Tuesday, 11 June 2013

What Are The Types Of Computer Processors?

Cisc Processors:

One of the earlier goals of CPU designers was to provide more and more instructions in the instruction set of a CPU to ensure that the CPU supports functions directly.This makes it easier to translate high-level languages programs to machine language and ensures that the machine language programs run more effectively.Of Course,every additional instruction in the instruction set of a CPU requires the necessary hardware circuitry to handle that instruction,adding more complexity to the CPU's hardware circuitry.Another goal of CPU designers was to optimize the usage of expensive memory.To achieve this,the designers tried to pack more instructions in memory by introducing the concept of variable length instructions such as half-word,one-and-half-word etc.For Example,an operand in an immediate instruction needs fewer bits and can be designed as a half-word instruction.

Additionally,CPU's were designed to support a variety of add of addressing modes (discussed later in the chapter during the discussion of memory).CPUs with large instruction set,variable length instructions, and a variety of addressing modes are said to employ CISC (Complex Instruction Set Computer) architecture.Since CISC processors possess so many processing features,they make the job of machine language; programmers easier.However,they are complex and expensive to produce.Most personal computers of today use CISC processors.

RISC Processors:

In early 1980,some CPU designers realised that many instructions supported by a CISC-based CPU are rarely used.Hence,an idea evolved that the design complexity of a CPU can be reduced greatly by implementing only bare minimum basic set of instructions and some some of the more frequently used instructions in the hardware circuitry of the CPU.Other complex instructions need not to be supported in the instruction set of the CPU because they can always be implemented in software by using the basic set of instructions.While working on simple CPU design,the designers also came up with the idea of making all instructions of uniform length so that the decoding and execution of all instructions becomes simple and fast.

                                                Furthermore,to speed up computation and reduce the complexity of handling a number of addressing modes they decided to design all the instructions in such a way that they retrieve operands stored in registers in CPU rather that from memory.These designs ideas resulted in producing faster and less expensive processors.CPU's with small business sex,fixed-length instructions and reduced references to memory to retrieve operands are said to employ RISC (Reduced Instruction Set Computer) architecture.RISC processors have small instruction set,they place extra demand on programmers who must consider how to implement complex instructions by combining simple instructions.Processors are faster for most applications,less complex and less expensive to produce than CIS processors because of simpler design.

EPIC Processors:

The Explicitly Parallel Instruction Computing (EPIC) technology breaks through the sequential nature of conventional processor architectures by allowing the software to communicate explicitly to the processor when operations can be done in parallel.For This,it uses tighter coupling between the compiler and the processor.It enables the compiler to extract maximum parallelism in the original code and explicitly describe it to the processor.Processors based on EPIC architecture are simpler and more powerful than traditional CISC or RISC processors.These processors are mainly targeted to next-generation,64-bit and high-end server and workstation market (not for personal computer market).

Multicore Processors:

Till recently,the approach used for building faster processors was to keep reducing the size of chips while increasing the number of transistors they contain.Although,this trend has driven the computing industry for several years,it has now been realised that transistors cannot shrink forever.Current Transistors technology limits the ability to continue making single-core processors more powerful due to following reasons:

1-As a transistor get smaller,the gate,which switches the electricity on or Off,gets thinner and less able to block the flow of electrons.Thus,small transistors tend to use electricity all the time,even when they are not switching.This wastes power.
2-Increasing clock speeds causes transistors to switch faster and generate more heat and consume more power.

These and other challenges have forced processors manufactures to research for new approach for building faster processors.In response,manufactures are not building multicore processor chips instead of increasingly powerful (faster) single-core processor chips.That is,in the new architecture,a processor chip has multiple cooler-running,more energy-efficient processing cores instead of one increasingly powerful one.The multicore chips do not necessarily run as fast as the highest performing single-core models,but they improve overall performance by handling more work in parallel.For Instance,a dual-core chip running multiple applications is about 1.5 times faster than a chip with just on comparable cores.

The Operating system (OS) controls the overall assignment of tasks in a multicore processor.In a multicore processor each core has its independent cache (though in some designs All cores share the same cache),thus providing the PS with sufficient resources to handle multiple applications in parallel.When a single-core chip runs multiple programs,the OS assigns a time slice to work on one program and then assigns different time slices for other programs.This can cause conflicts errors,or slowdowns when the processor must perform multiple tasks simultaneously.However,multiple programs can be run at the same time on a multicore chip with each core handling a separate program.The same logic holds for running multiple threads of a multi-threaded application at the same time on multicore chip with each core handling at separate thread.Based on this,either the OS or a multithread application parcels out work to the multiple cores.

Multicore processors have following advantages over single-core processors:

1-They enable building of computers with better overall system performance by handling more work in parallel.
2-For comparable performance,multicore chips consume less power and generate less heat that single-core chips.Hence,multicore technology is also referred to as energy-efficient or power aware processor technology.
3-Because the chips core are on the same die in case of multicore processors architecture,they can share architectural components,such as memory elements and memory managment.They thus have fewer components and lower costs than systems running multiple (each a single-core processor).
4-Also,the signalling between cores can be faster and use less electricity than on multichip systems.

Multicore Processors,however,currently have following limitations:

1-To take advantage of multicore chips,applications must be redesigned so that the processor can run them as multiple threads.Note that it is more challenging to create software that is multi-threaded.
2-To redesign applications,programmers must find good places to break up the applications,divide the work into roughly equal pieces that can run at the same time and determine the best times for the threads to communicate with one another.All these add to extra work for programmers.
3-Software's vendors often charge customers for each processor that will run the software (one software license processor).A customer running an application on an 8-Processor machine (multiprocessor computer) with single core-processors would thus pay for 8 licenses.A key issue with multicore chips is whether software vendors have different views regarding this issue.Some consider a processor as a unit that plugs into a single socket on motherboard,regardless of whether it has one or more cores.Hence,a single software license is sufficient for a multicore chip.On the other hand,others charge more to use their software on multicore chips for per-processing licensing.They are of opinion that customers get added performance benefit by running the software on a chip with multiple cores,so they should pay more.Multi-core chip makers are concerned that this type of non-uniform policy will hurt their products sales.

Chip makers like Intel,AMD,IBM and SUN have already introduced multicore chips for servers,desktops and laptops.The current multicore chips are dual-core (2 cores per chip),quad-core ( 4 cores per chip) , 8 cores per chip and 16 cores per chip.Industry experts predict that multicore processors will be useful immediately in server class machines but won't be very useful on the desktop systems until software vendors develop considerable more multi-threaded software.Until this occurs,single-core chips will continue to be used.Also,since single-core chips are inexpensive to manufacture,they will continue to be popular for low-priced Pcs for a while.

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