2.1.3 Main Memory
Main memory is located close to the CPU, but not on the CPU chip itself. Main memory
provides you with working storage your computer needs to process applications.
Memory devices contain thousands of circuits imprinted on a silicon chip. The circuit
either conduct electrical current or not. Data is stored in memory as a combination of on
or off circuit states. Computer stores data using the binary system. That is, it has only
two digits 0 and 1. Binary concept was originated from switching ON and OFF electricity.
In the computer, 0 represents the electrical current OFF and 1 by the current being ON.
It is important to know how many 0s and 1s can be stored in a computer? This is referred
to as the memory capacity. In a binary system, each 0 or 1 is called a bit, which is short
for binary digit. To represent numbers, letters or special characters such as & or ), bits
are combined into groups.
A group of 8 bits is called a byte, and a byte can represent one character, digit or other
value. For an example, 0100 0001 represents the letter A. The capacity of computer
memory or a diskette is expressed in numbers of bytes. Generally, it is given in kilobytes
or megabytes.
provides you with working storage your computer needs to process applications.
Memory devices contain thousands of circuits imprinted on a silicon chip. The circuit
either conduct electrical current or not. Data is stored in memory as a combination of on
or off circuit states. Computer stores data using the binary system. That is, it has only
two digits 0 and 1. Binary concept was originated from switching ON and OFF electricity.
In the computer, 0 represents the electrical current OFF and 1 by the current being ON.
It is important to know how many 0s and 1s can be stored in a computer? This is referred
to as the memory capacity. In a binary system, each 0 or 1 is called a bit, which is short
for binary digit. To represent numbers, letters or special characters such as & or ), bits
are combined into groups.
A group of 8 bits is called a byte, and a byte can represent one character, digit or other
value. For an example, 0100 0001 represents the letter A. The capacity of computer
memory or a diskette is expressed in numbers of bytes. Generally, it is given in kilobytes
or megabytes.
• Kilobyte :Kilo means one thousand, so a kilobyte (KB) is approximately one thousand bytes. Actually, a kilobyte is 1,024 (2 10) bytes. • Megabyte :Mega means one million, so a megabyte (MB) is approximately million bytes. Actually a megabyte is 1,048,576 (2 20) bytes. • Gigabyte :Giga means one billion, a gigabyte (GB) is actually 1,073,741,824 (2 30) bytes. • Terabyte :Tera means one trillion, a terabyte (TB) is actually 1,099, 511,627,776 (2 40) bytes.
To get a feel for these amounts, consider the following examples. If your computer has
32 MB of RAM (a type of primary storage explained later), it can store 33,554,432 bytes
of data. A written word might, on average, contain 6 bytes, so this translates to
approximately 5.6 million words
Types of Main (Primary) Memory
There are several forms of primary memory. They include Random Access Memory,
Read Only Memory and cache memory.
Random Access Memory (RAM)
RAM is made of several small storage locations which are Integrated Circuits and these
small storage locations are called cells. Each cell has a unique number assigned to it
and this unique number is called the address of the cell and is used to identify the cell.
This memory can be compared to a large filing cabinet containing as many drawers as
there are addresses in memory. Each drawer contains a word and the address is written
on the outside of the drawer as in figure 2.2.
This memory is referred to as the Random Access Memory because the memory
locations of the Random Access Memory can be directly accessed to store and retrieve
data (we discuss about storage access methods in section 2.1.4) Whenever the
computer wants to work with data or instructions, they are first loaded in to RAM where
they are held temporarily until the computer needs them. Therefore, the storage of data
and instructions inside RAM is temporary and data and information which have been
stored will disappear from RAM as soon as the computer is switched off. RAM is
therefore volatile ie. it loses its content when the power is switched off.
small storage locations are called cells. Each cell has a unique number assigned to it
and this unique number is called the address of the cell and is used to identify the cell.
This memory can be compared to a large filing cabinet containing as many drawers as
there are addresses in memory. Each drawer contains a word and the address is written
on the outside of the drawer as in figure 2.2.
This memory is referred to as the Random Access Memory because the memory
locations of the Random Access Memory can be directly accessed to store and retrieve
data (we discuss about storage access methods in section 2.1.4) Whenever the
computer wants to work with data or instructions, they are first loaded in to RAM where
they are held temporarily until the computer needs them. Therefore, the storage of data
and instructions inside RAM is temporary and data and information which have been
stored will disappear from RAM as soon as the computer is switched off. RAM is
therefore volatile ie. it loses its content when the power is switched off.
Read Only Memory (ROM)
Data and instructions stored in the ROM will not be lost on failure of power supply and
therefore, ROM is known as non-volatile memory.
Unlike RAM, ROM can only be read by the CPU and cannot be erased or overwritten. It
provides permanent storage for data and instructions that do not change.
Examples: Data from the computer manufacturer The instructions that tells the computer how to start up when power is turned on.
therefore, ROM is known as non-volatile memory.
Unlike RAM, ROM can only be read by the CPU and cannot be erased or overwritten. It
provides permanent storage for data and instructions that do not change.
Examples: Data from the computer manufacturer The instructions that tells the computer how to start up when power is turned on.
Cache Memory
Over the past few years, the speed of the CPU has increased. However, memory
performance has not been able to keep pace with this change. Therefore, processing
speed has not increased due to the slow speed of RAM. To reduce this mismatch in
operating speed, a small memory chip is attached between CPU and RAM. This memory
is called Cache Memory. The access time is close to the processing speed of CPU. The
CPU will first retrieve data from the Cache Memory. If the data is not available in the
Cache Memory, then the data is first loaded from RAM into the Cache Memory, and then
taken by the CPU from the Cache Memory as shown in Figure 2.3.
As the cache memory has got less amount of data, usually, the CPU can access the
desired data and instructions more quickly than if it were selecting from the larger set in
main memory.
As the Cache Memory is accessed much faster than conventional RAM, it is used to
store programs or data currently being executed or temporary data frequently used by
the CPU. So the Cache Memory makes the RAM to be faster and it also supplements
the RAM. As discussed above the cache memory increases the overall performance of
the computer system. However, cache Memory is expensive than Main Memory.
performance has not been able to keep pace with this change. Therefore, processing
speed has not increased due to the slow speed of RAM. To reduce this mismatch in
operating speed, a small memory chip is attached between CPU and RAM. This memory
is called Cache Memory. The access time is close to the processing speed of CPU. The
CPU will first retrieve data from the Cache Memory. If the data is not available in the
Cache Memory, then the data is first loaded from RAM into the Cache Memory, and then
taken by the CPU from the Cache Memory as shown in Figure 2.3.
As the cache memory has got less amount of data, usually, the CPU can access the
desired data and instructions more quickly than if it were selecting from the larger set in
main memory.
As the Cache Memory is accessed much faster than conventional RAM, it is used to
store programs or data currently being executed or temporary data frequently used by
the CPU. So the Cache Memory makes the RAM to be faster and it also supplements
the RAM. As discussed above the cache memory increases the overall performance of
the computer system. However, cache Memory is expensive than Main Memory.
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