DIT Computer Organization 3

Memory
We have already mentioned that digital computer works on stored programmed concept introduced by
Von Neumann. We use memory to store the information, which includes both program and data.

Due to several reasons, we have different kind of memories. We use different kind of memory at
different level.

The memory of computer is broadly categories into two categories:

 Internal and
 external

Internal memory is used by CPU to perform task and external memory is used to store bulk information,
which includes large software and data.

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)

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 cannot have a big volume of cache memory due to its higher cost and some constraints of the CPU.
Due to higher cost we cannot 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 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.

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.

Register, cache memory and main memory are internal memory. Magnetic Disk, removable media are
external memory. Internal memories are semiconductor memory. Semiconductor memories are
categoried as volatile memory and non-volatile memory.

RAM: Random Access Memories are volatile in nature. As soon as the computer is switched off, the
contents of memory are also lost.

ROM: Read only memories are non volatile in nature. The storage is permanent, but it is read only
memory. We can not store new information in ROM.

Several types of ROM are available:

 PROM: Programmable Read Only Memory; it can be programmed once as per

user requirements.

 EPROM: Erasable Programmable Read Only Memory; the contents of the

memory can be erased and store new data into the memory. In this case, we
have to erase whole information.

 EEPROM: Electrically Erasable Programmable Read Only Memory; in this type of

memory the contents of a particular location can be changed without effecting
the contents of other location.

Main Memory
The main memory of a computer is semiconductor memory. The main memory unit of computer is
basically consists of two kinds of memory:

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RAM : Random access memory; which is volatile in nature.

ROM : Read only memory; which is non-volatile.

The permanent information are kept in ROM and the user space is basically in RAM.

The smallest unit of information is known as bit (binary digit), and in one memory cell we can store one
bit of information. 8 bit together is termed as a byte.

The maximum size of main memory that can be used in any computer is determined by the addressing
scheme.

A computer that generates 16-bit address is capable of addressing upto 216 which is equal to 64K
memory location. Similarly, for 32 bit addresses, the total capacity will be 232 which is equal to 4G
memory location.

In some computer, the smallest addressable unit of information is a memory word and the machine is
called word-addressable.

In some computer, individual address is assigned for each byte of information, and it is called byte-
addressable computer. In this computer, one memory word contains one or more memory bytes which
can be addressed individually.

A byte addressable 32-bit computer, each memory word contains 4 bytes. A possible way of address
assignment is shown in figure3.1. The address of a word is always integer multiple of 4.

The main memory is usually designed to store and retrieve data in word length quantities. The word
length of a computer is generally defined by the number of bits actually stored or retrieved in one main
memory access.

Consider a machine with 32 bit address bus. If the word size is 32 bit, then the high order 30 bit will
specify the address of a word. If we want to access any byte of the word, then it can be specified by the
lower two bit of the address bus.

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Figure 3.1: Address assignment to a

4-byte word

The data transfer between main memory and the CPU takes place through two CPU registers.

 MAR : Memory Address Register

 MDR : Memory Data Register.

If the MAR is k-bit long, then the total addressable memory location will be 2k.

If the MDR is n-bit long, then the n bit of data is transferred in one memory cycle.

The transfer of data takes place through memory bus, which consist of address bus and data
bus. In the above example, size of data bus is n-bit and size of address bus is k bit.

It also includes control lines like Read, Write and Memory Function Complete (MFC) for
coordinating data transfer. In the case of byte addressable computer, another control line to be
added to indicate the byte transfer instead of the whole word.

For memory operation, the CPU initiates a memory operation by loading the appropriate data
i.e., address to MAR.

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If it is a memory read operation, then it sets the read memory control line to 1. Then the
contents of the memory location is brought to MDR and the memory control circuitry indicates
this to the CPU by setting MFC to 1.

If the operation is a memory write operation, then the CPU places the data into MDR and sets
the write memory control line to 1. Once the contents of MDR are stored in specified memory
location, then the memory control circuitry indicates the end of operation by setting MFC to 1.

A useful measure of the speed of memory unit is the time that elapses between the initiation of
an operation and the completion of the operation (for example, the time between Read and
MFC). This is referred to as Memory Access Time. Another measure is memory cycle time. This
is the minimum time delay between the initiation two independent memory operations (for
example, two successive memories read operation). Memory cycle time is slightly larger than
memory access time.

