Pentium Architecture by wia20667

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									               Pentium Architecture
• Recall our examination of the Intel 486 pipeline
   – variable length of instructions, variable complexity of
     operations, memory-register ALU operations, etc led to poor
     performance
• In order to improve performance using RISC features, the
  Pentium architects had to rethink things – they were stuck
  with their CISC instruction set (for backward
  compatibility)
   – in CISC architectures, a machine instruction is first translated
     into a sequence of microinstructions
   – each microinstruction is a lengthy string of 1s and 0s, each of
     which refer to one control signal in the machine
   – there needs to be a process to translate each machine instruction
     into microinstructions and execute each microinstruction – this
     is done by collecting machine instructions and their associated
     microinstructions into microprograms
           Why Microinstructions?
• First, since the Pentium architecture uses a
  microprogrammed control unit, there is already a
  necessary step of decoding a machine instruction into
  microcode
• Now, consider each microinstruction:
   – each is equal length
   – each executes in the same amount of time
      • unless there are structural hazards such as a cache miss
   – branches are at the microinstruction level and are more
     predictable than machine language level branching
• In a RISC architecture, each machine instruction is
  carried out directly in hardware because each
  instruction is simple and takes roughly 1 cycle to
  execute
   – to more efficiently pipeline a CISC architecture, we can
     pipeline the microinstructions (instead of machine
     instructions) to keep a pipeline running efficiently
      Control and Micro-Operations
• An example architecture
  is shown to the right
• Each of the various
  connections is controlled
  by a particular control
  signal
   – for instance, to send the
     MBR value to the AC, we
     would signal C11
       • note that this figure is
         incomplete
   – a microprogram is a
     sequence of micro-
     operations
   – each micro-operation is
     one or more control
     signals sent out in a clock
     cycle to move
     information from one
                                    this is not an x86 architecture!
     location to another
                              Example
• Consider a CISC instruction such as Add R1, X
   – this requires that X be moved into the MAR and a read
     signaled
   – the datum returned will be placed into the MBR
   – the adder is then sent the value in R1 and MBR, adding the two
     and storing the result back into R1
   – this sequence can be written in terms of micro-operations as:
      • t1: MAR  (IR (address) )
                                             the values t1, t2, etc denote
      • t2: MBR  Memory                     separately clock cycles
      • t3: R1  (R1) + (MBR)
          – There may be other sequences needed as well, for instance, if register
            results are stored in an accumulator temporarily, then we must change the
            above to include
      • t3: Acc  (R1) + (MBR)
      • t4: R1  (Acc)
   – we can then convert these into the actual control signals (for
     instance, MBR  Memory is C5 in the previous figure)
                        Control Memory
Each micro-                          ...
program consists of      Jump to Indirect or Execute
                                                         Fetch cycle routine
one or more micro-                   ...
instructions, each           Jump to Execute             Indirect Cycle routine
stored in a separate                 ...
entry of the control                                     Interrupt cycle routine
                               Jump to Fetch
memory
                           Jump to Op code routine       Execute cycle begin
                                     ...
The control                                               AND routine
memory itself is          Jump to Fetch or Interrupt
firmware, a                          ...
