The Memory unit is an essential component
in any digital computer. Memory is needed for storing programs and data. The
total memory capacity of a computer can be visualized as being a hierarchy of
components. The memory hierarchy system consists of all storage devices
employed in computer system from the slow but high capacity auxiliary memory to
a relatively faster main memory.
- The memory unit that directly communicate with CPU is called Main Memory. Only programs and data currently needed by the processor reside in main memory.
- Devices that provides backup storage called auxiliary memory. The most common auxiliary devices used in computer system are magnetic disks and tapes. They are used for storing system programs, large data files, and other backup information.
- All other information is stored in auxiliary memory and transferred to main memory when needed.
Very high speed memory is called Cache Memory. It is used to increase the speed of processing by
making current programs and data available to the CPU at a rapid rate. CPU
logic is usually faster than main memory access time. The cache is used for
storing segments of programs currently being executed in the CPU and temporary
data frequently needed in the present calculations. Cache organization is
concerned with the transfer of information between main memory and CPU.
Cache memory is: -
·
Very small.
·
Relatively expensive.
·
and has very high access speed.
·
Cache memory holds those parts that are not
presently used by the CPU.
Main
Memory: - Main
memory is central storage unit in computer system. The principal technology
used for the main memory is based on semiconductor integrated circuits.
Integrated circuits RAM chips are available in two possible operating modes,
static and dynamic. Main memory is: -
·
Fast.
·
Relatively large.
·
And used to store programs and data during the
computer operations.
The
static RAM consist of internal flip-flops that store the binary
information. The stored information remains valid as long as power is applied
to the unit. Static RAM is easier to use and has shorted read and writes
cycles.
The
dynamic RAM stores the binary information in the form of
electric charges that are applied to capacitors. The capacitors are provided
inside the chip by MOS transistors. The dynamic RAM offers reduced power
consumption and larger storage capacity in single memory chip. The dynamic
RAM’s are used to for implementing the main memory. Most of the computer
systems are dynamic RAMs with improved performance characteristics such as multi-bank
DRAM, synchronous DRAM, and Direct DRAM bus DRAM.
Memory
Connections to CPU: - RAM and ROM chips are connected to CPU
through the data and address buses. The data bus of ROM has only one output
capacity, whereas the data bus connected to RAMs can transfer information in
both directions.
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