UINT getNextPCAddress(MipsSimContext *pContext, UINT InstAddress)
{
BTBEntry *branchDetails = checkInstructionInBTB(pContext, InstAddress);
UINT NextPCAddress = InstAddress;
Instruction *pInst = getInstFromList(pContext, InstAddress);
BTBEntry *pBTBEntry = NULL;
if (branchDetails != NULL)
{
// Branch entry found in BTB!!
branchDetails->HitRate++;
if (branchDetails->PredState == BRANCH_TAKEN)
{
// Pred State is Taken
NextPCAddress = branchDetails->TargetAddress;
pInst->BranchPred = BRANCH_TAKEN;
}
else
{
// Either the Pred State is Not Taken or is not set
// Move to the next PC
NextPCAddress += 4;
pInst->BranchPred = BRANCH_NOT_TAKEN;
}
}
else
{
// Add a new entry in the BTB!
// if Branch or Jum Instruction, add an entry in BTB if not present
if (pInst->IsJumpInst || pInst->IsBranchInst)
{
if (checkInstructionInBTB(pContext, pInst->InstAddress) == NULL)
{
// Add entry in BTB
pBTBEntry = (BTBEntry*)malloc(sizeof(BTBEntry));
pBTBEntry->InstAddress = pInst->InstAddress;
if (pInst->IsJumpInst)
{
pBTBEntry->TargetAddress = pInst->InstructionDWord.JType.Target * 4;
}
else
{
pBTBEntry->TargetAddress = pInst->InstAddress + 4 + pInst->InstructionDWord.IType.SignedImmediate * 4;
}
pBTBEntry->PredState = NOT_SET;
pBTBEntry->HitRate = 1;
pBTBEntry->next = NULL;
insertNewPredAtISStage(pContext, pBTBEntry);
pInst->BranchPred = BRANCH_NOT_TAKEN;
}
}
NextPCAddress += 4;
}
return NextPCAddress;
}
void updatePredAtEXStage(MipsSimContext *pContext, UINT InstAddress, UINT PredState)
{
BTBEntry *pBTBEntry = NULL;
// Get the Instruction from the BTB table
pBTBEntry = checkInstructionInBTB(pContext, InstAddress);
pBTBEntry->PredState = PredState;
}
void executeInst(MipsSimContext *pContext)
{
UINT CurrRSEntry = 0;
Instruction *pInst = NULL;
UINT count = 0;
bool ComputeAddress = true;
UINT ROBCount = 0;
UINT RSCount = 0;
// Check if there is an instruction ready in RS to execute
for (CurrRSEntry = 0; CurrRSEntry < MAX_RS_ENTRIES; CurrRSEntry++)
{
if (pContext->RS[CurrRSEntry].Busy && pContext->RS[CurrRSEntry].pInst->PipelineStage == INSTRUCTION_EX)
{
pInst = pContext->RS[CurrRSEntry].pInst;
// To check whehter the instruction can start execution
// NOP and BREAK Instructions will not enter this stage
// For Jump Instruction start execution, operand will be available
// if both Rs & Rt were operands and both are available, then start
// if Rs is the only operand, then check for only Qj
// if Rt is the only operand, then check for only Qk
if ((pInst->IsRsUsedAsOperand && pInst->IsRtUsedAsOperand &&
pContext->RS[CurrRSEntry].Qj == MAX_ROB_ENTRIES && pContext->RS[CurrRSEntry].Qk == MAX_ROB_ENTRIES) ||
(pInst->IsRsUsedAsOperand && !pInst->IsRtUsedAsOperand && pContext->RS[CurrRSEntry].Qj == MAX_ROB_ENTRIES) ||
(!pInst->IsRsUsedAsOperand && pInst->IsRtUsedAsOperand && pContext->RS[CurrRSEntry].Qk == MAX_ROB_ENTRIES) ||
pInst->IsJumpInst)
{
// Operands availble, EXECUTE !!!!!
