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#define CHECK_FLAG(flags,bit)   ((flags) & (1 << (bit)))
#define MEMORY 4096000 /* Stack(bytes) (4mb) */

extern void write_cr3(unsigned long *page_directory);
extern void write_cr0(unsigned long page_directory);
extern int read_cr0();
extern int read_cr3();

static char allocbuffer[MEMORY]; /* Array to store all memory locations */
static char *allocp = &allocbuffer[0]; /* Pointer to next free location */

unsigned long *page_directory = (unsigned long *) 0x110000;
unsigned long *page_table = (unsigned long *) 0x111000;
unsigned long address = 0;

/* Pages used... this is used for page_directory array */
unsigned long page_number = 1;

typedef struct
{
       int access;
       unsigned long start;
       int used;
}page_t;

void enable_paging(int memory)
{
  int __RAM__= memory/4096; /* convert to amount of pages */
  __RAM__ -= 2; /* 8kb stack for OS */

  unsigned long *pagedir = (unsigned long *) 0x400000;
  unsigned long *pagetable = (unsigned long *) 0x401000;

  unsigned long addr = 0, i;

  for (i = 0; i < 1024; i++)
    {
      *(pagetable + i) = addr | 3;	/* Supervisor, R/W, Present */
      addr += 4096; /* 4kB / page */
    }

    /* For the first entry */
  *(pagedir) = (unsigned long int) pagetable;
  *(pagedir) |= 3; /* Set attributes */

  /* Set remaining pages to absent */
  for (i = 1; i < 1024; i++)
    *(pagedir + i) = 0 | 2;	/* Not present */

  write_cr3 ((unsigned long int) pagedir);
  write_cr0 (read_cr0 () | 0x80000000);	/* Set paging bit */
}

/* This function sets the amount of needed pages to present and updates the
   page_number global variable. It returns the pointer to the memory location */
unsigned long request_pages(int size)
{
    unsigned long ret = 0;
    
    if(size > 4096) /* More than one page */
    {
            int runs = size/4096;
            ret = page_directory[page_number+1];
            for(int i=0;i<(page_number+1);i++)
                    page_table[i+1] = address | 3;
            page_number += runs;        
    } else { /* only one */
           ret = page_directory[page_number+1];
           page_table[page_number+1] = address | 3; /* Set pages to present */
           page_number++;
    }
    
    return ret; /* Pointer to start of free space */
}

/* Resets unused pages to not present */
unsigned long free_pages(int size)
{
         int runs = size/4096;
         page_number -= runs;
         for(int i = runs;i>0;i--)
             page_table[i-1] = address | 2; /* Reset to not present */ 
         
         return 0;
}

/* Allocates n amount of memory using pages */
char* malloc(unsigned long n) /* Return pointer to n chars */
{
      unsigned long page = request_pages(n);
      if (page - MEMORY >= n) /* Space available */
      {
          allocp += n;
          return allocp - n; /* Return pointer to start of memory location */
      } else {
          return 0; /* No space */
      }
}

/* Frees memory pointer to by p */
void* free(char *p)
{
      int tmp = strlen(p);
      free_pages(tmp); /* Free pages */
      
      allocp = p; /* Reset pointer */
      return 0;
}