program stored in                                         ADD routine
                          Jump to Fetch or Interrupt
ROM, that is placed
inside of the control   Note: each micro-program ends with a branch to the
unit                    Fetch, Interrupt, Indirect or Execute micro-program
   Example of Three Micro-Programs
• Fetch:     t1: MAR  (PC)               C2
             t2: MBR  Memory             C0, C5, CR
                  PC  (PC) + 1           C*
             t3: IR  (MBR)               C4
• Indirect: t1: MAR  (IR (address) )     C8
             t2: MBR  Memory             C0, C5, CR
             t3: IR(address)  (MBR (address) )
                                          C4
• Interrupt: t1: MBR  (PC)               C1
             t2: MAR  save address       C*
                  PC  routine address    C*
             t3: Memory  (MBR)           C12, CW
  – CR – Read control to system bus
  – CW – write control to system bus
     • C0 – C12 refers to the previous figure
     • C* are signals not shown in the figure
    Horizontal vs. Vertical                           Micro-instruction address
                                                      points to a branch in the
     Micro-Instructions                               control memory and is
                                                      taken if the condition bit
                                                      is true

                                 Micro-instruction Address
       Function Codes
                             Jump       Horizontal micro-instructions contain
Vertical micro-instructions Condition   1 bit for every control signal
use function codes that                 controlled by the control unit
need additional decoding

Internal CPU Control Signals                   Micro-instruction Address

Because this micro-instruction requires
1 bit for every control line, it is longer         Jump Condition
than the vertical micro-instruction and    System Bus
therefore take more space to store, but Control Signals
does not require additional time to
decode by the control unit
   Micro-programmed Control Unit
• Decoder analyzes IR
   – delivers starting address of op
     code’s micro-program in control
     store
       • address placed in the to a micro-
         program counter (here, called a
         Control Address Register)
• Loop on the following
   – sequencer signals read of control
     memory using address in microPC
   – item in control memory moved to
     control buffer register
   – contents of control buffer register
     generate control signals and next
     address information
       • if the micro-instructions are
         vertical, decoding is required here
   – sequencer moves next address to
     control address register
       • next instruction (add 1 to current)
       • jump to new part of this
         microprogram
       • jump to new machine routine
        Pentium IV: RISC features
• All RISC features are implemented on the execution of
  microinstructions instead of machine instructions
   – microinstruction-level pipeline with dynamically scheduled
     microoperations
      • fetch machine instruction (3 stages)
      • decode machine instruction into microinstructions (2 stages)
      • superscalar issues multiple microinstructions (2 stages, register
        renaming occurs here, up to 3 microinstructions can be issued per cycle)
      • execute of microinstructions (1 stage, units are pipelined and can take
        from 1 to many cycles (up to 32?) to execute)
      • write back (3 stages)
      • commit (3 stages, up to 3 microinstructions can commit in any cycle)
   – reservation stations (128 registers available) and multiple
     functional units (7 of them)
   – branch speculation used (control of speculation is given to
     reservation stations rather than a reorder buffer, commit still
     occurs, controlled by reservation stations)
   – trace cache used
Pentium IV Architecture
                          Specifications
• There are 7 functional units:
   –   2 simple ALUs (for simple integer operations like add and compare)
   –   1 complex ALU (for integer multiplication and integer division)
   –   1 load unit
   –   1 store unit
   –   1 floating point move (register to register move and convert)
   –   1 floating point unit (addition, subtraction, multiplication, division)
        • the simple ALU units execute in half a clock cycle so each can accommodate up
          to two microoperations per cycle reducing latency
        • the load and store units have their own address calculation components so that
          the memory address can be computed first and then the memory access
          performed, along with aggressive data cache to lower load latencies
        • floating point and complex ALU take more than 1 cycle so are pipelined
        • floating point units can handle up to 2 FP operations at a time allowing for some
          SIMD execution and improving overall FP performance
• There are 128 registers for renaming
   – reservation stations are used rather than a re-order buffer (which was used in
     older versions of the Pentium pipeline)
   – this means that instructions must wait in reservation stations longer than in
     Tomasulo’s version, waiting for speculation results
                 Pentium IV Pipeline
• Pentium III (Pentium Pro) pipeline was 10 stages deep
   – taking a minimum of 10 clock cycles to complete the shortest instructions
     with a clock rate of 1.1 GHz or less
       • the figure below shows the Pentium III pipeline
• For the Pentium IV
   – pipeline depth was lengthened to 21 stages (minimum) in order to
     accommodate a faster clock rate of 1.5 GHz
   – by 2004, the pipeline was lengthened to 31 stages (minimum) and the
     clock rate up to 3.2 GHz
• The lengthening of the pipeline allowed for the faster clock rates
   – the clock rate is now so fast that it takes 2 complete cycles for an
     instruction or data to cross the chip so that at least 2 stages in the pipeline
     are needed for certain operations like data movement!
