// この単純な物理ページアロケータは、JOS が仮想メモリをセットアップしているときにのみ使われる。
 // page_alloc() が真のアロケータである。
 // n>0 ならば、'n' バイトを保持するのに必要な連続した物理メモリを割り当てる。
 // メモリの初期化はしない。カーネルの仮想アドレスを返す。
 //
 // n==0 ならば、アロケーションをせずに、次のフリーページのアドレスを返す。
 //
 // メモリ不足の場合は、boot_alloc は panic する。この関数は、page_free_list がセットアップ
 // される以前の初期化時のみ使われる。
 static void *
 boot_alloc(uint32_t n)
 {
    static char *nextfree;  // virtual address of next byte of free memory
    char *result;

    // Initialize nextfree if this is the first time.
    // 'end' is a magic symbol automatically generated by the linker,
    // which points to the end of the kernel's bss segment:
    // the first virtual address that the linker did *not* assign
    // to any kernel code or global variables.
    if (!nextfree) {
        extern char end[];
        nextfree = ROUNDUP((char *) end, PGSIZE);
    }

    // Allocate a chunk large enough to hold 'n' bytes, then update
    // nextfree.  Make sure nextfree is kept aligned
    // to a multiple of PGSIZE.
 -  //
 -  // LAB 2: Your code here.
 +  result = nextfree;
 +  nextfree += ROUNDUP(n, PGSIZE);
 +  if (nextfree < result) {
 +      panic("Memory limit exceeded.");
 +  }

 -  return NULL;
 +  return result;
  }

 // Set up a two-level page table:
 //    kern_pgdir is its linear (virtual) address of the root
 //
 // This function only sets up the kernel part of the address space
 // (ie. addresses >= UTOP).  The user part of the address space
 // will be setup later.
 //
 // From UTOP to ULIM, the user is allowed to read but not write.
 // Above ULIM the user cannot read or write.
 void
 mem_init(void)
 {
    uint32_t cr0;
    size_t n;

    // Find out how much memory the machine has (npages & npages_basemem).
    i386_detect_memory();

 -  // Remove this line when you're ready to test this function.
 -  panic("mem_init: This function is not finished\n");
 -
    //////////////////////////////////////////////////////////////////////
    // create initial page directory.
    kern_pgdir = (pde_t *) boot_alloc(PGSIZE);
    memset(kern_pgdir, 0, PGSIZE);

    //////////////////////////////////////////////////////////////////////
    // Recursively insert PD in itself as a page table, to form
    // a virtual page table at virtual address UVPT.
    // (For now, you don't have understand the greater purpose of the
    // following line.)

    // Permissions: kernel R, user R
    kern_pgdir[PDX(UVPT)] = PADDR(kern_pgdir) | PTE_U | PTE_P;

    //////////////////////////////////////////////////////////////////////
    // Allocate an array of npages 'struct PageInfo's and store it in 'pages'.
    // The kernel uses this array to keep track of physical pages: for
    // each physical page, there is a corresponding struct PageInfo in this
    // array.  'npages' is the number of physical pages in memory.  Use memset
    // to initialize all fields of each struct PageInfo to 0.
 -  // Your code goes here:
 -
 +  pages = (struct PageInfo*) boot_alloc((uint32_t) npages * sizeof(struct PageInfo));
 +  memset(pages, 0, (uint32_t) npages * sizeof(struct PageInfo));

    //////////////////////////////////////////////////////////////////////
    // Now that we've allocated the initial kernel data structures, we set
    // up the list of free physical pages. Once we've done so, all further
    // memory management will go through the page_* functions. In
    // particular, we can now map memory using boot_map_region
    // or page_insert
    page_init();