Hierarchy of Memory
Memory hierarchy Main Memory: memory unit that communicates directly
with the CPU (RAM). Hierarchy of Memory
Auxiliary Memory: device that provide backup storage (Disk Drives) Example of auxiliary
memory: magnetic disks and tapes. They are used for storing system programs,
large data files, and other backup information Only program programs and
data currently needed by the processor reside in main memory. All other
information is stored in auxiliary memory and transferred to main memory when
needed.
Memory hierarchy system consists of all storage devices from the slow
but high-capacity auxiliary memory to relatively faster main memory, to an
even smaller and faster cache memory.
Magnetic tapes used to store
removable files and disks used as backup storage. CPU communicates with
auxiliary memory through I/O processor.
Cache Memory: special very-high-speed memory to increase the processing speed (Cache RAM)
It is used to compensate the speed differential between main memory access
time and processor logic.
CPU logic is faster than main memory access.
So small cache
employed between CPU and main memory whose access time is close to processor
logic clock cycle.
So it is possible to increase the performance
rate of the computer.
As the storage capacity of the memory
increases, the cost per bit for storing binary information decreases and the
access time of the memory becomes longer.
Example
Auxiliary memory has a large storage capacity, is relatively
inexpensive, but has low access speed compared to main memory.
The
cache memory is very small, relatively expensive, and has very high access
speed.
Goal of memory hierarchy
To obtain the highest-possible average access speed while minimizing the
total cost.
Ratio
Access time ratio between cache and main memory is about 1 to 7.
For Example
Cache memory may have an access time of 100ns, while main memory access
time may be 700ns.
Auxiliary memory average access time is usually
1000 times that of main memory.
Block Size
Block size in auxiliary memory typically ranges from 256 to 2048 words,
while cache block size is typically from 1 to 16 words.
Multiprogramming
Multiprogramming refers to the existence of two or more programs in
different parts of the memory hierarchy at the same time.
For
example, when one program is waiting for input or output transfer, there is
another program ready to utilize the CPU.
Memory management
A program with its data normally resides in auxiliary memory. When the
program or a segment of the program is to be executed, it is transferred to
main memory to be executed by the CPU. It is the task of the operating
system to maintain in main memory a portion of this information that is
currently active.
3. The part of the computer system that supervises
the flow of information between auxiliary memory and main memory is called the
memory management system.
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