   – With the 128 reservation stations, 128 instructions could be in some state
     of operation simultaneously (as opposed to 40 in the Pentium III)
    Trace Cache and Branch Prediction
• We talk about the trace cache in chapter 5
   – for now, consider it to be an instruction cache that stores instruction not by
     address but by the order they are being executed
   – in this way, branches do not necessarily cost us cache misses because the
     instruction being branched to is not in the same cache block
• The trace cache stores microinstructions (not machine
  instructions)
   – repeated decoding is avoided, once a machine instruction has been
     decoded, the decoded version is placed in the trace cache, this greatly
     reduces time necessary to do instruction decoding
• A branch target buffer is used to store microinstruction branches
  (not machine instruction branches) within the trace cache
   – the target buffer uses a 2-level predictor to select between local and global
     histories
       • target buffer is 8 times the size of the target buffer used in the Pentium III
   – the misprediction rate for the target buffer is below .15%!
• The trace cache and branch target buffer combined mean that
   – microinstruction fetch and microinstruction decoding is rarely needed
     because, once fetched and decoded, the items are often found in the cache
     and because predictions rarely cause wrong instructions to be fetched
                      Source of Stalls
• This architecture is very complex and relies on being able
  to fetch and decode instructions quickly
   – the process breaks down when
      • less than 3 instructions can be fetched in 1 cycle
      • trace cache causes a miss, or branches are miss predicted
      • less than 3 instructions can be issued because instructions have different
        number of microoperations
          – e.g., one instruction has 4 and another has 1, staggering when each
            instruction issues and executes
      • limitation of reservation stations
      • data dependencies cause a functional unit to stall
      • data cache access results in a miss
   – in some of these cases, the issue stage must stall, in others the
     commit stage must stall
      • misprediction rates are very low, about .8% for integer benchmarks and
        .1% for floating point benchmarks (these are misprediction rates at the
        machine level of instructions, not microinstructions)
      • trace cache has nearly a 0% miss rate, the L1 and L2 data caches have
        miss rates of around 6% and .5% respectively
      • the machine’s effective CPI is around 2.2
                Pentium IV Comparison
• Comparing the Pentium IV to the Pentium III
   – P4 has over twice the performance in many SPEC benchmarks in spite of a
     clock speed that isn’t twice as fast (this info is not in this text edition)
• The text provides a comparison between the P4 and the AMD Opteron
   – the Opteron uses dynamic scheduling, speculation, a shallower pipeline, issue
     and commit of up to 3 instructions per cycle, 2-level cache, and the chip has a
     similar transistor count although is only 2.8 GHz
       • the Opteron is a RISC instruction set, so instructions are machine instructions, not
         microinstructions
   – P4 has a higher CPI on all benchmarks except mcf (in which the AMD is more
     than twice the P4)
       • so for the most case, instructions take less clock time in the AMD than in the P4 but
         the P4 is a slightly faster clock
• The text provides a briefer comparison between the P4 and the IBM
  Power5
   – the Power5 is only 1.9 GHz
   – P5 is significantly better on most floating point benchmarks and slightly worse
     on most integer benchmarks with a clock speed half that of the P4
       • see figures 2.28 – 2.34 for specific comparisons
                     A Balancing Act
• Improving one aspect of our processor does not necessarily
  improve performance
   – in fact, it might harm performance
       • consider lengthening the pipeline depth and increasing clock speed (as with
         the P4) but without adding reservation stations or using the trace cache
• Modern processor design takes a lot of effort to balance out the
  factors
   – without accurate branch prediction and speculation hardware, stalls from
     miss-predicted branches will drop performance greatly
   – as clock speeds increase, stalls from cache misses create a bigger impact
     on CPI, so larger caches and cache optimization techniques are needed
     (we cover the latter in chapter 5)
   – to support multiple issue of instructions, we need a larger cache-to-
     processor bandwidth, which can take up valuable space
   – as we increase the number of instructions that can be issued, we need to
     increase the number of reservation stations and reorder buffer size
• For even greater improvement, we might need to turn to software
  approaches instead of or in addition to hardware enhancements –
  in appendix G, we will visit several compiler-based ideas
                 Sample Problem #1
• We see how complex an architecture can become in the
  case of the Pentium IV
  – assume that we have additional space on the CPU and want to
    enhance some element(s), what should we pick and why?
