K.J.Somaiya Institute Of Engineering
And Information Technology,Sion
What is a Microprocessor?
The microprocessor, also known as the Central Processing Unit
(CPU), is the brain of all computers and many household and
A microprocessor is an electronic component that is used by a
computer to do its work. It is a central processing unit on a single
integrated circuit chip containing millions of very small
components including transistors, resistors, and diodes that work
together. Microprocessors help to do everything from writing to
searching the Web. Microprocessors carry out these instructions at
incredible speed—many millions of times a second.
Multiple microprocessors, working together, are the "hearts" of
datacenters, super-computers, communications products, and other
The first microprocessor was the Intel 4004, introduced in 1971.
The 4004 was not very
powerful; it was primarily used to perform simple mathematical
Microprocessors use three steps commonly called Fetch, Decode, and
In the Fetch step, an instruction is copied from the computer
memory into the microprocessor.
In the Decode step, the microprocessor figures out what operation
the instruction is meant to do.
In the Execute step, this operation is performed.
Now,let’s talk something about RISC and CISC.
The Reduced Instruction Set Computer(RISC) Architecture
was discovered by John Cocke in 1974.The new architecture
design enabled computers to run much faster than
For his efforts, Cocke received the
Technology in 1991.
US National Medal of
RISC, or Reduced Instruction Set Computer. is a type of microprocessor
architecture that utilizes a small, highly-optimized set of instructions,
rather than a more specialized set of instructions which are often found
in other types of architectures
At that time, the complex instruction set computer (CISC) architecture
was the norm.The goal of the CISC design was to complete a task in as
few lines of assembly code as possible. Architects would build complex
instructions directly into the hardware—a microprocessor would come
with a specific instruction set in which each single instruction executed a
series of operations. In this design, the compiler had to do very little
work to translate a high-level language statement into assembly
language. It was later found that many small, short instructions could
compute complex instructions more efficiently. This led to a design
called Reduced Instruction Set Computing (RISC).
Cocke and his team reduced the size of the instruction set, eliminating
certain instructions that were rarely used. "He said that we wanted a
computer with a simple architecture and a set of simple instructions that
could be executed in a single machine cycle—making the resulting
machine significantly more efficient than possible with other, more
complex computer designs.
With the new design, the CPU was only able to execute a limited set of
instructions, but it could execute them much faster because the
instructions were so simple. Each task, such as fetching an instruction,
accessing memory or writing data, could be completed within a single
machine cycle, or electronic pulse; with CISC, tasks often required
multiple machine cycles, taking at least twice as long to execute a task.
• RISC architectures represent an important innovation in the area of
• The RISC architecture is an attempt to produce more CPU power by
simplifying the instruction set of the CPU.
• One of the main concerns of RISC designers was to maximize the
efficiency of pipelining.
Characteristics Of RISC Architecture
1. The CPU takes less silicon area to implement,and also runs faster.
2. Simple Addressing Modes.
3. Simple Instruction formats.
4. Complex Operations are executed as sequence of simple
Advantages Of RISC Architecture
1. Less Design Complexity
2. Reducing Design Cost
3. Reducing the time between Designing And Marketing.
Disadvantages Of RISC
1. It usually leads to longer programs,which needs larger memory
space to store
2. Time Consuming
3. More Memory Access may be needed
Stands for "Complex Instruction Set Computing." This is a
type of microprocessor design. The CISC architecture contains
a large set of computer instructions that range from very
simple to very complex and specialized. CISC are chips that
are easy to program and which make efficient use of memory.
The first PC microprocessors developed were CISC chips,
because all the instructions the processor could execute were
built into the chip.
CISC was developed to make compiler development simpler. It
shifts most of the burden of generating machine instructions
to the processor. For example, instead of having to make a
compiler write long machine instructions to calculate a
square-root, a CISC processor would have a built-in ability
to do this.
The three decisions that led to the CISC philosophy are:
Use Microcode: simple logic to control the data paths between
the various elements of the processor.
In a micro programmed system, the main processor has some
built-in memory (typically ROM) that contains groups of
microcode instructions which correspond with each machinelanguage instruction.
Since the microcode memory can be much faster than main
memory, an instruction set can be implemented in microcode
without losing much speed over a purely hard-wired
Build rich instruction sets: By using a micro programmed
design, designers could build more functionality into each
This design cut down on the total number of instructions
required to implement a program, so it made more efficient
use of a slow main memory.
Made the job for assembly-language programmer simpler
Build high-level instruction sets :After the programmerfriendly instruction sets were built, designers started to
build instruction sets which map directly from high-level
Because microprogram instruction sets can be written to match
the constructs of high-level languages, the compiler does not
have to be as complicated.
Allows compilers to emit fewer instructions per line of
CISC tries to reduce the number of instructions for a program
Advantages of CISC
As each instruction became more capable, fewer instructions
could be used to implement a given task. This made more
efficient use of the relatively slow main memory.
Because microprogram instruction sets can be written to
match the constructs of high-level languages, the compiler
does not have to be as complicated.
Memory was expensive in the early days of PCs, and CISC chips
saved memory because their programming could be fed directly
into the processor.
Microprogramming is as easy as assembly language to
implement, and much less expensive than hardwiring a control
Disadvantages Of CISC
As many instructions as possible could be stored in memory
with the least possible wasted space, individual instructions
could be of almost any length this means that different
instructions will take different amounts of clock time to
execute, slowing down the overall performance of the
Many specialized instructions aren't used frequently enough
to justify their existence --- approximately 20% of the
available instructions are used in a typical program.