Evaluation of Microprocessor

Evaluation of Microprocessor

The first microprocessor introduced in 1971 was a 4-bit microprocessor with 4m5KB memory and had a set of 45 instructions. In the past 5 decades microprocessor speed has doubled every two years, as predicted by Gordon Moore, Intel co-founder. Current microprocessors can access 64 GB memory. Depending on width of data microprocessors can process, they are of these categories−

• 8-bit
• 16-bit
• 32-bit
• 64-bit

Size of instruction set is another important consideration while categorizing microprocessors. Initially, microprocessors had very small instructions sets because complex hardware was expensive as well as difficult to build.

As technology developed to overcome these issues, more and more complex instructions were added to increase functionality of the microprocessor. However, soon it was realized that having large instruction sets was counterproductive as many instructions that were rarely used sat idle on precious memory space. So the old school of thought that supported smaller instruction sets gained popularity.

Let us learn more about the two types of microprocessors based on their instruction set.

RISC

RISC stands for Reduced Instruction Set Computers. It has a small set of highly optimized instructions. Complex instruction are also implemented using simpler instructions, reducing the size of instruction set. The designing philosophy for RISC incorporates these salient points −

• Number of instructions should be minimum.
• Instructions should be of same length.
• Simple addressing modes should be used
• Reduce memory references to retrieve operands by adding registers

Some of the techniques used by RISC architecture include −

·        Pipelining− A sequence of instructions is fetched even if it means overlapping of instructions in fetching and execution.

·        Single cycle execution − Most of RISC instructions take one CPU cycle to execute.

Examples of RISC processors are Intel P6, Pentium4, AMD K6 and K7, etc.

CISC

CISC stands for Complex Instruction Set Computers. It supports hundreds of instructions. Computers supporting CISC can accomplish wide variety of tasks, making them ideal for personal computers. These are some characteristics of CISC architecture −

• Larger set of instructions
• Instructions are of variable length
• Complex addressing modes
• Instructions take more than one clock cycle
• Work well with simpler compilers

Examples of CISC processors are Intel 386 & 486, Pentium, Pentium II and III, Motorola 68000, etc.

EPIC

EPIC stands for Explicitly Parallel Instruction Computing. It is a computer architecture that is a cross between RISC and CISC, trying to provide the best of both. Its important features include −

• Parallel instructions rather than fixed width
• Mechanism to communication compiler’s execution plan to hardware
• Programs must have sequential semantics

Some EPIC processors are Intel IA-64, Itanium, etc.

Microprocessor Concepts

Microprocessor is the brain of computer, which does all the work. It is a computer processor that incorporates all the functions of CPU (Central Processing Unit) on a single IC (Integrated Circuit) or at the most a few ICs. Microprocessors were first introduced in early 1970s. 4004 was the first general purpose microprocessor used by Intel in building personal computers. Arrival of low cost general purpose microprocessors has been instrumental in development of modern society the way it has.

We will study the characteristics and components of a microprocessor in detail.

Microprocessors Characteristics

Microprocessors are multipurpose devices that can be designed for generic or specialized functions. The microprocessors of laptops and smartphones are general purpose whereas ones designed for graphical processing or machine vision are specialized ones. There are some characteristics that are common to all microprocessors.

These are the most important defining characteristics of a microprocessor −

• Clock speed
• Instruction set
• Word size

Clock Speed

Every microprocessor has an internal clock that regulates the speed at which it executes instructions and also synchronizes it with other components. The speed at which the microprocessor executes instructions is called clock speed. Clock speeds are measured in MHz or GHz where 1 MHz means 1 million cycles per second whereas 1 GHz equals to 1 billion cycles per second. Here cycle refers to single electric signal cycle.

Currently microprocessors have clock speed in the range of 3 GHz, which is maximum that current technology can attain. Speeds more than this generate enough heat to damage the chip itself. To overcome this, manufacturers are using multiple processors working in parallel on a chip.

Word Size

Number of bits that can be processed by a processor in a single instruction is called its word size. Word size determines the amount of RAM that can be accessed at one go and total number of pins on the microprocessor. Total number of input and output pins in turn determines the architecture of the microprocessor.

First commercial microprocessor Intel 4004 was a 4-bit processor. It had 4 input pins and 4 output pins. Number of output pins is always equal to the number of input pins. Currently most microprocessors use 32-bit or 64-bit architecture.

Instruction Set

A command given to a digital machine to perform an operation on a piece of data is called an instruction. Basic set of machine level instructions that a microprocessor is designed to execute is called its instruction set. These instructions do carry out these types of operations −

• Data transfer
• Arithmetic operations
• Logical operations
• Control flow
• Input/output and machine control

Microprocessor Components

Compared to the first microprocessors, today’s processors are very small but still they have these basic parts right from the first model −

• CPU
• Bus
• Memory

CPU

CPU is fabricated as a very large scale integrated circuit (VLSI) and has these parts −

·        Instruction register − It holds the instruction to be executed.

·        Decoder − It decodes (converts to machine level language) the instruction and sends to the ALU (Arithmetic Logic Unit).

·        ALU − It has necessary circuits to perform arithmetic, logical, memory, register and program sequencing operations.

·        Register − It holds intermediate results obtained during program processing. Registers are used for holding such results rather than RAM because accessing registers is almost 10 times faster than accessing RAM.

Bus

Connection lines used to connect the internal parts of the microprocessor chip is called bus. There are three types of buses in a microprocessor −

·        Data Bus − Lines that carry data to and from memory are called data bus. It is a bidirectional bus with width equal to word length of the microprocessor.

·        Address Bus − It is a unidirectional responsible for carrying address of a memory location or I/O port from CPU to memory or I/O port.

·        Control Bus − Lines that carry control signals like clock signals, interrupt signal or ready signal are called control bus. They are bidirectional. Signal that denotes that a device is ready for processing is called ready signal. Signal that indicates to a device to interrupt its process is called an interrupt signal.

Memory

Microprocessor has two types of memory

·        RAM − Random Access Memory is volatile memory that gets erased when power is switched off. All data and instructions are stored in RAM.

·        ROM − Read Only Memory is non-volatile memory whose data remains intact even after power is switched off. Microprocessor can read from it any time it wants but cannot write to it. It is preprogrammed with most essential data like booting sequence by the manufacturer.