  – choices are to:
     •   add more reservation stations
     •   add more ALU functional units
     •   add another FP functional unit
     •   add more load/store units
     •   add a larger branch target buffer (either more entries, or more
         prediction bits)
     •   attempt to speed up the system clock and lengthen the pipeline (the
         additional space will be used for pipeline latches, control logic, etc)
     •   add more memory to the trace cache
     •   add more memory to the L1 cache
     •   increase the microoperation queue size to store more microoperations
         at any time
                          Solution
• Let’s consider each not from the perspective of how
  useful it might be but how much that particular hardware
  is limiting instruction issue and CPI
   – add more reservation stations – because we can issue no more
     than 3 microoperations per cycle, and assuming that the
     average microoperation executes for under 10 cycles, the 128
     registers should be sufficient
   – add more ALU/FP functional units – since these are pipelined,
     additional units are not necessary
   – add more load/store units – limiting the number of loads may
     be a source of data dependencies, and so an additional load unit
     might help, an additional store unit is probably not necessary
   – add a larger branch target buffer (either more entries, or more
     prediction bits) – prediction accuracy is extremely high, more
     entries or bits are not needed
                  Solution Continue
   – attempt to speed up the system clock and lengthen the pipeline
     (the additional space will be used for pipeline latches, control
     logic, etc) – there is little that we can do to further lengthen the
     pipeline, this may not be feasible
   – add more memory to the trace cache – similar to the branch
     target buffer, this will probably have very little impact because
     of the low miss rate of the current trace cache
   – add more L1 cache – this can make a significant impact since
     the miss rate is currently fairly high, this would be my top
     choice
   – increase the microoperation queue size to store more
     microoperations at any time – although it is unclear how many
     stalls arise from running out of microoperations, because of the
     trace cache’s performance, this is probably not necessary
• Top choices: increase L1 cache and add another load
  unit
                 Sample Problem #2
• Two fallacies cited in the chapter are:
   – “Processors with lower CPI will always be faster”
   – “Processors with faster clock rates will always be faster”
• Why are these not necessarily true?
   – recall our CPU time formula: CPU Time = IC*CPI*CCT
      • if CPI is lower, the CPU Time is lower and thus the processor is faster
      • if clock rate is higher, then CCT is lower and CPU Time is lower, thus
        the processor is faster
   – BUT, we see from our examination of various processors that
      • deeper pipelines can have a larger impact than faster clock rates
      • multiple issue superscalars have a significant impact on CPI but only if
        supported by reservation stations, reorder buffers, and accurate branch
        speculation
      • in the Pentium IV, the CPI might be lower than other machines but its
        IC can be higher because, in this case, IC is at the microinstruction level
      • additionally, a very low CPI with a slow clock rate may not outperform
        a higher CPI with a faster clock rate
       Limitations on ILP (Chapter 3)
• From mid 80s through 2000, architects focused on
  promoting ILP
   –   deeper pipelines
   –   multiple instruction issue
   –   dynamic scheduling
   –   Speculation
• Hardware needs increased
   – multiple function units
        • cost grows linearly with the number of units
   – increase (possibly very large) in memory bandwidth
   – more register-file bandwidth
        • which might take up significant space on the chip and may require larger
          system bus sizes which turns into more pins
   – more complex memory system
        • possibly independent memory banks
                        Limitations
• By 2000, architects found limitations in just how much
  ILP there is to exploit
   – inherent limitations to multiple-issue are the limited amount of
     ILP of a program:
      • how many instructions are independent of each other?
      • how much distance is available between loading an operand and using
        it? between using and saving it?
