static private int translate(int virtAddr, int size, boolean writing)
throws MachineException {
int i = 0;
long vpn, offset;
TranslationEntry entry;
long pageFrame;
int physAddr;
if (Debug.isEnabled('a')) {
if (writing)
Debug.printf('a', "\tTranslate 0x%x, %s: ", new Integer(virtAddr),
"write");
else
Debug.printf('a', "\tTranslate 0x%x, %s: ", new Integer(virtAddr),
"read");
}
// check for alignment errors
if (((size == 4) && (virtAddr & 0x3) != 0) ||
((size == 2) && (virtAddr & 0x1) != 0)) {
Debug.println('a', "alignment problem at " + virtAddr + ", size " + size);
throw new MachineException(exceptionNames[AddressErrorException],
AddressErrorException);
}
// we must have either a TLB or a page table, but not both!
Debug.ASSERT(tlb == null || pageTable == null);
Debug.ASSERT(tlb != null || pageTable != null);
// calculate the virtual page number, and offset within the page,
// from the virtual address
vpn = ((long) virtAddr & LOW32BITS) / PageSize;
offset = ((long) virtAddr & LOW32BITS) % PageSize;
if (tlb == null) { // => page table => vpn is index into table
if (vpn >= pageTableSize) {
Debug.println('a', "virtual page # " + virtAddr +
" too large for page table size " + pageTableSize);
throw new MachineException(exceptionNames[AddressErrorException],
AddressErrorException);
} else if (!pageTable[(int)vpn].valid) {
Debug.println('a', "virtual page # " + virtAddr +
" too large for page table size " + pageTableSize);
throw new MachineException(exceptionNames[PageFaultException],
PageFaultException);
}
entry = pageTable[(int)vpn];
} else {
for (entry = null, i = 0; i < TLBSize; i++)
if (tlb[i].valid && (tlb[i].virtualPage == vpn)) {
entry = tlb[i]; // FOUND!
break;
}
if (entry == null) { // not found
Debug.println('a', "** no valid TLB entry found for this virtual page!");
// really, this is a TLB fault,
// the page may be in memory,
// but not in the TLB
throw new MachineException(exceptionNames[PageFaultException],
PageFaultException);
}
}
if (entry.readOnly && writing) { // trying to write to a read-only page
Debug.println('a', virtAddr + " mapped read-only at " + i + " in TLB!");
throw new MachineException(exceptionNames[ReadOnlyException],
ReadOnlyException);
}
pageFrame = entry.physicalPage;
// if the pageFrame is too big, there is something really wrong!
// An invalid translation was loaded into the page table or TLB.
if (pageFrame >= NumPhysPages) {
Debug.println('a', "*** frame " + pageFrame + " > " + NumPhysPages);
throw new MachineException(exceptionNames[BusErrorException],
BusErrorException);
}
entry.use = true; // set the use, dirty bits
if (writing)
entry.dirty = true;
physAddr = (int) (pageFrame * PageSize + offset);
Debug.ASSERT((physAddr >= 0) && ((physAddr + size) <= MemorySize));
if (Debug.isEnabled('a')) {
Debug.printf('a', "phys addr = 0x%x\n", new Integer(physAddr));
}
return physAddr;
}