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Because the instruction set of a RISC processor is so simple, it uses up much less chip space extra functions, such as memory management units or floating point arithmetic units, can also be placed on the same chip. Since a simplified instruction set allows for a pipelined, superscalar design RISC processors often achieve 2 to 4 times the performance of CISC processors using comparable semiconductor technology and the same clock rates. The advantages of RISC Implementing a processor with a simplified instruction set design provides several advantages over implementing a comparable CISC design: (1) Speed. Pipelining is a design technique where the computer's hardware processes more than one instruction at a time, and doesn't wait for one instruction to complete before starting the next.ģ4 Implementing a processor with a simplified instruction set design RISC designers are concerned primarily with creating the fastest chip possible, and so they use a number of techniques, including pipelining. Good performance: With a simpler instruction set, it should possible for a processor to execute its instruction in a single clock cycle. Therefor it should be possible to design a more performance processor with less cost. For the CISC machine, all the effort invested in processor design to provide complex instructions and thereby reduce the compiler workload was being wasted.Ģ9 Less cost: Since only the simpler instructions are needed, the processor hardware required to implement them could be reduced in complexity. Further analysis shows that these instructions tend to perform the simpler operations and use only the simpler addressing modes. It was an obvious conclusion that if this 20% of instruction was speeded up, the performance benefits would be far greater. CISC processors are backward compactable.Ģ8 Why RISC is better The 80/20 rule: Analysis of the instruction mix generated by CISC compilers, shows that more than 80% of the instructions generated and executed used only 20% of an instruction set. CISC are complex but it doesn’t necessarily increase the cost. RISC CPU need more instructions than CISC CPU. However, When we compiled high-level language. Also, it allow higher clock speed than CISC. CISC RISC have fewer and simpler instructions, therefore, they are less complex and easier to design. A Three states RISC pipelines is : Fetch instruction Decode and select registers Execute the instruction Clock Stage 1 2 3 4 5 6 7 i1 i2 i3 i4 i5 i6 i7 -Ģ7 RISC vs. It is to prepare the next instruction while the current instruction is still executing. This pipelining is a key technique used to speed up RISC machines.
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Most instructions complete in one machine cycle, which allows the processor to handle several instructions at the same time. Same length instructions.Ģ5 RISC characteristics Each instruction is the same length, so that it may be fetched in a single operation.
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In a RISC machine, the instruction set contains simple, basic instructions, from which more complex instructions can be composed. The effective address is computed in a single clock cycle.Ģ0 Instruction Pipeline Similar to a manufacturing assembly lineįetch an instruction Decode the instruction Execute the instruction Store results Each stage processes simultaneously (after initial latency) Execute one instruction per clock cycleĢ1 Pipeline Stages Some processors use 3, 4, or 5 stagesĢ4 RISC characteristics Simple instruction set. Reduced Instruction Set Computers-RISC machines-were the result.ġ9 Addressing modes Limited number of addressing modes One of their key realizations was that a sequence of simple instructions produces the same results as a sequence of complex instructions, but can be implemented with a simpler (and faster) hardware design. Sin-Min Lee Department of Computer Science San Jose State Universityġ8 The Basis for RISC Use of simple instructions CS147 Lecture 20 RISC Architecture and Super Computer Prof.