Fix comment.

[pintos-anon] / src / threads / mmu.h

diff --git a/src/threads/mmu.h b/src/threads/mmu.h
index 2af85bc335848d445c7b283c01983374b10e7c15..a1f05ae7b489a5f61f0f6f2e5f36b9bd320f5afa 100644 (file)
--- a/src/threads/mmu.h
+++ b/src/threads/mmu.h
@@ -9,19 +9,19 @@
 
 /* Virtual to physical translation works like this on an x86:
 
-   - The top 10 bits of the virtual address (bits 22:31) are used
+   - The top 10 bits of the virtual address (bits 22:32) are used
      to index into the page directory.  If the PDE is marked
      "present," the physical address of a page table is read from
      the PDE thus obtained.  If the PDE is marked "not present"
      then a page fault occurs.
 
-   - The next 10 bits of the virtual address (bits 12:21) are
+   - The next 10 bits of the virtual address (bits 12:22) are
      used to index into the page table.  If the PTE is marked
      "present," the physical address of a data page is read from
      the PTE thus obtained.  If the PTE is marked "not present"
      then a page fault occurs.
 
-   - The bottom 12 bits of the virtual address (bits 0:11) are
+   - The bottom 12 bits of the virtual address (bits 0:12) are
      added to the data page's physical base address, producing
      the final physical address.
 
@@ -57,21 +57,22 @@
 
 #define MASK(SHIFT, CNT) (((1ul << (CNT)) - 1) << (SHIFT))
 
-/* Page offset (bits 0:11). */
-#define PGSHIFT         0                  /* First offset bit. */
-#define PGBITS          12                 /* Number of offset bits. */
-#define PGMASK          MASK(PGSHIFT, PGBITS)
-#define PGSIZE          (1 << PGBITS)
+/* Page offset (bits 0:12). */
+#define PGSHIFT 0                       /* Index of first offset bit. */
+#define PGBITS  12                      /* Number of offset bits. */
+#define PGMASK  MASK(PGSHIFT, PGBITS)   /* Page offset bits (0:12). */
+#define PGSIZE  (1 << PGBITS)           /* Bytes in a page. */
 
-/* Page table (bits 12:21). */
-#define        PTSHIFT         PGBITS             /* First page table bit. */
-#define PTBITS          10                 /* Number of page table bits. */
-#define PTMASK          MASK(PTSHIFT, PTBITS)
+/* Page table (bits 12:22). */
+#define        PTSHIFT PGBITS                  /* Index of first page table bit. */
+#define PTBITS  10                      /* Number of page table bits. */
+#define PTMASK  MASK(PTSHIFT, PTBITS)   /* Page table bits (12:22). */
+#define PTSPAN  (1 << PTBITS << PGBITS) /* Bytes covered by a page table. */
 
-/* Page directory (bits 22:31). */
-#define PDSHIFT         (PTSHIFT + PTBITS) /* First page dir bit. */
-#define PDBITS          10                 /* Number of page dir bits. */
-#define PDMASK          MASK(PDSHIFT, PDBITS)
+/* Page directory (bits 22:32). */
+#define PDSHIFT (PTSHIFT + PTBITS)      /* First page dir bit. */
+#define PDBITS  10                      /* Number of page dir bits. */
+#define PDMASK  MASK(PDSHIFT, PDBITS)   /* Page directory bits (22:32). */
 
 /* Offset within a page. */
 static inline unsigned pg_ofs (const void *va) {
@@ -93,6 +94,16 @@ static inline void *pg_round_down (const void *va) {
   return (void *) ((uintptr_t) va & ~PGMASK);
 }
 
+/* Base address of the 1:1 physical-to-virtual mapping.  Physical
+   memory is mapped starting at this virtual address.  Thus,
+   physical address 0 is accessible at PHYS_BASE, physical
+   address address 0x1234 at (uint8_t *) PHYS_BASE + 0x1234, and
+   so on.
+
+   This address also marks the end of user programs' address
+   space.  Up to this point in memory, user programs are allowed
+   to map whatever they like.  At this point and above, the
+   virtual address space belongs to the kernel. */
 #define        PHYS_BASE ((void *) LOADER_PHYS_BASE)
 
 /* Returns kernel virtual address at which physical address PADDR
@@ -180,7 +191,7 @@ static inline uint32_t pte_create_user (uint32_t *page, bool writable) {
 }
 
 /* Returns a pointer to the page that page table entry PTE, which
-   must "present", points to. */
+   must be "present", points to. */
 static inline void *pte_get_page (uint32_t pte) {
   ASSERT (pte & PG_P);
   return ptov (pte & ~PGMASK);