Microprocessor and its types: RISC, CISC and EPIC

Microprocessor

A microprocessor is a Central Processing Unit (CPU) on a single chip. It is a multipurpose programmable device constructed using Metal Oxide Semiconductor (MOS) technology. A microprocessor is designed to perform arithmetic and logic operations that make use of small number holding areas called Registers. In 1971 Intel Corporation fabricated the first microprocessor- 4004. It could do only add and subtract operations and that too it could process only 4 bits at a time. But Intel 4004 powered one of the first portable electronic calculators and since then the microprocessor technology has come a long way. Today we have processors with which can process up to 128 bits at a time at the speed of billion instructions per second.    

Registers

A register is a very small amount of very fast memory that is built into the CPU (central processing unit) in order to speed up its operations by providing quick access to commonly used values. All data must be represented in a register before it can be processed. For example, if two numbers are to be multiplied, both numbers must be in registers, and the result is also placed in a register. 

Instruction Set

It is the set of instructions that the microprocessor executes at a time.

Word Length

The number of bits processed in a single instruction is called word length or word size. The word size is directly proportional to the processing power of the CPU. During the processing, the internal general purpose registers hold data. So if internal registers can hold data upto 8 bits, the word length is 8 bits. If it can process 16 bits at a time, then the internal registers can hold upto 16 bits at a time and so on. Hence a 32 bit processor is about 4 times faster than an 8 bit processor. Examples of word lengths are 16 bit, 32 bit, 64 bit. The terms 16-bit CPU, 32-bit CPU, 64-bit CPU are used very often while talking about CPUs.

Clock Speed 

The microprocessor’s speed is controlled by the System Clock. The System Clock is an electronic circuit that generates pulses which are measured in million of cycles per second (MHz). The number of pulses generated by the clock per unit of time is its Clock speed. Each microprocessor is characterized by its clock speed. Nowadays microprocessors have clock speed of several GHz. 

Classification of Microprocessors

Based on architecture microprocessor can be of two types RISC and CISC

RISC:- It stands for Reduced Instruction Set Comptuter. It is a type of microprocessor architecture that uses a small set of instructions of uniform length. These are simple but primitive instructions which execute in one clock cycle. For this reason, RISC chips are less complex and also less expensive to produce. The instructions are of uniform length which interface with about 32-36 registers. The program size in case of RISC architecture is more but more memory cycles are needed to access data. To reduce the number of memory cycles, RISC keeps the necessary data in the processor itself. The drawback of RISC design is that the computer must combine or repeat operations to complete a large program consisting of many processing operations. Since instructions are simple , RISC processors are relatively simple to design. Examples of RISC processor is SPARC, POWER PC etc.

CISC:- It stands for Complex Instruction Set Computer. A CISC chip such as Intel Pentium provides programmers with hundreds of instructions of variable sizes, and the processing circuitry includes many special purpose circuits that carry out these instructions at high speeds. These instructions interface with memory in multiple mechanisms with complex addressing modes. In this case the program
size is reduced and hence lesser number of memory cycles are required to execute the instruction. So fewer general purpose registers(8-12) are present in CISC processors. Also less number of memory cycles result in faster execution of the program.

EPIC:- It stands for Explicitly Parallel Instruction Computing. It is a computer architecture that combines the best feature of both RISC and CISC. It does not use instructions of any fixed length but rather aims at parallel processing of instructions. It uses a bundle of complex instructions that in addition to basic instruction also contain information on how to run the instruction in parallel with other instructions. This greatly increases the efficiency of an EPIC processor. IA-64 (Intel Architecture-64) is Intel’s first 64 bit processor based on EPIC.