily by the speed of main memory.
4. The cache is used for storing segments of programs currently being executed in the CPU and temporary
data frequently needed in the present calculations

7. Register:
This is a part of Central Processor Unit, so they reside inside the CPU. The information from main memory is brought
to CPU and keep the information in register. Due to space and cost constraints, we have got a limited number of
registers in a CPU. These are basically faster devices.
Cache Memory:
Cache memory is a storage device placed in between CPU and main memory. These are semiconductor memories.
These are basically fast memory device, faster than main memory.
We can not have a big volume of cache memory due to its higher cost and some constraints of the CPU. Due to
higher cost we can not replace the whole main memory by faster memory. Generally, the most recently used
information is kept in the cache memory. It is brought from the main memory and placed in the cache memory. Now a
days, we get CPU with internal cache.
Main Memory:
Like cache memory, main memory is also semiconductor memory. But the main memory is relatively slower memory.
We have to first bring the information (whether it is data or program), to main memory. CPU can work with the
information available in main memory only.
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Magnetic Disk:
This is bulk storage device. We have to deal with huge amount of data in many application. But we don't have so much
semiconductor memory to keep these information in our computer.
On the other hand, semiconductor memories are volatile in nature. It loses its content once we switch off the
computer. For permanent storage, we use magnetic disk. The storage capacity of magnetic disk is very high.
Magnetic Tape (Removable media):
For different application, we use different data. It may not be possible to keep all the information in magnetic disk.
So, which ever data we are not using currently, can be kept in removable media. Magnetic tape is one kind of
removable medium. CD is also a removable media, which is an optical device.

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12. RAM: 12
RAM (Random Access Memory) was invented by Robert Heath Dennard. At IBM Thomas J Watson
Research Center in 1968.

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Block Diagram of RAM:
1. The above block diagram related to main interface to RAM.
2. If a Chip select, CS enables or disables the RAM.
3. ADRS specifies the address or location to read from or write.
4. WR selects between reading from or writing to the memory.
5. To read from memory, WR should be set to 0.OUT will be the n-bit
value stored at ADRS.
6. To write to memory, we set WR=1.
7. DATA is the n-bit value to save in memory.

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15. DRAM: (Dynamic Random Access Memory): 15
 A DRAM is made with cells that store data as charge on capacitors is interpreted as binary 0 or 1.
 If 1 bit of the DRAM cell it consist of 1 Transistor and 1 capacitor.
 Then the memory bits are stored charge across the capacitor.
 By charge or discharge of the capacitor then have data it can store either 0 or 1.

16. By accessing the capacitor should use the transistor. When the transistor turn on then we can read or write
the capacitor data.
If the transistor turn off then charge across the capacitor should be same. In ideal case the
capacitor should not loss its charge.
But actual case there is a leakage current because of that capacitor will lose its charge gradually
then that is the reason this Dynamic Ram/Dynamic Cell requires the periodic refresh Cycle.
For the write operation, a voltage signal is applied to the bit line B, a high voltage represents 1
and a low voltage represents 0. A signal is then applied to the address line, which will turn on the transistor
T, allowing a charge to be transferred to the capacitor.
For the read operation, when a signal is applied to the address line, the transistor T turns on and
the charge stored on the capacitor is fed out onto the bit line B.
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17. SRAM(Static Random Access Memory): 17
 In an SRAM, binary values are stored using traditional
flip-flop constructed with the help of transistors.
 A static RAM will hold its data as long as power is
supplied to it.
 A typical SRAM constructed with transistors is shown in
the figure.

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 Four transistors (T1, T2, T3, T4) are cross connected in an arrangement that produces a stable logic
state.
 In logic state 1, point C1 is high and point C2 is low; in this state T1 and T4 are off, and T2 and T3
are on .
 In logic state 0, point C1 is low and point C2 is high; in this state T1 and T4 are on, and T2 and T3
are off .
Both states are stable as long as the dc supply voltage is applied.
 The address line is used to open or close a switch which is nothing but another transistor. The
address line controls two transistors(T5 and T6).When a signal is applied to this line, the two
transistors are switched on, allowing a read or write operation.
 For a write operation, the desired bit value is applied to line B, and its complement is applied to
line. This forces the four transistors(T1, T2, T3, T4) into the proper state.
 For a read operation, the bit value is read from the line B bar. When a signal is applied to the
address line, the signal of point A1 is available in the bit line B.

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20. Registers: 20
 Registers are high-speed storage locations inside the microprocessor.
 Designed to be accessed at much higher speed than conventional memory.
 Registers are classified according to the functions they perform.
Types of Registers:
 Data Registers: To hold data for an operation.
 Address Registers: To hold the address of an instruction or data.
 Status/Flag Register: Keeps the current status of the processor or result of an arithmetic operation.

21. Thank You 21