Binary Storage Cell:

The binary storage cell is the basic building block of a memory unit.

The binary storage cell that stores one bit of information can be modeled by an SR latch with
associated gates. This model of binary storage cell is shown in the figure 3.2.

Figure 3.2: Binary Storage cell made up of SR-Latch

1 bit Binary Cell (BC)

The binary cell stores one bit of information in its internal latch.

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Control input to binary cell

Select Read/Write Memory Operation

0 X None
1 0 Write
1 1 Read
The storage part is modeled here with SR-latch, but in reality it is an electronics circuit made up of
transistors.
The memory constructed with the help of transistors is known as semiconductor memory. The
semiconductor memories are termed as Random Access Memory (RAM), because it is possible to access
any memory location in random.
Depending on the technology used to construct a RAM, there are two types of RAM -

SRAM: Static Random Access Memory.

DRAM: Dynamic Random Access Memory.

Dynamic Ram (DRAM):

A DRAM is made with cells that store data as charge on capacitors. The presence or absence of charge in
a capacitor is interpreted as binary 1 or 0.
Because capacitors have a natural tendency to discharge due to leakage current, dynamic RAM require
periodic charge refreshing to maintain data storage. The term dynamic refers to this tendency of the
stored charge to leak away, even with power continuously applied.
A typical DRAM structure for an individual cell that stores one bit information is shown in the figure 3.3.

Figure 3.3: Dynamic RAM (DRAM) cell

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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 and to a sense amplifier.

The sense amplifier compares the capacitor voltage to a reference value and determines if the cell
contains a logic 1 or a logic 0.

The read out from the cell discharges the capacitor, widh must be restored to complete the read
operation.

Due to the discharge of the capacitor during read operation, the read operation of DRAM is termed as
destructive read out.

Static RAM (SRAM):

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 3.4.

Figure 3.4: Static RAM (SRAM) cell

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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 A1 is high and point A2 is low; in this state T1 and T4 are off, and T2 and T3 are on .
In logic state 0, point A1 is low and point A2 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. When a signal is applied to the address line,
the signal of point A1 is available in the bit line B.

SRAM Versus DRAM :

 Both static and dynamic RAMs are volatile, that is, it will retain the information as long
as power supply is applied.
 A dynamic memory cell is simpler and smaller than a static memory cell. Thus a DRAM is
more dense,
i.e., packing density is high (more cell per unit area). DRAM is less expensive than
corresponding SRAM.
 DRAM requires the supporting refresh circuitry. For larger memories, the fixed cost of
the refresh circuitry is more than compensated for by the less cost of DRAM cells
 SRAM cells are generally faster than the DRAM cells. Therefore, to construct faster
memory modules (like cache memory) SRAM is used.

Internal Organization of Memory Chips

A memory cell is capable of storing 1-bit of information. A number of memory cells are
organized in the form of a matrix to form the memory chip. One such organization is shown in
the Figure 3.5.

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Figure 3.5: 16 X 8 Memory Organization

Each row of cells constitutes a memory word, and all cell of a row are connected to a common line
which is referred as word line. An address decoder is used to drive the word line. At a particular instant,
one word line is enabled depending on the address present in the address bus. The cells in each column
are connected by two lines. These are known as bit lines. These bit lines are connected to data input line
and data output line through a Sense/Write circuit. During a Read operation, the Sense/Write circuit
sense, or read the information stored in the cells selected by a word line and transmit this information
to the output data line. During a write operation, the sense/write circuit receive information and store it
in the cells of the selected word.

A memory chip consisting of 16 words of 8 bits each, usually referred to as 16 x 8 organization. The data
input and data output line of each Sense/Write circuit are connected to a single bidirectional data line in
order to reduce the pin required. For 16 words, we need an address bus of size 4. In addition to address
and data lines, two control lines, and CS, are provided. The line is to used to specify the
required operation about read or write. The CS (Chip Select) line is required to select a given chip in a
multi chip memory system.

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Consider a slightly larger memory unit that has 1K (1024) memory cells...

