cpu/beta_cpu/btb.cc

   1 #include <math.h>
   2
   3 #include "cpu/beta_cpu/btb.hh"
   4 #include "base/trace.hh"
   5
   6 DefaultBTB::DefaultBTB(unsigned _numEntries,
   7                        unsigned _tagBits,
   8                        unsigned _instShiftAmt)
   9     : numEntries(_numEntries),
  10       tagBits(_tagBits),
  11       instShiftAmt(_instShiftAmt)
  12 {
  13     // @todo Check to make sure num_entries is valid (a power of 2)
  14
  15     DPRINTF(Fetch, "BTB: Creating BTB object.\n");
  16
  17     btb = new BTBEntry[numEntries];
  18
  19     for (int i = 0; i < numEntries; ++i)
  20     {
  21         btb[i].valid = false;
  22     }
  23
  24     idxMask = numEntries - 1;
  25
  26     tagMask = (1 << tagBits) - 1;
  27
  28     tagShiftAmt = instShiftAmt + (int)log2(numEntries);
  29 }
  30
  31 inline
  32 unsigned
  33 DefaultBTB::getIndex(const Addr &inst_PC)
  34 {
  35     // Need to shift PC over by the word offset.
  36     return (inst_PC >> instShiftAmt) & idxMask;
  37 }
  38
  39 inline
  40 Addr
  41 DefaultBTB::getTag(const Addr &inst_PC)
  42 {
  43     return (inst_PC >> tagShiftAmt) & tagMask;
  44 }
  45
  46 bool
  47 DefaultBTB::valid(const Addr &inst_PC)
  48 {
  49     unsigned btb_idx = getIndex(inst_PC);
  50
  51     Addr inst_tag = getTag(inst_PC);
  52
  53     assert(btb_idx < numEntries);
  54
  55     if (btb[btb_idx].valid && inst_tag == btb[btb_idx].tag) {
  56         return true;
  57     } else {
  58         return false;
  59     }
  60 }
  61
  62 // @todo Create some sort of return struct that has both whether or not the
  63 // address is valid, and also the address.  For now will just use addr = 0 to
  64 // represent invalid entry.
  65 Addr
  66 DefaultBTB::lookup(const Addr &inst_PC)
  67 {
  68     unsigned btb_idx = getIndex(inst_PC);
  69
  70     Addr inst_tag = getTag(inst_PC);
  71
  72     assert(btb_idx < numEntries);
  73
  74     if (btb[btb_idx].valid && inst_tag == btb[btb_idx].tag) {
  75         return btb[btb_idx].target;
  76     } else {
  77         return 0;
  78     }
  79 }
  80
  81 void
  82 DefaultBTB::update(const Addr &inst_PC, const Addr &target)
  83 {
  84     unsigned btb_idx = getIndex(inst_PC);
  85
  86     assert(btb_idx < numEntries);
  87
  88     btb[btb_idx].valid = true;
  89     btb[btb_idx].target = target;
  90     btb[btb_idx].tag = getTag(inst_PC);
  91 }