William Stallings Computer Organization and Architecture 8th EditionMajor Advances in Computers(1)Major Advances in Computers(2)The Next Step - RISCComparison of processorsDriving force for CISCIntention of CISCExecution CharacteristicsOperationsWeighted Relative Dynamic Frequency of HLL Operations [PATT82a] (CISC)Operands (Dynamic Percentage of Operand References)Procedure CallsImplicationsLarge Register FileRegisters for Local VariablesRegister WindowsRegister Windows cont.Overlapping Register WindowsCircular Buffer diagramOperation of Circular BufferGlobal VariablesRegisters v CacheReferencing a Scalar - Window Based Register FileReferencing a Scalar - CacheCompiler Based Register OptimizationGraph ColoringGraph Coloring ApproachWhy CISC (1)?Why CISC (2)?RISC CharacteristicsRISC v CISCRISC PipeliningEffects of PipeliningOptimization of PipeliningLoop Unrolling Twice ExampleNormal and Delayed BranchUse of Delayed BranchControversyRequired ReadingWilliam Stallings Computer Organization and Architecture8th EditionChapter 13Reduced Instruction Set ComputersMajor Advances in Computers(1)•The family concept—IBM System/360 1964—DEC PDP-8—Separates architecture from implementation•Microprogrammed control unit—Idea by Wilkes 1951—Produced by IBM S/360 1964•Cache memory—IBM S/360 model 85 1969Major Advances in Computers(2)•Solid State RAM—(See memory notes)•Microprocessors—Intel 4004 1971•Pipelining—Introduces parallelism into fetch execute cycle•Multiple processorsThe Next Step - RISC•Reduced Instruction Set Computer•Key features—Large number of general purpose registers—or use of compiler technology to optimize register use—Limited and simple instruction set—Emphasis on optimising the instruction pipelineComparison of processorsDriving force for CISC•Software costs far exceed hardware costs•Increasingly complex high level languages•Semantic gap•Leads to:—Large instruction sets—More addressing modes—Hardware implementations of HLL statements–e.g. CASE (switch) on VAXIntention of CISC•Ease compiler writing•Improve execution efficiency—Complex operations in microcode•Support more complex HLLsExecution Characteristics•Operations performed•Operands used•Execution sequencing•Studies have been done based on programs written in HLLs•Dynamic studies are measured during the execution of the programOperations•Assignments—Movement of data•Conditional statements (IF, LOOP)—Sequence control•Procedure call-return is very time consuming•Some HLL instruction lead to many machine code operationsWeighted Relative Dynamic Frequency of HLL Operations [PATT82a] (CISC)Dynamic Occurrence(Relative frequency of Occurrence)Machine-Instruction Weighted(Surrogate measures of actual time spent executing)Memory-Reference Weighted(Surrogate measures of actual time spent referencing memory) Pascal C Pascal C Pascal CASSIGN 45% 38% 13% 13% 14% 15%LOOP 5% 3% 42% 32% 33% 26%CALL 15% 12% 31% 33% 44% 45%IF 29% 43% 11% 21% 7% 13%GOTO — 3% — — — —OTHER 6% 1% 3% 1% 2% 1%Operands (Dynamic Percentage of Operand References)•Mainly local scalar variables•Optimisation should concentrate on accessing local variables Pascal C AverageInteger Constant 16% 23% 20%Scalar Variable(80% Local Variables)58% 53% 55%Array/Structure(+ a reference to an index or a pointer @ item)26% 24% 25%Procedure Calls•Very time consuming•Depends on number of parameters passed•Depends on level of nesting•Most programs do not do a lot of calls followed by lots of returns•Most variables are local•(c.f. locality of reference)Implications•Best support is given by optimising most used and most time consuming features•Large number of registers—Operand referencing•Careful design of pipelines—Branch prediction etc.•Simplified (reduced) instruction setLarge Register File•Software solution—Require compiler to allocate registers—Allocate based on most used variables in a given time—Requires sophisticated program analysis•Hardware solution—Have more registers—Thus more variables will be in registersRegisters for Local Variables•Store local scalar variables in registers•Reduces memory access•Every procedure (function) call changes locality•Parameters must be passed•Results must be returned•Variables from calling programs must be restoredRegister Windows•Only few parameters•Limited range of depth of call•Use multiple small sets of registers•Calls switch to a different set of registers•Returns switch back to a previously used set of registersRegister Windows cont.•Three areas within a register set—Parameter registers—Local registers—Temporary registers—Temporary registers from one set overlap parameter registers from the next—This allows parameter passing without moving dataOverlapping Register WindowsCircular Buffer diagramOperation of Circular Buffer•When a call is made, a current window pointer is moved to show the currently active register window•If all windows are in use, an interrupt is generated and the oldest window (the one furthest back in the call nesting) is saved to memory•A saved window pointer indicates where the next saved windows should restore toGlobal Variables•Allocated by the compiler to memory—Inefficient for frequently accessed variables•Have a set of registers for global variablesRegisters v CacheLarge Register File CacheAll local scalars Recently-used local scalarsIndividual variables Blocks of memoryCompiler-assigned global variables Recently-used global variablesSave/Restore based on procedure nesting depth Save/Restore based on cache replacement algorithmRegister addressing Memory addressingReferencing a Scalar - Window Based Register FileReferencing a Scalar - CacheCompiler Based Register Optimization•Assume small number of registers (16-32)•Optimizing use is up to compiler•HLL programs have no explicit references to registers—usually - think about C - register int•Assign symbolic or virtual register to each candidate variable •Map (unlimited) symbolic registers to real registers•Symbolic registers that do not overlap can share real registers•If you run out of real registers some variables use memoryGraph Coloring•Given a graph of nodes and edges•Assign a color to each node•Adjacent nodes have different colors•Use minimum number of colors•Nodes are symbolic registers•Two registers that are live in the same program