JamesM's kernel development tutorials (The Heap)

paging.c

void
init_paging(void)
{
	int i;
	uint32_t mem_end_page;
	/* in heap.c */
	extern struct vm_heap *kernel_heap;

	/*
	 * The size of physical memory.
	 * For the moment we assume it is 16MB big.
	 */
	mem_end_page = 0x1000000;

	nframes = mem_end_page / 0x1000;
	frames  = kmalloc0(INDEX_FROM_BIT(nframes));

	/* Let's make a page directory. */
	kernel_directory  = kmalloc0_a(sizeof(struct vm_page_directory));

	/*
	 * Map some pages in the kernel heap area.
	 * Here we call get_page but not alloc_frame. This causes page_table_t's
	 * to be created where necessary. We can't allocate frames yet because
	 * they they need to be identity mapped first below, and yet we can't
	 * increase placement_address between identity mapping and enabling the
	 * heap.
	 */
	i = 0;
	for (i = VM_KERN_HEAP_START; i < VM_KERN_HEAP_START + VM_KERN_HEAP_INITIAL_SIZE; i += 0x1000)
		get_page(i, 1, kernel_directory);

	/*
	 * We need to identity map (phys addr = virt addr) from 0x0 to the end
	 * of used memory, so we can access this transparently, as if paging
	 * wasn't enabled.  NOTE that we use a while loop here deliberately.
	 * inside the loop body we actually change placement_address by calling
	 * kmalloc(). A while loop causes this to be computed on-the-fly rather
	 * than once at the start.
	 */
	i = 0;
	while (i < placement_address + sizeof(struct vm_heap)) {
		/* Kernel code is readable but not writeable from userspace. */
		alloc_frame(get_page(i, 1, kernel_directory), 0, 0);
		i += 0x1000;
	}

	// Now allocate those pages we mapped earlier.
	for (i = VM_KERN_HEAP_START; i < VM_KERN_HEAP_START + VM_KERN_HEAP_INITIAL_SIZE; i += 0x1000)
		alloc_frame(get_page(i, 0, kernel_directory), 0, 0);

	/* Before we enable paging, we must register our page fault handler. */
	register_interrupt_handler(14, page_fault_handler);

	/* Now, enable paging! */
	switch_page_directory(kernel_directory);

	// Initialise the kernel heap.
	kernel_heap = new_heap(VM_KERN_HEAP_START, VM_KERN_HEAP_START + VM_KERN_HEAP_INITIAL_SIZE, 0xCFFFF000, 0, 0);
}