NachOS/code/userprog/exception.cc

// exception.cc 
//      Entry point into the Nachos kernel from user programs.
//      There are two kinds of things that can cause control to
//      transfer back to here from user code:
//
//      syscall -- The user code explicitly requests to call a procedure
//      in the Nachos kernel.  Right now, the only function we support is
//      "Halt".
//
//      exceptions -- The user code does something that the CPU can't handle.
//      For instance, accessing memory that doesn't exist, arithmetic errors,
//      etc.  
//
//      Interrupts (which can also cause control to transfer from user
//      code into the Nachos kernel) are handled elsewhere.
//
// For now, this only handles the Halt() system call.
// Everything else core dumps.
//
// Copyright (c) 1992-1993 The Regents of the University of California.
// All rights reserved.  See copyright.h for copyright notice and limitation 
// of liability and disclaimer of warranty provisions.

#include "copyright.h"
#include "system.h"
#include "syscall.h"

//----------------------------------------------------------------------
// UpdatePC : Increments the Program Counter register in order to resume
// the user program immediately after the "syscall" instruction.
//----------------------------------------------------------------------
static void
UpdatePC ()
{
    int pc = machine->ReadRegister (PCReg);
    machine->WriteRegister (PrevPCReg, pc);
    pc = machine->ReadRegister (NextPCReg);
    machine->WriteRegister (PCReg, pc);
    pc += 4;
    machine->WriteRegister (NextPCReg, pc);
}


//----------------------------------------------------------------------
// ExceptionHandler
//      Entry point into the Nachos kernel.  Called when a user program
//      is executing, and either does a syscall, or generates an addressing
//      or arithmetic exception.
//
//      For system calls, the following is the calling convention:
//
//      system call code -- r2
//              arg1 -- r4
//              arg2 -- r5
//              arg3 -- r6
//              arg4 -- r7
//
//      The result of the system call, if any, must be put back into r2. 
//
// And don't forget to increment the pc before returning. (Or else you'll
// loop making the same system call forever!
//
//      "which" is the kind of exception.  The list of possible exceptions 
//      are in machine.h.
//----------------------------------------------------------------------
int copyStringFromMachine(int from, char *to, unsigned size)
{
	unsigned i = 0;
	int res;
    
    // Need to read size-1 to put final /0 if needed
	while((i<size-1) && (machine->ReadMem(from+i,1,&res))){
		*(to+i) = (char)res;
		if ((char)res == '\0'){
		  return i;
		}
		i++;
	}
	*(to+i)='\0';
	return size;
}

int copyStringToMachine(int to, char* from, unsigned size)
{
    unsigned i;
    for (i=0; i < size - 1; i++) {
      if (from[i] == '\0') {
        break;  
      }
      machine->WriteMem(to+i,1,(int)from[i]);
    }

    // Write the last /0
    machine->WriteMem(to+i,1,'\0');
    return i;
}

void
ExceptionHandler (ExceptionType which)
{
    int type = machine->ReadRegister (2);
    int address = machine->registers[BadVAddrReg];

    switch (which)
      {
	case SyscallException:
          {
	    switch (type)
	      {
		case SC_Halt:
		  {
		    DEBUG ('s', "Shutdown, initiated by user program.\n");
		    interrupt->Powerdown ();
		    break;
		  }
        #ifdef CHANGED
        case SC_Exit:
          {
            int ret = machine->ReadRegister(4);
            printf("Exit code %d\n", ret);
            interrupt->Powerdown();
            break;
          }
        case SC_PutChar:
          {
            DEBUG ('s', "PutChar.\n");
            consoledriver->PutChar((char)machine->ReadRegister(4));
            break;
          }
        case SC_PutString:
          {

            DEBUG ('s', "PutString.\n");
            int readsize;
            int addr_start = machine->ReadRegister(4);
            do {
                char* read = new char[MAX_STRING_SIZE];
                readsize = copyStringFromMachine( addr_start, read, MAX_STRING_SIZE);
                consoledriver->PutString(read);
                //  beware, the last element of our buffer was added by 
                //  copyStringFromMachine(), we need to remove one memory case 
                //  or the first char of our next read will be
                //  forgotten
                addr_start += readsize - 1;
                delete [] read;
            } while(readsize == MAX_STRING_SIZE);
            break;
          }
        case SC_GetChar:
          {
            DEBUG ('s', "GetChar.\n");
            // Get the char input an write it to the register n.2
            int c = consoledriver->GetChar();
            machine->WriteRegister((2), c);
            break;
          }

        case SC_GetString:
          {
            DEBUG ('s', "GetString.\n");
            int writesize = 0;
            int addr = machine->ReadRegister(4);
            int size = machine->ReadRegister(5);
            DEBUG ('s', "size:%d\n", size);
            do {
                char* write = new char[MAX_STRING_SIZE];
                consoledriver->GetString(write, MAX_STRING_SIZE);
                DEBUG('s', "write:%s:\n", write);
                writesize = copyStringToMachine(addr, write, MAX_STRING_SIZE);
                addr += writesize;
                DEBUG('s', "Writed:%d\n", writesize);
                delete [] write;
            } while( writesize == MAX_STRING_SIZE - 1 );
            break;
          }
        #endif
		default:
		  {
		    printf("Unimplemented system call %d\n", type);
		    ASSERT(FALSE);
		  }
	      }

	    // Do not forget to increment the pc before returning!
	    UpdatePC ();
	    break;
	  }

	case PageFaultException:
	  if (!address) {
	    printf("NULL dereference at PC %x!\n", machine->registers[PCReg]);
	    ASSERT (FALSE);
	  } else {
	    printf ("Page Fault at address %x at PC %x\n", address, machine->registers[PCReg]);
	    ASSERT (FALSE);	// For now
	  }
	  break;

	case ReadOnlyException:
	  printf ("Read-Only at address %x at PC %x\n", address, machine->registers[PCReg]);
	  ASSERT (FALSE);	// For now
	  break;

	case BusErrorException:
	  printf ("Invalid physical address at address %x at PC %x\n", address, machine->registers[PCReg]);
	  ASSERT (FALSE);	// For now
	  break;

	case AddressErrorException:
	  printf ("Invalid address %x at PC %x\n", address, machine->registers[PCReg]);
	  ASSERT (FALSE);	// For now
	  break;

	case OverflowException:
	  printf ("Overflow at PC %x\n", machine->registers[PCReg]);
	  ASSERT (FALSE);	// For now
	  break;

	case IllegalInstrException:
	  printf ("Illegal instruction at PC %x\n", machine->registers[PCReg]);
	  ASSERT (FALSE);	// For now
	  break;

	default:
	  printf ("Unexpected user mode exception %d %d %x at PC %x\n", which, type, address, machine->registers[PCReg]);
	  ASSERT (FALSE);
	  break;
      }
}