   – multi-cycle latency for certain types of operations that cause
     inconsistencies in the amount of issuing that can be
     simultaneous
• Architects more recently have concentrated
   – on further optimizations of current architectures
   – and achieving higher clock rates without increasing issue rates
            Limitations on Issue Size
• Ideally, we would like to issue as many independent instructions
  simultaneously as possible, but this is not practical because we
  would have to:
   –   look arbitrarily far ahead to find an instruction to issue
   –   rename all registers when needed to avoid WAR/WAW
   –   determine all register and memory dependences
   –   predict all branches (conditional, unconditional, returns)
   –   provide enough functional units to ensure all ready instructions can be
       issued
• What is a possible maximum window size?
   – to determine register dependences over n instructions requires n2-n
     comparisons
        • 2000 instructions  4,000,000 comparisons
        • 50 instructions  2450 comparisons
   – window sizes have ranged between 4 and 32 with some recent machines
     having sizes of 2-8
        • a machine with window size of 32 achieves about 1/5 of the ideal speedup for
          most benchmarks (see figure on next slide)
Window Size Impact on
   Instruction Issue
            Realistic Branch Prediction
• Types of predictions
   – Perfect branch prediction
       • impossible to achieve so we won’t bother with this
   – Selective history prediction using
       • correlating two-bit predictor
       • non-correlating two-bit predictor
       • selector between them
   – Standard two-bit predictor with 512 two-bit entries
   – Static predictor
       • uses program profile history
   – None                                                       Misprediction      Issue
                                                                Rate               Rate
• Experimental results shown                 Selective:         ~3%                ~24
  to the right                               Standard:          ~17%               ~20
   – notice that issue rate is not           Static:            ~10%               ~21
     significantly different and that
     the static predictor is the
                                             see the figures on the next slide for details
     easiest so might be a
     reasonable approach
Branch Predictor Performance
           Effects of Finite Registers
• With infinite registers, register
  renaming can eliminate all
  WAW and WAR hazards
   – with Tomasulo’s approach, the
     reservation stations offer virtual
     registers
• Power 5 has 88 additional FP
  and 88 additional integer
  registers for reservation
  stations
   – surprisingly though, the number
     of registers does not have a
     dramatic impact as long as there
     are at least 64 + 64 registers
     available
                     Alias Analysis
• Aside from register
  renaming, we have
  name dependencies on
  memory references
• Three models are:
  – global (perfect analysis
    of all global vars)
  – stack perfect (perfect
    analysis of all stack
    references)
  – inspection (examine
    accesses for
    interference at compile
    time)
  – none (assume all
    references conflict)
             A Realizable Processor
• The authors describe an ambitious but realistic processor
  that could be available with today’s technology:
   – issue up to 64 instructions / cycle with no restrictions on what
     instructions can be issued in the same cycle
   – tournament branch predictor with 1K entries and 16 entry
     return predictor
   – perfect memory reference disambiguation performed
     dynamically
   – register renaming with 64 int and 64 FP registers
      • with a 64 instruction / cycle issue capability, the average number of
        instructions issued per cycle is estimated to be around 20
      • if there are no stalls for limited hardware, cache misses and miss-
        speculation, this would result in a CPI of .05!
