text: #include "param.h"
  #include "types.h"
  #include "defs.h"
  #include "x86.h"
  #include "memlayout.h"
  #include "mmu.h"
  #include "proc.h"
  #include "elf.h"
  
  extern char data[];  // defined by kernel.ld
  pde_t *kpgdir;  // for use in scheduler()
  
  // Set up CPU's kernel segment descriptors.
  // Run once on entry on each CPU.
  void
  seginit(void)
  {
    struct cpu *c;
  
    // Map "logical" addresses to virtual addresses using identity map.
    // Cannot share a CODE descriptor for both kernel and user
    // because it would have to have DPL_USR, but the CPU forbids
    // an interrupt from CPL=0 to DPL=3.
    c = &cpus[cpuid()];
    c->gdt[SEG_KCODE] = SEG(STA_X|STA_R, 0, 0xffffffff, 0);
    c->gdt[SEG_KDATA] = SEG(STA_W, 0, 0xffffffff, 0);
    c->gdt[SEG_UCODE] = SEG(STA_X|STA_R, 0, 0xffffffff, DPL_USER);
    c->gdt[SEG_UDATA] = SEG(STA_W, 0, 0xffffffff, DPL_USER);
    lgdt(c->gdt, sizeof(c->gdt));
  }
  
  // Return the address of the PTE in page table pgdir
  // that corresponds to virtual address va.  If alloc!=0,
  // create any required page table pages.
  static pte_t *
  walkpgdir(pde_t *pgdir, const void *va, int alloc)
  {
    pde_t *pde;
    pte_t *pgtab;
  
    pde = &pgdir[PDX(va)];
    if(*pde & PTE_P){
      pgtab = (pte_t*)P2V(PTE_ADDR(*pde));
    } else {
      if(!alloc || (pgtab = (pte_t*)kalloc()) == 0)
        return 0;
      // Make sure all those PTE_P bits are zero.
      memset(pgtab, 0, PGSIZE);
      // The permissions here are overly generous, but they can
      // be further restricted by the permissions in the page table
      // entries, if necessary.
      *pde = V2P(pgtab) | PTE_P | PTE_W | PTE_U;
    }
    return &pgtab[PTX(va)];
  }
  
  // Create PTEs for virtual addresses starting at va that refer to
  // physical addresses starting at pa. va and size might not
  // be page-aligned.
  int
  mappages(pde_t *pgdir, void *va, uint size, uint pa, int perm)
  {
    char *a, *last;
    pte_t *pte;
   
  //  cprintf("SIZE: %x\n", size);
    a = (char*)PGROUNDDOWN((uint)va);
    last = (char*)PGROUNDDOWN(((uint)va) + size - 1);
    for(;;){
      if((pte = walkpgdir(pgdir, a, 1)) == 0)
        return -1;
      if(*pte & PTE_P)
        panic("remap");
      *pte = pa | perm | PTE_P;
      if(a == last)
        break;
      a += PGSIZE;
      pa += PGSIZE;
    }
    return 0;
  }
  
  // There is one page table per process, plus one that's used when
  // a CPU is not running any process (kpgdir). The kernel uses the
  // current process's page table during system calls and interrupts;
  // page protection bits prevent user code from using the kernel's
  // mappings.
  //
  // setupkvm() and exec() set up every page table like this:
  //
  //   0..KERNBASE: user memory (text+data+stack+heap), mapped to
  //                phys memory allocated by the kernel
  //   KERNBASE..KERNBASE+EXTMEM: mapped to 0..EXTMEM (for I/O space)
  //   KERNBASE+EXTMEM..data: mapped to EXTMEM..V2P(data)
  //                for the kernel's instructions and r/o data
  //   data..KERNBASE+PHYSTOP: mapped to V2P(data)..PHYSTOP,
  //                                  rw data + free physical memory
  //   0xfe000000..0: mapped direct (devices such as ioapic)
  //
  // The kernel allocates physical memory for its heap and for user memory
  // between V2P(end) and the end of physical memory (PHYSTOP)
  // (directly addressable from end..P2V(PHYSTOP)).
  
  // This table defines the kernel's mappings, which are present in
  // every process's page table.
  static struct kmap {
    void *virt;
    uint phys_start;
    uint phys_end;
    int perm;
  } kmap[] = {
   { (void*)KERNBASE, 0,             EXTMEM,    PTE_W}, // I/O space
   { (void*)KERNLINK, V2P(KERNLINK), V2P(data), 0},     // kern text+rodata
   { (void*)data,     V2P(data),     PHYSTOP,   PTE_W}, // kern data+memory
   { (void*)DEVSPACE, DEVSPACE,      0,         PTE_W}, // more devices
  };
  