switch (pInst->Opcode)
{
case OPCODE_SPECIAL:
switch (pInst->InstructionDWord.RType.Function)
{
case FUNC_SRL:
case FUNC_SRA:
pContext->RS[CurrRSEntry].Result = pContext->RS[CurrRSEntry].Vj >> pContext->RS[CurrRSEntry].Vk;
break;
case FUNC_SLL:
pContext->RS[CurrRSEntry].Result = pContext->RS[CurrRSEntry].Vj << pContext->RS[CurrRSEntry].Vk;
break;
case FUNC_ADD:
case FUNC_ADDU:
pContext->RS[CurrRSEntry].Result = pContext->RS[CurrRSEntry].Vj + pContext->RS[CurrRSEntry].Vk;
break;
case FUNC_SUB:
case FUNC_SUBU:
pContext->RS[CurrRSEntry].Result = pContext->RS[CurrRSEntry].Vj - pContext->RS[CurrRSEntry].Vk;
break;
case FUNC_AND:
pContext->RS[CurrRSEntry].Result = pContext->RS[CurrRSEntry].Vj & pContext->RS[CurrRSEntry].Vk;
break;
case FUNC_OR:
pContext->RS[CurrRSEntry].Result = pContext->RS[CurrRSEntry].Vj | pContext->RS[CurrRSEntry].Vk;
break;
case FUNC_XOR:
pContext->RS[CurrRSEntry].Result = pContext->RS[CurrRSEntry].Vj ^ pContext->RS[CurrRSEntry].Vk;
break;
case FUNC_NOR:
pContext->RS[CurrRSEntry].Result = ~(pContext->RS[CurrRSEntry].Vj | pContext->RS[CurrRSEntry].Vk);
break;
case FUNC_SLT:
case FUNC_SLTU:
pContext->RS[CurrRSEntry].Result = (pContext->RS[CurrRSEntry].Vj < pContext->RS[CurrRSEntry].Vk) ? 1 : 0;
break;
default:
// should not reach here!
break;
}
break;
case OPCODE_REGIMM:
switch (pInst->InstructionDWord.IType.Rt)
{
case RT_BLTZ:
if (pContext->RS[CurrRSEntry].Vj < 0)
{
// Branch Taken
pInst->BranchOutcome = BRANCH_TAKEN;
}
else
{
// Branch Not Taken
pInst->BranchOutcome = BRANCH_NOT_TAKEN;
}
// Mark the instruction to be ready
pContext->ROB[pContext->RS[CurrRSEntry].Dest].Ready = true;
break;
case RT_BGEZ:
if (pContext->RS[CurrRSEntry].Vj >= 0)
{
// Branch Taken
pInst->BranchOutcome = BRANCH_TAKEN;
}
else
{
// Branch Not Taken
pInst->BranchOutcome = BRANCH_NOT_TAKEN;
}
// Mark the instruction to be ready
pContext->ROB[pContext->RS[CurrRSEntry].Dest].Ready = true;
break;
default:
// shoudld not reach here!
break;
}
case OPCODE_J:
// Branch Taken
pInst->BranchOutcome = BRANCH_TAKEN;
// Mark the instruction to be ready
pContext->ROB[pContext->RS[CurrRSEntry].Dest].Ready = true;
break;
case OPCODE_BEQ:
if (pContext->RS[CurrRSEntry].Vj == pContext->RS[CurrRSEntry].Vk)
{
// Branch Taken
pInst->BranchOutcome = BRANCH_TAKEN;
}
else
{
// Branch Not Taken
pInst->BranchOutcome = BRANCH_NOT_TAKEN;
}
// Mark the instruction to be ready
pContext->ROB[pContext->RS[CurrRSEntry].Dest].Ready = true;
break;
case OPCODE_BNE:
if (pContext->RS[CurrRSEntry].Vj != pContext->RS[CurrRSEntry].Vk)
{
// Branch Taken
pInst->BranchOutcome = BRANCH_TAKEN;
}
else
{
// Branch Not Taken
pInst->BranchOutcome = BRANCH_NOT_TAKEN;
}
// Mark the instruction to be ready
pContext->ROB[pContext->RS[CurrRSEntry].Dest].Ready = true;
break;
case OPCODE_BLEZ:
if (pContext->RS[CurrRSEntry].Vj <= 0)
{
// Branch Taken
pInst->BranchOutcome = BRANCH_TAKEN;
}
else
{
// Branch Not Taken
pInst->BranchOutcome = BRANCH_NOT_TAKEN;
}
// Mark the instruction to be ready
pContext->ROB[pContext->RS[CurrRSEntry].Dest].Ready = true;
break;
case OPCODE_BGTZ:
if (pContext->RS[CurrRSEntry].Vj > 0)
{
// Branch Taken
pInst->BranchOutcome = BRANCH_TAKEN;
}
else
{
// Branch Not Taken
pInst->BranchOutcome = BRANCH_NOT_TAKEN;
}
// Mark the instruction to be ready
pContext->ROB[pContext->RS[CurrRSEntry].Dest].Ready = true;
break;
case OPCODE_ADDI:
case OPCODE_ADDIU:
pContext->RS[CurrRSEntry].Result = pContext->RS[CurrRSEntry].Vj + pContext->RS[CurrRSEntry].Vk;
break;
case OPCODE_SLTI:
pContext->RS[CurrRSEntry].Result = (pContext->RS[CurrRSEntry].Vj < pContext->RS[CurrRSEntry].Vk) ? 1 : 0;
break;
case OPCODE_LW:
// For Address Generation, all Load/Stores ahead of this store should have their address generated!