   – we might question whether a 64 instruction window is
     reasonable given the complexity needed in comparing up to 64
     instructions together in each cycle, today we find most
     computers limit window sizes to 8 at most
                           Example
• Let’s compare three hypothetical processors and
  determine their MIPS rating for the gcc benchmark
   – processor 1: simple MIPS 2-issue superscalar pipeline with
     clock rate of 4 GHz, CPI of 0.8, cache system with .005 misses
     per instruction
   – processor 2: deeply pipelined MIPS with a clock rate of 5
     GHz, CPI of 1.0, smaller cache yielding .0055 misses per
     instruction
   – processor 3: speculative superscalar with 64-entry window
     that achieves 50% of its ideal issue rate (see figure 3.7) with a
     clock rate of 2.5 GHz, a small cache yielding .01 misses per
     instruction (although 25% of the miss penalty is not visible due
     to dynamic scheduling)
      • assume memory access time (miss penalty) is 50 ns
   – to solve this problem, we have to determine each processor’s
     CPI, which is a combination of processor CPI and the impact
     of memory (cache misses)
• Processor 1:                     Solution
  –   4 GHz clock = .25 ns per clock cycle
  –   memory access of 50 ns so miss penalty = 50 / .25 = 200 cycles
  –   cache penalty = .005 * 200 = 1.0 cycles per instruction
  –   overall CPI = 0.8 + 1.0 = 1.8
  –   MIPS = 4 GHz / 1.8 = 2222 MIPS
• Processor 2:
  –   5 GHz clock = .2 ns per clock cycle
  –   miss penalty = 50 / .2 = 250 cycles
  –   cache penalty = .0055 * 250 = 1.4 cycles per instruction
  –   overall CPI = 1.0 + 1.4 = 2.4
  –   MIPS = 5 GHz / 2.4 = 2083 MIPS
• Processor 3:
  – 2.5 GHz clock = .4 ns per clock cycle
  – miss penalty takes affect only 75% of the time, so miss penalty = .75 * 50 /
    .4 = 94 cycles
  – cache penalty = .01 * 94 = 0.94
  – CPU portion of the CPI is based on half the ideal issue rate of a 64-entry
    window, which is 1 / (9 * 2) = 0.22
  – overall CPI = 0.94 + 0.22 = 1.16
  – MIPS = 2.5 / 1.16 = 2155 MIPS
               Sample Problem #1
• For the li benchmark
   – compare a perfect processor from one that has a 128 window
     size, tournament branch predictor, 64 integer and 64 FP
     renaming registers and inspection alias analysis
• The perfect processor can issue 18 instructions per cycle
   – but the branch prediction only permits up to 16 instructions
     per cycle and an infinite number of registers and perfect alias
     analysis can only accommodate 12 instructions per cycle
   – so the perfect processor can achieve an issue rate of 12
     instructions per cycle, or a CPI = 1 / 12 = .083
• The more realistic processor is most limited by alias
  analysis (4 instructions per cycle), so a CPI = .25
   – the perfect machine is then .25 / .083 = 3 times faster on this
     benchmark
                 Sample Problem #2
• Architects are considering one of three enhancements to the next
  generation of computer
   – more on-chip cache to reduce the impact of memory access
   – faster memories
   – faster clock rates
• Explain, using the example on pages 167-169, how each of these
  would impact the three hypothetical processors
   – more on-chip cache lowers cache CPI – depending on the current miss
     rate, this might be useful, but for processor 1 and 2, the miss rates are
     already < .1%
   – faster memory reduces cache CPI (it decreases the number of cycles
     needed for any cache miss) – since all three processors’ CPIs are roughly
     half from cache miss and half from processor performance, this could have
     a significant impact
   – faster clock rates increases cache CPI, possibly will have no effect on
     execution CPI – by merely increasing clock rate, the stalls for memory
     accesses will increase, however if this increase is coupled with a longer
     pipeline, then execution CPI might decrease and so overall performance
     might improve
                   Sample Problem #3
• Consider a speculative superscalar with a window size of 32
   – with proper hardware support, the superscalar can issue 70% of the
     expected issue rate (see figure 3.2)
       • the processor has a 3.33 GHz clock rate
       • the processor stalls when all functional units are busy (which arises once in
         every 12 cycles)
       • when there is a misprediction, the processor require 6 complete cycles to flush
         the reorder buffer and begin again (profile-based prediction is used)
       • memory accesses take 40 ns, 40% of the instructions are loads or stores and the
         instruction cache has a miss rate of .5% and the data cache has a miss rate of
         .03%
   – determine this machine’s MIPS rating for the doduc benchmark
• Solution:
   – cache miss penalty = 40 ns / 3.33 GHz = 120 cycles
   – memory CPI = .005 * 120 + .40 * .0003 * 120 = .614
       • CPU CPI = 1 / 6.3 + 1 / 12 + 6 * .05 = .542
       • CPI = .614 + .542 = 1.156
   – MIPS rating = 3.33 GHz / 1.156 = 2881 MIPS

								
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