Feat: implement page_get_swap and page_set_swap functions

src/vm/page.c

@@ -1,4 +1,9 @@
 #include "page.h"
+#include "userprog/pagedir.h"
+#include "threads/pte.h"
+
+#define SWAP_FLAG_BIT 9
+#define ADDR_START_BIT 12
 
 /* Updates the 'owner' thread's page table entry for virtual address 'upage'
    to have a present bit of 0 and stores the specified swap slot value in the
@@ -6,6 +11,17 @@
 void
 page_set_swap (struct thread *owner, void *upage, size_t swap_slot)
 {
+   uint32_t *pte = lookup_page (owner->pagedir, upage, false);
+   *pte &= ~PTE_P; //clears the first bit (present bit) to be 0
+
+   //ASSERT (swap_slot < (1 << 20)); //not sure if this is needed
+
+   //shifts the swap slot addr to take up bits 31 to 12
+   //uses bitwise & to make sure it does not affect flags
+   //then applies it to pte
+   *pte |= (1 << SWAP_FLAG_BIT); // sets the 9th bit
+   uint32_t swap_slot_bits = (swap_slot << ADDR_START_BIT) & PTE_ADDR;
+   *pte = (*pte & PTE_FLAGS) | swap_slot_bits;
 
 }
 
@@ -15,6 +31,13 @@ page_set_swap (struct thread *owner, void *upage, size_t swap_slot)
 size_t
 page_get_swap (struct thread *owner, void *upage)
 {
+   uint32_t *pte = lookup_page (owner->pagedir, upage, false);
 
+   //these should always be checked and true before using this func
+   ASSERT ((*pte & PTE_P) == 0);
+   ASSERT ((*pte & (1 << 9)) == 1);
+
+   //masks address bits and returns truncated value
+   return ((*pte & PTE_ADDR) >> 12);
 }