128 x 8 memory chips:

If it is organized as a 128 x 8 memory chips, then it has got 128 memory words of size 8 bits. So
the size of data bus is 8 bits and the size of address bus is 7 bits ( ). The storage
organization of 128 x 8 memory chip is shown in the figure 3.6.

Figure 3.6: 128 x 8 Memory Chip

1024 x 1 memory chips:

If it is organized as a 1024 x 1 memory chips, then it

has got 1024 memory words of size 1 bit only.

Therefore, the size of data bus is 1 bit and the size of

address bus is 10 bits ( ).

A particular memory location is identified by the

contents of memory address bus. A decoder is used
to decode the memory address. There are two ways
of decoding of a memory address depending upon
the organization of the memory module.

In one case, each memory word is organized in a row.

In this case whole memory address bus is used
together to decode the address of the specified
location. The memory organization of 1024 x 1
memory chip is shown in the figure 3.7. Figure 3.7: 1024 x 1 Memory chip

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In second case, several memory words are organized in one row. In this case, address bus is divided into
two gropus.

One group is used to form the row address and the second group is used to form the column address.
Consider the memory organization of 1024 x 1 memory chip. The required 10-bit address is divided into
two groups of 5 bits each to form the row and column address of the cell array. A row address selects a
row of 32 cells, all of which are accessed in parallel. However, according to the column address, only one
of these cells is connected to the external data line via the input output multiplexers. The arrangement
for row address and column address decoders is shown in the figure 3.8.

Figure 3.8: Organizaion of 1k x 1 Memory chip

The commercially available memory chips contain a much larger number of cells. As for example, a

memory unit of 1MB (mega byte) size, organised as 1M x 8, contains memory cells. It has got
220 memory location and each memory location contains 8 bits information. The size of address bus is
20 and the size of data bus is 8.
The number of pins of a memory chip depends on the data bus and address bus of the memory module.
To reduce the number of pins required for the chip, we use another scheme for address decoding. The
cells are organized in the form of a square array. The address bus is divided into two groups, one for
column address and other one is for row address. In this case, high- and low-order 10 bits of 20-bit
address constitute of row and column address of a given cell, respectively. In order to reduce the

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number of pin needed for external connections, the row and column addresses are multiplexed on ten
pins. During a Read or a Write operation, the row address is applied first. In response to a signal pulse
on the Row Address Strobe (RAS) input of the chip, this part of the address is loaded into the row
address latch.
All cell of this particular row is selected. Shortly after the row address is latched, the column address is
applied to the address pins. It is loaded into the column address latch with the help of Column Address
Strobe (CAS) signal, similar to RAS. The information in this latch is decoded and the appropriate
Sense/Write circuit is selected.

Figure 3.9: 1 MB(Mega Byte) Memory Chip

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For a Write operation, the information at the input lines are transferred to the selected circuits.

Figure 3.10: Organization of a 1M x 1 Memory chip.

The 1MB (Mega byte) memory chip with 20 address lines as shown in the figure 3.9. The same memory
chip (1MB) with 10 address lines ( where row & column address are multiplexed) is shown in Figure
3.10.

Now we discuss the design of memory subsystem using memory chips. Consider a memory chips of
capacity 16K x 8. The requirement is to design a memory subsystem of capacity 64K x 16. Each memory
chip has got eight lines for data bus, but the data bus size of memory subsytem is 16 bits.
The total requiremet is for 64K memory location, so four such units are required to get the 64K memory
location. For 64K memory location, the size of address bus is 16. On the other hand, for 16K memory
location, size of address bus is 14 bits.
Each chip has a control input line called Chip Select (CS). A chip can be enabled to accept data input or to
place the data on the output bus by setting its Chip Select input to 1. The address bus for the 64K
memory is 16 bits wide. The high order two bits of the address are decoded to obtain the four chip
select control signals. The remaining 14 address bits are connected to the address lines of all the chips.
They are used to access a specific location inside each chip of the selected row. The inputs of all
chips are tied together to provide a common control.

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Figure 3.11: 16k x 8 Memory chip

The block diagram of a 16k x 8 memory chip is shown in the figure 3.11. The block diagram of a 64k x 16
memory module constructed with the help of eight 16k x 8 memory chips is shown in the figure 3.12.

Figure 3.12: 64k x 16 Memory chip

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