From c9ea0b73492bf116c836c54199898f29d9664bc0 Mon Sep 17 00:00:00 2001
From: Gabe Black <gblack@eecs.umich.edu>
Date: Sun, 12 Oct 2008 16:59:55 -0700
Subject: CPU: Make the highest order bit in the micro pc determine if it's
 combinational or from the ROM.

---
 src/cpu/simple/base.cc | 29 ++++++++++++++++-------------
 src/cpu/static_inst.hh | 20 ++++++++++++++++++++
 2 files changed, 36 insertions(+), 13 deletions(-)

diff --git a/src/cpu/simple/base.cc b/src/cpu/simple/base.cc
index 48e5db347..d207bda14 100644
--- a/src/cpu/simple/base.cc
+++ b/src/cpu/simple/base.cc
@@ -368,9 +368,13 @@ BaseSimpleCPU::preExecute()
     // decode the instruction
     inst = gtoh(inst);
 
-    //If we're not in the middle of a macro instruction
-    if (!curMacroStaticInst) {
+    MicroPC upc = thread->readMicroPC();
 
+    if (isRomMicroPC(upc)) {
+        stayAtPC = false;
+        curStaticInst = microcodeRom.fetchMicroop(upc, curMacroStaticInst);
+    } else if (!curMacroStaticInst) {
+        //We're not in the middle of a macro instruction
         StaticInstPtr instPtr = NULL;
 
         //Predecode, ie bundle up an ExtMachInst
@@ -401,15 +405,13 @@ BaseSimpleCPU::preExecute()
         //out micro ops
         if (instPtr && instPtr->isMacroop()) {
             curMacroStaticInst = instPtr;
-            curStaticInst = curMacroStaticInst->
-                fetchMicroop(thread->readMicroPC());
+            curStaticInst = curMacroStaticInst->fetchMicroop(upc);
         } else {
             curStaticInst = instPtr;
         }
     } else {
         //Read the next micro op from the macro op
-        curStaticInst = curMacroStaticInst->
-            fetchMicroop(thread->readMicroPC());
+        curStaticInst = curMacroStaticInst->fetchMicroop(upc);
     }
 
     //If we decoded an instruction this "tick", record information about it.
@@ -469,22 +471,23 @@ BaseSimpleCPU::advancePC(Fault fault)
     if (fault != NoFault) {
         curMacroStaticInst = StaticInst::nullStaticInstPtr;
         predecoder.reset();
-        thread->setMicroPC(0);
-        thread->setNextMicroPC(1);
+        thread->setMicroPC(normalMicroPC(0));
+        thread->setNextMicroPC(normalMicroPC(1));
         fault->invoke(tc);
     } else {
         //If we're at the last micro op for this instruction
         if (curStaticInst && curStaticInst->isLastMicroop()) {
-            //We should be working with a macro op
-            assert(curMacroStaticInst);
+            //We should be working with a macro op or be in the ROM
+            assert(curMacroStaticInst ||
+                    isRomMicroPC(thread->readMicroPC()));
             //Close out this macro op, and clean up the
             //microcode state
             curMacroStaticInst = StaticInst::nullStaticInstPtr;
-            thread->setMicroPC(0);
-            thread->setNextMicroPC(1);
+            thread->setMicroPC(normalMicroPC(0));
+            thread->setNextMicroPC(normalMicroPC(1));
         }
         //If we're still in a macro op
-        if (curMacroStaticInst) {
+        if (curMacroStaticInst || isRomMicroPC(thread->readMicroPC())) {
             //Advance the micro pc
             thread->setMicroPC(thread->readNextMicroPC());
             //Advance the "next" micro pc. Note that there are no delay
diff --git a/src/cpu/static_inst.hh b/src/cpu/static_inst.hh
index 8a1b3e749..c9eb6ff24 100644
--- a/src/cpu/static_inst.hh
+++ b/src/cpu/static_inst.hh
@@ -74,6 +74,26 @@ namespace Trace {
 
 typedef uint32_t MicroPC;
 
+static const MicroPC MicroPCRomBit = 1 << (sizeof(MicroPC) * 8 - 1);
+
+static inline MicroPC
+romMicroPC(MicroPC upc)
+{
+    return upc | MicroPCRomBit;
+}
+
+static inline MicroPC
+normalMicroPC(MicroPC upc)
+{
+    return upc & ~MicroPCRomBit;
+}
+
+static inline bool
+isRomMicroPC(MicroPC upc)
+{
+    return MicroPCRomBit & upc;
+}
+
 /**
  * Base, ISA-independent static instruction class.
  *
-- 
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