  // Set up kernel part of a page table.
  pde_t*
  setupkvm(void)
  {
    pde_t *pgdir;
    struct kmap *k;
  
    if((pgdir = (pde_t*)kalloc()) == 0)
      return 0;
    memset(pgdir, 0, PGSIZE);
    if (P2V(PHYSTOP) > (void*)DEVSPACE)
      panic("PHYSTOP too high");
    for(k = kmap; k < &kmap[NELEM(kmap)]; k++)
      if(mappages(pgdir, k->virt, k->phys_end - k->phys_start,
                  (uint)k->phys_start, k->perm) < 0) {
        freevm(pgdir);
        return 0;
      }
    return pgdir;
  }
  
  // Allocate one page table for the machine for the kernel address
  // space for scheduler processes.
  void
  kvmalloc(void)
  {
    kpgdir = setupkvm();
    switchkvm();
  }
  
  // Switch h/w page table register to the kernel-only page table,
  // for when no process is running.
  void
  switchkvm(void)
  {
    lcr3(V2P(kpgdir));   // switch to the kernel page table
  }
  
  // Switch TSS and h/w page table to correspond to process p.
  void
  switchuvm(struct proc *p)
  {
    if(p == 0)
      panic("switchuvm: no process");
    if(p->kstack == 0)
      panic("switchuvm: no kstack");
    if(p->pgdir == 0)
      panic("switchuvm: no pgdir");
  
    pushcli();
    mycpu()->gdt[SEG_TSS] = SEG16(STS_T32A, &mycpu()->ts,
                                  sizeof(mycpu()->ts)-1, 0);
    mycpu()->gdt[SEG_TSS].s = 0;
    mycpu()->ts.ss0 = SEG_KDATA << 3;
    mycpu()->ts.esp0 = (uint)p->kstack + KSTACKSIZE;
    // setting IOPL=0 in eflags *and* iomb beyond the tss segment limit
    // forbids I/O instructions (e.g., inb and outb) from user space
    mycpu()->ts.iomb = (ushort) 0xFFFF;
    ltr(SEG_TSS << 3);
    lcr3(V2P(p->pgdir));  // switch to process's address space
    popcli();
  }
  
  // Load the initcode into address 0 of pgdir.
  // sz must be less than a page.
  void
  inituvm(pde_t *pgdir, char *init, uint sz)
  {
    char *mem;
  
    if(sz >= PGSIZE)
      panic("inituvm: more than a page");
    mem = kalloc();
    memset(mem, 0, PGSIZE);
    mappages(pgdir, 0, PGSIZE, V2P(mem), PTE_W|PTE_U);
    memmove(mem, init, sz);
  }
  
  // Load a program segment into pgdir.  addr must be page-aligned
  // and the pages from addr to addr+sz must already be mapped.
  int
  loaduvm(pde_t *pgdir, char *addr, struct inode *ip, uint offset, uint sz)
  {
    uint i, pa, n;
    pte_t *pte;
  
    if((uint) addr % PGSIZE != 0)
      panic("loaduvm: addr must be page aligned");
    for(i = 0; i < sz; i += PGSIZE){
      if((pte = walkpgdir(pgdir, addr+i, 0)) == 0)
        panic("loaduvm: address should exist");
      pa = PTE_ADDR(*pte);
      if(sz - i < PGSIZE)
        n = sz - i;
      else
        n = PGSIZE;
      if(readi(ip, P2V(pa), offset+i, n) != n)
        return -1;
    }
    return 0;
  }
  
  // Allocate page tables and physical memory to grow process from oldsz to
  // newsz, which need not be page aligned.  Returns new size or 0 on error.
  int
  allocuvm(pde_t *pgdir, uint oldsz, uint newsz)
  {
    char *mem;
    uint a;
  
    if(newsz >= KERNBASE)
      return 0;
    if(newsz < oldsz)
      return oldsz;
  
    a = PGROUNDUP(oldsz);
  //  a = oldsz;
  //  cprintf("ALLOC TOP: %x\n", newsz);
  //  cprintf("ALLOC BOTTOM: %x\n", a);
    for(; a < newsz; a += PGSIZE){
      mem = kalloc();
      if(mem == 0){
  //	cprintf("SP: %x\n", myproc()->tf->esp);
        cprintf("allocuvm out of memory\n");
        deallocuvm(pgdir, newsz, oldsz);
        return 0;
      }
  //   cprintf("MEM: %x\n", mem);
      memset(mem, 0, PGSIZE);
      if(mappages(pgdir, (char*)a, PGSIZE, V2P(mem), PTE_W|PTE_U) < 0){
        cprintf("allocuvm out of memory (2)\n");
        deallocuvm(pgdir, newsz, oldsz);
        kfree(mem);
        return 0;
      }
    }
  //  cprintf("TOPPAGE: %x\n", newsz);
    return newsz;
  }
  
