Memory
The information (instruction and data) required is stored in memory. The computer's memory is constructed out of semi-conducting material and stores information in binary form. Binary information is composed of two symbols 0 and 1, called binary digits(bits). All information within the computer is represented by two digits. The memory is organized into equal sized units (usually a collection of 8 bits, called a byte). These units are arranged a sequence and are identified by numbers called addresses. The memory of a computer can be divided into distinct parts as below.
Registers
Registers are locations within the microprocessor where data is stored temporarily during processing.These internal high speed registers are used in arithmetic and logic operations for holding data operands. Some registers are accessible by the user through instructions. Other s are reserved for the use of the CPU to perform it's activities.
Internal Cache
Cache is a small high speed memory thata contains frequently used data. The use of cache avoids repeated reading of data from the slower main memory. Internal cache is located within the main
microprocessor.
External Cache
External
cache is used to supplement the internal cache. It is used when an internal cache
is not present.It is placed between the CPU and the main memory.
Main Memory
Main memory
stores data and instructions required by the main microprocessor. The main
memory is also called RAM(Random Access Memory). Microprocessor instructions
can directly access main memory locations. Main Memory is fast but expensive.
However it is volatile the content stored will be lost when the power supply is
cut off.
Secondary Memory
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| Memory levels in computer |
All the
data and programs required by the computer cannot be stored in the main memory
because it is small in size and volatile. Secondary memory is slower but less
expensive than main memory. It is also non-volatile and larger in size.The
microprocessor can access the various types of memory in the memory
hierarchy up-to the main memory. The microprocessor cannot
access the secondary memory directly. Therefore, data from the
secondary memory has to be brought to the main memory so that the
processor can use it.Memory management techniques are required to transfer
information between the main memory and secondary memory and this
function is performed by the operating system. Some examples of secondary
storage devices are hard disks, floppy disks, CD-ROM's, etc.
The
various types of memory are shown in the order of increasing size,
decreasing speed and decreasing cost. Registers are the fastest memory devices
but their high cost does not make it feasible to have them
in large numbers. Similarly secondary memory is the least expensive but is very
slow. The hierarchy therefore achieves optimal performance at reasonable costs
as the amount of memory of each type that is commonly used is proportional to
the size of its rectangle. The cost and speed of different type of memory are
compared in figure.
