Multiple memory accessesinstruction microprogrammed

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→ Multiple memory accesses/instruction - Microprogrammed control is necessity → Microprogram control storage takes substantial portion of CPU chip area → Semantic Gap is large between machine instruction and microinstruction - General purpose instruction set includes all the features required by individually different applications → When any one application is running, all the features required by the other applications are extra burden to the application
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Central Processing Unit 45 RISC – Reduced Instruction Set Computers In the late ‘70s and early ‘80s there was a reaction to the shortcomings of the CISC style of processors Reduced Instruction Set Computers (RISC) were proposed as an alternative The underlying idea behind RISC processors is to simplify the instruction set and reduce instruction execution time RISC processors often feature: Few instructions Few addressing modes Only load and store instructions access memory All other operations are done using on-processor registers Fixed length instructions Single cycle execution of instructions The control unit is hardwired, not microprogrammed
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Central Processing Unit 46 RISC – Reduced Instruction Set Computers Since all but the load and store instructions use only registers for operands, only a few addressing modes are needed By having all instructions the same length, reading them in is easy and fast The fetch and decode stages are simple The instruction and address formats are designed to be easy to decode Unlike the variable length CISC instructions, the opcode and register fields of RISC instructions can be decoded simultaneously The control logic of a RISC processor is designed to be simple and fast The control logic is simple because of the small number of instructions and the simple addressing modes The control logic is hardwired, rather than microprogrammed, because hardwired control is faster
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