  // Deallocate user pages to bring the process size from oldsz to
  // newsz.  oldsz and newsz need not be page-aligned, nor does newsz
  // need to be less than oldsz.  oldsz can be larger than the actual
  // process size.  Returns the new process size.
  int
  deallocuvm(pde_t *pgdir, uint oldsz, uint newsz)
  {
    pte_t *pte;
    uint a, pa;
  
    if(newsz >= oldsz)
      return oldsz;
  
    a = PGROUNDUP(newsz);
    for(; a  < oldsz; a += PGSIZE){
      pte = walkpgdir(pgdir, (char*)a, 0);
      if(!pte)
        a = PGADDR(PDX(a) + 1, 0, 0) - PGSIZE;
      else if((*pte & PTE_P) != 0){
        pa = PTE_ADDR(*pte);
        if(pa == 0)
          panic("kfree");
        char *v = P2V(pa);
        kfree(v);
        *pte = 0;
      }
    }
    return newsz;
  }
  
  // Free a page table and all the physical memory pages
  // in the user part.
  void
  freevm(pde_t *pgdir)
  {
    uint i;
  
    if(pgdir == 0)
      panic("freevm: no pgdir");
    deallocuvm(pgdir, KERNBASE, 0);
    for(i = 0; i < NPDENTRIES; i++){
      if(pgdir[i] & PTE_P){
        char * v = P2V(PTE_ADDR(pgdir[i]));
        kfree(v);
      }
    }
    kfree((char*)pgdir);
  }
  
  // Clear PTE_U on a page. Used to create an inaccessible
  // page beneath the user stack.
  void
  clearpteu(pde_t *pgdir, char *uva)
  {
    pte_t *pte;
  
    pte = walkpgdir(pgdir, uva, 0);
    if(pte == 0)
      panic("clearpteu");
    *pte &= ~PTE_U;
  }
  
  // Given a parent process's page table, create a copy
  // of it for a child.
  pde_t*
  copyuvm(pde_t *pgdir, uint sz, uint lp, uint pn)
  {
    pde_t *d;
    pte_t *pte;
    uint pa, i, flags;
    char *mem;
  
    if((d = setupkvm()) == 0)
      return 0;
    for(i = 0; i < sz; i += PGSIZE){
      if((pte = walkpgdir(pgdir, (void *) i, 0)) == 0)
        panic("copyuvm: pte should exist");
      if(!(*pte & PTE_P))
        panic("copyuvm: page not present");
      pa = PTE_ADDR(*pte);
      flags = PTE_FLAGS(*pte);
      if((mem = kalloc()) == 0)
        goto bad;
      memmove(mem, (char*)P2V(pa), PGSIZE);
      if(mappages(d, (void*)i, PGSIZE, V2P(mem), flags) < 0)
        goto bad;
    }
  
    //Added second loop so that the stack is correctly copied over to the new process CS153
  // cprintf("COPUVM SP2 : %x\n", lp-pn*PGSIZE);
   for(i = PGROUNDDOWN(lp-1); i < KERNBASE; i += PGSIZE){
      if((pte = walkpgdir(pgdir, (void *) i, 0)) == 0)
        panic("copyuvm: pte should exist");
      if(!(*pte & PTE_P))
        panic("copyuvm: page not present");
      pa = PTE_ADDR(*pte);
      flags = PTE_FLAGS(*pte);
      if((mem = kalloc()) == 0)
        goto bad;
      memmove(mem, (char*)P2V(pa), PGSIZE);
      if(mappages(d, (void*)i, PGSIZE, V2P(mem), flags) < 0)
        goto bad;
    }
  
    return d;
  
  bad:
    freevm(d);
    return 0;
  }
  
  //PAGEBREAK!
  // Map user virtual address to kernel address.
  char*
  uva2ka(pde_t *pgdir, char *uva)
  {
    pte_t *pte;
  
    pte = walkpgdir(pgdir, uva, 0);
    if((*pte & PTE_P) == 0)
      return 0;
    if((*pte & PTE_U) == 0)
      return 0;
    return (char*)P2V(PTE_ADDR(*pte));
  }
  
  // Copy len bytes from p to user address va in page table pgdir.
  // Most useful when pgdir is not the current page table.
  // uva2ka ensures this only works for PTE_U pages.
  int
  copyout(pde_t *pgdir, uint va, void *p, uint len)
  {
    char *buf, *pa0;
    uint n, va0;
  
    buf = (char*)p;
    while(len > 0){
      va0 = (uint)PGROUNDDOWN(va);
      pa0 = uva2ka(pgdir, (char*)va0);
      if(pa0 == 0)
        return -1;
      n = PGSIZE - (va - va0);
      if(n > len)
        n = len;
      memmove(pa0 + (va - va0), buf, n);
      len -= n;
      buf += n;
      va = va0 + PGSIZE;
    }
    return 0;
  }
  
  //PAGEBREAK!
  // Blank page.
  //PAGEBREAK!
  // Blank page.
  //PAGEBREAK!
  // Blank page.
  
  