memlib.c

实现一个简单的单进程内核,。该内核启用二级虚拟页表映射

/* memlib.c - functions for modeling the memory system */
/* $beg
in memlib */

#include <syscall.h>

#ifndef NULL
#define NULL 0
#endif

/* private global variables */
static char *mem_start_brk;  /* points to first byte of the heap */
static char *mem_brk;        /* points to first invalid byte (byte after
				end of heap) */
static char *mem_max_addr;   /* max virtual address for the heap */

/* 
 * mem_init - initializes the memory system model
 */
void mem_init(int max_heap_addr)
{
  /* Find out start of heap. */
  mem_start_brk = (char*)brk((void*)NULL);
  /* Heap starts out being zero-sized. */
  mem_brk = mem_start_brk;
  /* The max address for the heap. */
  mem_max_addr = (void*)max_heap_addr;
}

/* 
 * mem_sbrk - simply uses the the sbrk function. Extends the heap 
 *    by incr bytes and returns the start address of the new area. In
 *    this model, the heap cannot be shrunk.
 */
void *mem_sbrk(int incr)
{
    char *old_brk = mem_brk;

    /* Error check the request. */
    if ( (incr < 0) || ((mem_brk + incr) > mem_max_addr)) {
      return (void *)-1;
    }


    /* Issue a SBRK for more memory. */
    if (!((int)brk((void*)(mem_brk + incr)) - (int)mem_brk)) {
      return (void *)-1;
    }
    mem_brk += incr;
    return (void *)old_brk;
}
/* $end memlib */