//
// Initialize page structure and memory free list.
// After this is done, NEVER use boot_alloc again. ONLY use the page
// allocator functions below to allocate and deallocate physical
// memory via the page_free_list.
//
void
page_init(void)
{
// LAB 4:
// Change your code to mark the physical page at MPENTRY_PADDR
// as in use
// The example code here marks all physical pages as free.
// However this is not truly the case. What memory is free?
// 1) Mark physical page 0 as in use.
// This way we preserve the real-mode IDT and BIOS structures
// in case we ever need them. (Currently we don't, but...)
// 2) The rest of base memory, [PGSIZE, npages_basemem * PGSIZE)
// is free.
// 3) Then comes the IO hole [IOPHYSMEM, EXTPHYSMEM), which must
// never be allocated.
// 4) Then extended memory [EXTPHYSMEM, ...).
// Some of it is in use, some is free. Where is the kernel
// in physical memory? Which pages are already in use for
// page tables and other data structures?
//
// Change the code to reflect this.
// NB: DO NOT actually touch the physical memory corresponding to
// free pages!
//bluesea
//注意！
//上述注释中说的都是针对物理内存的，因为我们对pages[]中的PageInfo进行初始化，而
//PageInfo是描述指定物理地址的页使用情况。
//所以需要弄清楚的是：在extended memory上的内核及数据结构占据了哪些物理地址,
//而不是虚拟地址。
page_free_list = 0;
size_t i, mpentry = MPENTRY_PADDR/PGSIZE;
for (i = 0; i < npages; i++) {
if (i == 0 || i == mpentry)
set_page_used(&pages[i]);
else if (i >= 1 && i < npages_basemem)
set_page_free(&pages[i]);
else if (i >= IOPHYSMEM / PGSIZE && i < EXTPHYSMEM / PGSIZE)
set_page_used(&pages[i]);
//这一段比较蛋疼... 没写的原因是一直不确定kernel除了自身加载进来的4M
//以外，还有没有其他的变量占用内存。如PageInfo* pages, kern_pgdir等。
//后来才发现，pages需要自己在mem_init()初始化。。。
else if (i >= EXTPHYSMEM / PGSIZE && i < 0x400000 / PGSIZE)
set_page_used(&pages[i]);
else
set_page_free(&pages[i]);
}
}
//find first page that is 0
int find_first_free_page(struct file_system *fs)
{
//finds first free page in free list
int pos = 0; //position in array with an open array
int i;
int j=0; //position of first zero in number
for (i=0; i < NUM_FREE_LIST_BYTES; i++)
{
if (fs->free_list[i]!=255)
{
pos = i;
break;
}
}
while(fs->free_list[pos] & (1<<(7-j))) {
j++;
}
set_page_used(fs, pos*8+j); //set the page as taken
return (int)sizeof(struct file_system) + (PAGE_SIZE*(pos*8+j)); //first free page number
}
