Risc and cisc casestudy


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Risc and cisc casestudy

  1. 1. ---From Jayvijay.V.Shah K.J.Somaiya Institute Of Engineering And Information Technology,Sion
  2. 2. CASE STUDY What is a Microprocessor?  The microprocessor, also known as the Central Processing Unit (CPU), is the brain of all computers and many household and electronic devices.  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 digital devices.  The first microprocessor was the Intel 4004, introduced in 1971. The 4004 was not very powerful; it was primarily used to perform simple mathematical operations. Microprocessor operation Microprocessors use three steps commonly called Fetch, Decode, and Execute.  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.
  3. 3.  In the Execute step, this operation is performed. Now,let’s talk something about RISC and CISC. RISC 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 previously. 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
  4. 4. 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 computer organization. • 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 instructions. 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
  5. 5. 3. More Memory Access may be needed CISC  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 implementation.
  6. 6. Build rich instruction sets: By using a micro programmed design, designers could build more functionality into each instruction. 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 languages. 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 source  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
  7. 7. unit. 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 machine.  Many specialized instructions aren't used frequently enough to justify their existence --- approximately 20% of the available instructions are used in a typical program.