MIT6.828 lab2

1. Lab 2
Memory Management

2. Virtual Memory
When we into protected mode
Selector
Software Segmentation Paging RAM
Offset
Virtual Linear Physical

3. Virtual Memory
Hence the “selector” has no effect and
the linear address always equals the
offset of the virtual address.
so, we only care ....
Virtual
Paging RAM
address
Linear Physical

4. Virtual Memory
Linear address Physical address
10 10 12 20 12
Dir Table Offset PPN Offset
1023 20 12
20 12 PPN Offset
1023
0
PPN Offset Page Table
0
Page Directory

5. Virtual Memory
0x0fffffff
256MB
Linear address
10 10 12
Extended
Dir Table Offset
Memory
0x00100000
BIOS ROM
4GB ROM
VGA Display
0x000A0000
RAM
0x00000000

6. Virtual Memory
Linear address
10 10 12
Dir Table Offset
pde_t *kern_pgdir;
kern_pgdir = (pde_t *)
PDX(va) boot_alloc(PGSIZE);
1023 20 12
PPN Offset
0
Page Directory

7. Virtual Memory
0x0fffffff
256MB
kern_pgdir 4KB
Kernel ?KB
0x00100000
BIOS ROM
ROM
VGA Display
0x000A0000
RAM
0x00000000

8. Virtual Memory
Linear address
10 10 12
Dir Table Offset
1023 20 12
PDX(va) PTX(va)
20 12 PPN Offset
1023
0
PPN Offset Page Table
0
Page Directory

9. Virtual Memory
Linear address
10 10 12 struct Page *pages;
Dir Table Offset static struct Page *page_free_list;
1023 20 12
PDX(va) PTX(va)
20 12 PPN Offset
1023
0
PPN Offset Page Table
0
Page Directory

10. Virtual Memory
0x0fffffff
256MB
page_init
pages
kern_pgdir 4KB
kernel ?KB
0x00100000
BIOS
0x000A0000(IOPHYSMEM)
page_free_list 0x00000000

11. Virtual Memory
struct Page *pages;
pages = boot_alloc(npages * (sizeof(struct Page)));
0x0fffffff
256MB
page_init
pages
kern_pgdir 4KB
kernel ?KB
0x00100000
BIOS
0x000A0000(IOPHYSMEM)
page_free_list 0x00000000

12. Virtual Memory
page_alloc :
get page from page_free_list
page_free :
return page to page_free_list

13. Virtual Memory
Linear address
10 10 12
Physical address
20 12
目前我們完
Dir Table Offset PPN Offset
成了page的
1023 20 12
架構，但是
20 12 PPN Offset
1023
未提供一個
0
PPN Offset Page Table
外部使用的
0
Page Directory 介面

14. Interface
pte_t *
pgdir_walk(pde_t *pgdir, const void *va, int create)
{
// Fill this function in
//return NULL;
pde_t *dir = &pgdir[PDX(va)];
if(*dir & PTE_P){
pte_t *table = (pte_t*) KADDR(PTE_ADDR(*dir));
return &table[PTX(va)];
}else{
if(!create){
return NULL;
}
struct Page *page_tmp = page_alloc(ALLOC_ZERO);
if(!page_tmp){
return NULL;
}else{
pte_t *table = (pte_t*) page2kva(page_tmp);
*dir = PADDR(table) | PTE_P | PTE_W | PTE_U;
page_tmp->pp_ref = 1;
return &table[PTX(va)];
}
}
}

15. Interface
int
page_insert(pde_t *pgdir, struct Page *pp, void *va, int perm)
{
// Fill this function in
// return 0;
if(!pp){
return -E_NO_MEM;
}
pte_t *page_entry = pgdir_walk(pgdir, va, 1);
if(!page_entry){
return -E_NO_MEM;
}
pgdir[PDX(va)] |= perm;
if(page2pa(pp) != PTE_ADDR(*page_entry)){
page_remove(pgdir, va);
pp->pp_ref++;
}
*page_entry = page2pa(pp) | perm | PTE_P;
return 0;
}

16. Interface
struct Page *
page_lookup(pde_t *pgdir, void *va, pte_t **pte_store)
{
// Fill this function in
// return NULL;
*pte_store = pgdir_walk(pgdir, va, 0);
if(!(*pte_store) || !(**pte_store & PTE_P)){
return NULL;
}else{
return pa2page(PTE_ADDR(**pte_store));
}
}

17. Interface
void
page_remove(pde_t *pgdir, void *va)
{
// Fill this function in
pte_t *page_entry;
struct Page *page = page_lookup(pgdir, va, &page_entry);
if(page){
page_decref(page);
}
*page_entry = 0;
tlb_invalidate(pgdir, va);
}

18. Thanks a lot.