// Go through current ROB entries and make this check
for (count = 0; count < MAX_ROB_ENTRIES; count++)
{
if (pContext->ROB[count].Busy &&
pContext->ROB[count].pInst != pInst &&
pContext->ROB[count].pInst->IsLoadOrStoreInst &&
pContext->ROB[count].pInst->InstIssueNum < pInst->InstIssueNum &&
!pContext->ROB[count].pInst->IsLoadOrStoreAddrComputed)
{
ComputeAddress = false;
break;
}
}
if (ComputeAddress)
{
// Proceed with Address Computation for Load
pContext->RS[CurrRSEntry].Addr = pContext->RS[CurrRSEntry].Vj + pContext->RS[CurrRSEntry].Addr;
pInst->IsLoadOrStoreAddrComputed = true;
}
break;
case OPCODE_SW:
// For Address Generation, all Load/Stores ahead of this store should have their address generated!
// Go through current ROB entries and make this check
for (count = 0; count < MAX_ROB_ENTRIES; count++)
{
if (pContext->ROB[count].Busy &&
pContext->ROB[count].pInst->InstAddress != pInst->InstAddress &&
pContext->ROB[count].pInst->IsLoadOrStoreInst &&
pContext->ROB[count].pInst->InstIssueNum < pInst->InstIssueNum &&
!pContext->ROB[count].pInst->IsLoadOrStoreAddrComputed)
{
ComputeAddress = false;
break;
}
}
if (ComputeAddress)
{
// Proceed with Address Computation for Store
pContext->RS[CurrRSEntry].Addr = pContext->RS[CurrRSEntry].Vj + pContext->RS[CurrRSEntry].Addr;
pInst->IsLoadOrStoreAddrComputed = true;
}
// Update the value in ROB
pContext->ROB[pContext->RS[CurrRSEntry].Dest].Value = pContext->RS[CurrRSEntry].Vk;
// Mark the instruction to be ready
pContext->ROB[pContext->RS[CurrRSEntry].Dest].Ready = true;
// Write the result of the Address to the ROB
pContext->ROB[pContext->RS[CurrRSEntry].Dest].Addr = pContext->RS[CurrRSEntry].Addr;
break;
default:
// should not reach here!
break;
}
// Update PC here itself
if (pInst->IsBranchInst || pInst->IsJumpInst)
{
if (pInst->BranchOutcome == BRANCH_NOT_TAKEN && pInst->BranchPred == BRANCH_TAKEN)
{
// Update PC to PC + 4
pContext->CurrentPC = pInst->InstAddress + 4;
// Cancel all Instructions fetched after Branch in ROB
for (ROBCount = 0; ROBCount < MAX_ROB_ENTRIES; ROBCount++)
{
if (pContext->ROB[ROBCount].pInst->InstIssueNum > pInst->InstIssueNum)
{
// Free up ROB and RS
pContext->ROB[ROBCount].UpdateBusyStatus = true;
for (RSCount = 0; RSCount < MAX_RS_ENTRIES; RSCount++)
{
if (pContext->RS[RSCount].Dest == ROBCount)
{
pContext->RS[RSCount].UpdateBusyStatus = true;
break;
}
}
}
}
// Also remove the last instruction from Instruction Queue
pContext->RemoveInstFromQueue = true;
}
if (pInst->BranchOutcome == BRANCH_TAKEN && (pInst->BranchPred == BRANCH_NOT_TAKEN || pInst->BranchPred == NOT_SET))
{
// Update PC to new Target
pContext->CurrentPC = checkInstructionInBTB(pContext, pInst->InstAddress)->TargetAddress;
// Cancel all Instructions fetched after Branch in ROB
for (ROBCount = 0; ROBCount < MAX_ROB_ENTRIES; ROBCount++)
{
if (pContext->ROB[ROBCount].pInst->InstIssueNum > pInst->InstIssueNum)
{
// Free up ROB and RS
pContext->ROB[ROBCount].UpdateBusyStatus = true;
for (RSCount = 0; RSCount < MAX_RS_ENTRIES; RSCount++)
{
if (pContext->RS[RSCount].Dest == ROBCount)
{
pContext->RS[RSCount].UpdateBusyStatus = true;
break;
}
}
}
}
// Also remove the last instruction from Instruction Queue
pContext->RemoveInstFromQueue = true;
}
}
if (pInst->IsBranchInst || pInst->IsJumpInst)
{
pInst->UpdatePred = true;
}
// Move to the next stage in next cycle
pContext->RS[CurrRSEntry].pInst->UpdatePipelineStage = true;
}
}
}
