The CISC microprocessors have the instruction built into the microprocessor. These instruction take up space in the microprocessor and leave less space for program processing. Some microprocessor perform  specific tasks, such as those used for  performing scientific calculation. The extra instructions built into the microprocessor take up valuable space in the microprocessor. The processing time taken to execute the extra instruction is also wasted.

                        To overcome these problem, a new architecture called RISC came into picture. The microprocessors using RISC architecture have limited instruction built into it. This requires few transistors to be built into the microprocessor to make up for the time delay in processing the instruction . The reduction in instructions also saves the space in the microprocessor. The RISC microprocessor is cheaper to build than the CISC processor. This processor is favorable for scientific purposes where limited instruction are required.

Complex Instruction Set Computer (CISC)

The Microprocessors using CISC architecture have many instruction built into it. This saves the processing time for performing tasks. The time is saved because the required instructions are available in the microprocessor itself and it does not have to retrieve the instruction from the program stored in the external memory, such as the RAM.

                            The CISC architecture helps the microprocessor to speed up the execution of programs requiring instruction. However, the instruction built into the microprocessor affects the performance of the microprocessor. This is because more time is taken to process the instruction and also the space available on the microprocessor for processing reduces. To overcome this problem, more transistors need to be built into the microprocessor to maintain the speed of microprocessor. This type of processor is useful for general purpose computing. It is generally used in personal computers (PCs).

   

                         ReducedInstruction Set Computer (RISC) :-

Caches that are used in computers are fast pools of memory that are designed to speed up the transfer of data between fast and slower devices. Aside from being used to buffer data, caches can possess software support logic, enabling them to begin processing procedures in advance.
Most of the modern computer  use two or three cache levels so that processor does not have to wait for longer time for information from the memory.

                 Different types of cache levels are:-

L1 Cache :-  L1 or Level 1 cache is referred to the cache which is built in the processor. This is the fastest cache in the computer. This cache is also known as primary cache or internal cache. The most common size of this type of cache memory is 8 KB to 64 KB.

L2 Cache :-  L2 or Level 2 cache was located outside the processor i.e. commonly present on the motherboard. The most common size of this type of cache memory is 64 KB to 8 KB. L2 cache can also reduce data access time by buffering the data that processor is about to request from memory, as well as program instructions. L2 cache is secondary to the CPU and is slower than L1 cache, despite often being much larger. In addition, data that is requested from L2 cache is copied to L1 cache. Requested data is removed from L2 cache if it is an exclusive cache, and stays there if it is an inclusive cache. L2 cache is most often unified, which means that it is used to store both program data and instructions.

L3 Cache :-   L3 or Level 3 cache all modern processor have L2 cache on them, thus the cache on the motherboard is referred to as L3 cache. Often only high end workstations and servers need L3 cache. Currently only the Pentium 4 Extreme Edition features L3 cache. L3 has been part of the CPU or externally mounted near the CPU on the motherboard. It comes in sizes and speeds.

            L3 cache is a memory cache that is built into the motherboard. It is used to feed the L2 cache, and is typically faster than the system's main memory, but still slower than the L2 cache.

  

  

Different Cache Levels

   BSB refers to the bus that connects the CPU to the L2 cache. The speed of BSB is totally dependent on the speed of the CPU. 

                 The L2 cache stores the instruction and data that need to be processed. Whenever these data and instructions are needed by the processor ,the information is transferred using BSB. Since BSB transfers data directly to the CPU, it is very fast as compared to Front Side Bus(FSB). BSB describes the interface between the L1 cache on the processor with the L2 cache.

Front Side Bus:-

  FSB is also known as the Processor Bus, Memory Bus, or System Bus and connects the CPU (chipset) with the main memory and L2 cache. 

The FSB can range from speeds of 66 MHz, 133 MHz, 100 MHz, 266 MHz, 400 MHz, and up. The FSB is now another important consideration when looking at purchasing a computer Motherboard or a new computer.

            The FSB communicates with components through the computer’s chipset. The chipset consists of two processors, known as the Northbridge and the Southbridge. These two chips collect data from specific components, channeling all information to the CPU through the front side bus.

                          The speed of the front side bus is measured in Megahertz or Gigahertz, just like the processor. Most computers' processors run faster than their system buses, so the FSB speed is typically a ratio of the processor speed. For Example :- a Pentium 4 processor that runs at 2.4 GHz may have an FSB speed of only 400 MHz . The CPU to FSB ratio would be 6:1. A Power Mac G5, however, with a 2.0 GHz processor, has a 1.0 GHz front side bus. 
 Therefore, its CPU to FSB ratio is 2:1.


                                                                  Back Side Bus :-

  
 Microprocessor is an electronic component that is used by a computer to do its work. microprocessor is an integrated circuit that engineers imprint onto a small  Silicon Semiconductor Chip.

        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. Everything a computer does is described by lots of precise instructions, and microprocessors carry out these instructions at incredible speed-many millions of times a second.

          Microprocessors are responsible for performing the majority of a computer's processing tasks, which include making calculations, managing data and following input instructions. With personal computers, the term "microprocessor" is synonymous with the term "central processing unit" or CPU.

Example :-  Microprocessor include 8085, 8086, Pentium, i3, i5 etc.

              

        Three basic characteristics differentiate Microprocessors :-

 Instruction Set :-  An Instruction Set or Command Set is the basic set of commands understood by the microprocessor. When compiled the High-Level instructions of the programming language are transformed into the Machine-Level commands for a specific microprocessor. Two important  factors in instruction set design are decoder simplicity and code density.

                    The decoder reads the next instruction from memory, and then routes the component pieces of that instruction appropriately. The design of the instruction set can dramatically affect the complexity and, therefore, the speed of decoding the instructions. Code density is the combined size of the instructions needed to perform a particular task. Higher density improves memory utilization.

Bandwidth :-  In order to carry out instructions, microprocessors transmit data in bits, or units of binary notation that appear as either ones or zeros. The maximum number of bits that a microprocessor can process while carrying out a single instruction is known as bandwidth. The higher a microprocessor's bandwidth, the more processing power a microprocessor has. For example, a microprocessor with a bandwidth of 32 bits is more powerful than a microprocessor with a bandwidth of 16 bits.

Clock-Speed :-   Alternatively referred to as Clock Rate and Processor Speed, Clock Speed is the speed that the microprocessor executes each instruction or each vibration of the clock. The CPU requires a fixed number of  cycles to execute each instruction. The faster the clocks rate, the faster the CPU, or the faster it can execute instructions. Clock Speeds are usually determined in MHz, 1 MHz representing 1 million cycles per second, or in GHz, 1 GHz representing 1 thousand million cycles per second. The higher the CPU speed the better a computer will perform in a general sense.

                              Other components can improve a computer's performance as well like memory (RAM) and the motherboard as well as the number of cores that a processor has like dual core or quad core.