Memory hierarchy

1. Memory Hierarchy &I/O Techniques
UNIT 1,3

2. NEED FOR MEMORY HIERARCHY
The memory unit is a essential component in a digital computer since it is
needed for storing program and data.
A small computer with a limited application may be able to fulfill its intended
task without the need of additional storage capacity. Most general-purpose
computers would run more efficiently if they were equipped with additional
storage beyond the capacity of the main memory. there is just not enough
space in one memory unit to accommodate all the programs used in a
typical computer.
 It is more economical to use low-cost storage devices to serve as a backup
for storing the information that is not currently used by the CPU.
The memory unit that communicates directly with the CPU is called the
main memory.
Devices that provide backup storage are called auxiliary memory. The
most common auxiliary memory devices used in computer systems are
magnetic tapes and magnetic disks
They are used for storing system programs, large data files, and other
backup information. Only programs and data currently needed by the
processor reside in main memory.

3. The total memory capacity of a computer can be visualized as hierarchy of
components.
The memory hierarchy system consists of all storage devices employed in a
computer system from the slow but high-capacity auxiliary memory to a
relatively faster main memory, to an even smaller and faster cache memory
accessible to the high-speed processing logic.
The overall goal of Memory Hierarchy is to obtain the highest possible
average access speed while minimizing the total cost of the entire memory
system.
I/O Processor
Magnetic disks
Magnetic tapes
Main Memory
CPU
Cache
Memory

4. At the bottom of the hierarchy are the relatively slow magnetic tapes used to
store removable files. Next are the magnetic disks used as backup storage.
The main memory occupies a central position by being able to communicate
directly with the CPU and with auxiliary memory devices through an I/O
processor.
Cache Memory :- L1 cache & L2 cache
5.A special very high speed memory called cache.
6.It is sometimes used to increase the speed of processing by making current
programs and data available to the CPU at a rapid rate.
7.The cache memory is employed in computer system to compensate for the
speed differential between main memory access time and processor logic.
8.The Cache memory is concerned with the transfer of information between
main memory & CPU.
L1 cache is a small, fast memory cache that is built in to a CPU and helps
speed access to important and frequently-used data.
L1 cache is typically smaller and faster than L2 cache.
L1 cache is an abbreviation of Level 1 cache.

5. While I/O processor manages data transfers between auxiliary
memory and main memory, the cache organization is concerned
with the transfer of information between main memory and CPU.
Many operating systems are designed to enable the CPU to process
a number of independent program concurrently . This concept is
called multiprogramming, refers to the existence of two or more
programs in different parts of the memory hierarchy at the same
time.
The part of the computer system that supervises the flow of
information between auxiliary memory and main memory is called
the memory management system.

6. Main Memory
The technology used for main memory is based on Semiconductor
Integrated Circuits.
Main Memory : RAM & ROM
RAM (volatile) : SRAM & DRAM
SRAM consist of internal flip-flops to store binary information. The
stored information remains valid as long as power is applied to the
unit.
SRAM is easier to use & has shorter read/write cycles.
DRAM stores information in the form of electrical charges that are
applied to capacitors.
The stored charge on the capacitors tend to discharge with time.
It needs refreshing.
DRAM offers reduced power consumption & larger storage capacity
in a single memory chip.

7. ROM : Read only Memory
It is also Random Access, permanent.
ROM is required to store initial program called bootstrap
loader.
Its function is to start the computer software operating
when power is turned on.
The hardware of the computer sets the Program Counter
(PC) to the first address of the bootstrap loader.
It loads a portion of the O.S. from disk to main memory &
then control is transferred to the O.S.

8. CAO Model Question Paper
Unit – 1
Q: 1 Explain Stored program control concept of various types of
computers.
Q: 2 On what bases Flynn has classified computers? Explain.
Q: 3 What are the characteristics of CISC architecture ? How is it
different from RISC architecture?
Q: 4 Explain Immediate and indexed addressing mode. Discuss their
advantages and disadvantages.
Q: 5 How do you classify instructions in an instruction set? Explain
any two instructions in each category with suitable examples.
Q: 6 Discuss the two metrics to measure the performance of a
computer system. What are their advantages & Disadvantages?
Q: 7 Distinguish between SRAM and DRAM.
Q: 8 Discuss computer Instruction Cycle.