machine.h

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// machine.h 
//      Data structures for simulating the execution of user programs
//      running on top of Nachos.
//
//      User programs are loaded into "mainMemory"; to Nachos,
//      this looks just like an array of bytes.  Of course, the Nachos
//      kernel is in memory too -- but as in most machines these days,
//      the kernel is loaded into a separate memory region from user
//      programs, and accesses to kernel memory are not translated or paged.
//
//      In Nachos, user programs are executed one instruction at a time, 
//      by the simulator.  Each memory reference is translated, checked
//      for errors, etc.
//
//  DO NOT CHANGE -- part of the machine emulation
//
// 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.

#ifndef MACHINE_H
#define MACHINE_H

#include "copyright.h"
#include "utility.h"
#include "translate.h"
#include "disk.h"

// Definitions related to the size, and format of user memory

#define PageSize        SectorSize      // set the page size equal to
                                        // the disk sector size, for
                                        // simplicity

#define NumPhysPages    32
#define MemorySize      (NumPhysPages * PageSize)
#define TLBSize         4               // if there is a TLB, make it small

enum ExceptionType { NoException,           // Everything ok!
                     SyscallException,      // A program executed a system call.
                     PageFaultException,    // No valid translation found
                     ReadOnlyException,     // Write attempted to page marked 
                                            // "read-only"
                     BusErrorException,     // Translation resulted in an 
                                            // invalid physical address
                     AddressErrorException, // Unaligned reference or one that
                                            // was beyond the end of the
                                            // address space
                     OverflowException,     // Integer overflow in add or sub.
                     IllegalInstrException, // Unimplemented or reserved instr.
                     
                     NumExceptionTypes
};

// User program CPU state.  The full set of MIPS registers, plus a few
// more because we need to be able to start/stop a user program between
// any two instructions (thus we need to keep track of things like load
// delay slots, etc.)

#define StackReg        29      // User's stack pointer
#define RetAddrReg      31      // Holds return address for procedure calls
#define NumGPRegs       32      // 32 general purpose registers on MIPS
#define HiReg           32      // Double register to hold multiply result
#define LoReg           33
#define PCReg           34      // Current program counter
#define NextPCReg       35      // Next program counter (for branch delay) 
#define PrevPCReg       36      // Previous program counter (for debugging)
#define LoadReg         37      // The register target of a delayed load.
#define LoadValueReg    38      // The value to be loaded by a delayed load.
#define BadVAddrReg     39      // The failing virtual address on an exception

#define NumTotalRegs    40

// The following class defines an instruction, represented in both
//      undecoded binary form
//      decoded to identify
//          operation to do
//          registers to act on
//          any immediate operand value

class Instruction {
  public:
    void Decode();      // decode the binary representation of the instruction

    unsigned int value; // binary representation of the instruction

    char opCode;     // Type of instruction.  This is NOT the same as the
                     // opcode field from the instruction: see defs in mips.h
    char rs, rt, rd; // Three registers from instruction.
    int extra;       // Immediate or target or shamt field or offset.
                     // Immediates are sign-extended.
};

// The following class defines the simulated host workstation hardware, as 
// seen by user programs -- the CPU registers, main memory, etc.
// User programs shouldn't be able to tell that they are running on our 
// simulator or on the real hardware, except 
//      we don't support floating point instructions
//      the system call interface to Nachos is not the same as UNIX 
//        (10 system calls in Nachos vs. 200 in UNIX!)
// If we were to implement more of the UNIX system calls, we ought to be
// able to run Nachos on top of Nachos!
//
// The procedures in this class are defined in machine.cc, mipssim.cc, and
// translate.cc.

class Machine {
  public:
    Machine(bool debug);        // Initialize the simulation of the hardware
                                // for running user programs
    ~Machine();                 // De-allocate the data structures

// Routines callable by the Nachos kernel
    void Run();                 // Run a user program

    int ReadRegister(int num);  // read the contents of a CPU register

    void WriteRegister(int num, int value);
                                // store a value into a CPU register


// Routines internal to the machine simulation -- DO NOT call these 

    void OneInstruction(Instruction *instr);    
                                // Run one instruction of a user program.
    void DelayedLoad(int nextReg, int nextVal);         
                                // Do a pending delayed load (modifying a reg)
    
    bool ReadMem(int addr, int size, int* value);
    bool WriteMem(int addr, int size, int value);
                                // Read or write 1, 2, or 4 bytes of virtual 
                                // memory (at addr).  Return FALSE if a 
                                // correct translation couldn't be found.
    
    ExceptionType Translate(int virtAddr, int* physAddr, int size,bool writing);
                                // Translate an address, and check for 
                                // alignment.  Set the use and dirty bits in 
                                // the translation entry appropriately,
                                // and return an exception code if the 
                                // translation couldn't be completed.

    void RaiseException(ExceptionType which, int badVAddr);
                                // Trap to the Nachos kernel, because of a
                                // system call or other exception.  

    void Debugger();            // invoke the user program debugger
    void DumpState();           // print the user CPU and memory state 


// Data structures -- all of these are accessible to Nachos kernel code.
// "public" for convenience.
//
// Note that *all* communication between the user program and the kernel 
// are in terms of these data structures.

    char *mainMemory;           // physical memory to store user program,
                                // code and data, while executing
    int registers[NumTotalRegs]; // CPU registers, for executing user programs


// NOTE: the hardware translation of virtual addresses in the user program
// to physical addresses (relative to the beginning of "mainMemory")
// can be controlled by one of:
//      a traditional linear page table
//      a software-loaded translation lookaside buffer (tlb) -- a cache of 
//        mappings of virtual page #'s to physical page #'s
//
// If "tlb" is NULL, the linear page table is used
// If "tlb" is non-NULL, the Nachos kernel is responsible for managing
//      the contents of the TLB.  But the kernel can use any data structure
//      it wants (eg, segmented paging) for handling TLB cache misses.
// 
// For simplicity, both the page table pointer and the TLB pointer are
// public.  However, while there can be multiple page tables (one per address
// space, stored in memory), there is only one TLB (implemented in hardware).
// Thus the TLB pointer should be considered as *read-only*, although 
// the contents of the TLB are free to be modified by the kernel software.

    TranslationEntry *tlb;              // this pointer should be considered 
                                        // "read-only" to Nachos kernel code

    TranslationEntry *pageTable;
    unsigned int pageTableSize;

  private:
    bool singleStep;            // drop back into the debugger after each
                                // simulated instruction
    int runUntilTime;           // drop back into the debugger when simulated
                                // time reaches this value
};

extern void ExceptionHandler(ExceptionType which);
                                // Entry point into Nachos for handling
                                // user system calls and exceptions
                                // Defined in exception.cc


// Routines for converting Words and Short Words to and from the
// simulated machine's format of little endian.  If the host machine
// is little endian (DEC and Intel), these end up being NOPs.
//
// What is stored in each format:
//      host byte ordering:
//         kernel data structures
//         user registers
//      simulated machine byte ordering:
//         contents of main memory

unsigned int WordToHost(unsigned int word);
unsigned short ShortToHost(unsigned short shortword);
unsigned int WordToMachine(unsigned int word);
unsigned short ShortToMachine(unsigned short shortword);

#endif // MACHINE_H

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