IA32 Paging Scheme

1. IA32 Paging Scheme Introduction to the Pentium’s support for “virtual” memory

2. What is ‘paging’? • It’s a scheme for dynamically remapping addresses for fixed-size memory-blocks Physical address-space Virtual address-space

3. What’s ‘paging’ good for? • For efficient ‘time-sharing’ among multiple tasks, an operating system needs to have several programs residing in main memory at the same time • To accomplish this using actual physical memory-addressing would require doing address-relocation calculations each time a program was loaded (to avoid conflicting with any addresses already being used)

4. Why use ‘Paging’? • Use of ‘paging’ allows ‘relocations’ to be done just once (by the linker), and every program can ‘reuse’ the same addresses Task #3 Task #1 physical memory Task #2

5. How to enable paging Control Register CR0 P G C D N W A M W P N E E T T S E M M P P E Protected-Mode must be enabled (PE=1) Then ‘Paging’ can be enabled (set PG=1) # Here is how you can enable paging (must be in protected-mode) mov %cr0, %eax # get current machine status bts $31, %eax # turn on the PE-bit’s image mov %eax, %cr0 # put modified status in CR0 jmp pg # now flush the prefetch queue pg: # but you had better prepare the ‘mapping’ beforehand!

6. Several ‘paging’ schemes • Intel’s design for ‘paging’ has continued to evolve since its introduction in 80386 CPU • Pentiums support the initial design, as well as several optional extensions • We shall describe the initial design which is simplest and remains as the ‘default’ • It is based on subdividing the entire 4GB virtual address-space into 4KB blocks

7. Terminology • The 4KB memory-blocks are called ‘page frames’ -- and they are non-overlapping • Therefore each page-frame begins at a memory-address which is a multiple of 4K • Remember: 4K = 4 x 1024 = 4096 = 212 • So the address of any page-frame will have its lowest 12-bits equal to zeros • Example: page six begins at 0x00006000

8. Control Register CR3 • Register CR3 is used by the CPU to find the tables in memory which will define the address-translation that it should employ • This table is called the ‘Page Directory’ and its address must be ‘page-aligned’ 31 0 Physical Address of the Page-Directory

9. Page-Directory • The Page-Directory occupies one frame, so it has room for 1024 4-byte entries • Each page-directory entry may contain a pointer to a further data-structure, called a Page-Table (also page-aligned 4KB size) • Each Page-Table occupies one frame and has enough room for 1024 4-byte entries • Page-Table entries may contain pointers

10. Two-Level Translation Scheme PAGE TABLES PAGE DIRECTORY PAGE FRAMES CR3

11. Address-translation • The CPU examines any virtual address it encounters, subdividing it into three fields 31 22 21 12 11 0 index into page-directory index into page-table offset into page-frame 10-bits 10-bits 12-bits This field selects one of the 1024 array-entries in the Page-Directory This field selects one of the 1024 array-entries in that Page-Table This field provides the offset to one of the 4096 bytes in that Page-Frame

12. Page-Level ‘protection’ • Each entry in a Page-Table can assign a collection of ‘attributes’ to the Page-Frame that it points to; for example: • The P-bit (page is ‘present’) can be used by the operating system to support its implementation of “demand paging” • The W/R-bit can be used to mark a page as ‘Writable’ or as ‘Read-Only’ • The U/S-bit can be used to mark a page as ‘User accessible’ or as ‘Supervisor-Only’

13. Format of a Page-Table entry 31 12 11 10 9 8 7 6 5 4 3 2 1 0 PAGE-FRAME BASE ADDRESS AVAIL 0 0 D A P C D P W T U W P LEGEND P = Present (1=yes, 0=no) W = Writable (1 = yes, 0 = no) U = User (1 = yes, 0 = no) A = Accessed (1 = yes, 0 = no) D = Dirty (1 = yes, 0 = no) PWT = Page Write-Through (1=yes, 0 = no) PCD = Page Cache-Disable (1 = yes, 0 = no)

14. Format of a Page-Directory entry 31 12 11 10 9 8 7 6 5 4 3 2 1 0 PAGE-TABLE BASE ADDRESS AVAIL 0 P S 0 A P C D P W T U W P LEGEND P = Present (1=yes, 0=no) W = Writable (1 = yes, 0 = no) U = User (1 = yes, 0 = no) A = Accessed (1 = yes, 0 = no) PS = Page-Size (0=4KB, 1 = 4MB) PWT = Page Write-Through (1=yes, 0 = no) PCD = Page Cache-Disable (1 = yes, 0 = no)

15. Violations • When a task violates the page-attributes of any Page-Frame, the CPU will generate a ‘Page-Fault’ Exception (interrupt 0x0E) • Then the operating system’s page-fault exception-handler gets control and can take whatever action it deems is suitable • The CPU will provide help to the OS in determining why a Page-Fault occurred

16. The Error-Code format • The CPU will push an Error-Code onto the operating system’s stack 3 2 1 0 reserved (=0) U / S W / R P Legend: P (Present): 0=attempted to access a ‘not-present’ page W/R (Write/Read): 1=attempted to write to a ‘read-only’ page U/S (User/Supervisor): 1=user attempted to access a ‘supervisor’ page User means that CPL = 3; Supervisor means that CPL = 0, 1, or 2

17. Control Register CR2 • Whenever a ‘Page-Fault’ is encountered, the CPU will save the virtual-address that caused that fault into the CR2 register • If the CPU was trying to modify the value of an operand in a ‘read-only’ page, then that operand’s virtual address is written into CR2 • If the CPU was trying to read the value of an operand in a supervisor-only page (or was trying to fetch-and-execute an instruction) while CPL=3, the relevant virtual address will be written into CR2

18. Identity-mapping • When the CPU first turns on the ‘paging’ capability, it must be executing code from an ‘identity-mapped’ page (or it crashes!) • We have created a demo-program that shows how to create the Page-Directory and Page-Tables for an identity-mapping of the entire bottom megabyte of RAM • The demo is named ‘trypages.s’

19. Page-Table initialization (1MB) • Only the first 256 page-frames need to be mapped (so one Page-Table will suffice): # This code-fragment shows how to construct a loop in assembly language # that will initialize the first 256 Page-Table entries for an identity-mapping # of the conventional memory area (i.e., the bottom megabyte of memory) .code32 xor %ebx, %ebx # Page-Table’s initial array-index mov $256, %ecx # number of entries to be defined mov $0x00000007, %eax # entry 0: P=W=U=1 nxpte: mov %eax, pgtbl(, %ebx, 4) # write this table-entry add $0x1000, %eax # compute next entry inc %ebx # increment array-index loop nxpte # setup all the entries

20. CR3 and Task-Switching 32-bits 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96 100 link esp0 ss0 esp1 ss1 esp2 Page-Table Directory Base ss2 PTDB EIP This value will get loaded into register CR3 as part of the context-switching mechanism when paging has been enabled (PG=1) So the ‘incoming task’ will automatically have its own individual mapping of its ‘virtual’ address-space to page-frames in the CPU’s ‘physical’ address-space 26 longwords EFLAGS ss0 ss0 ss0 EAX ss0 ECX ss0 ss0 EDX ss0 ss0 ss0 EBX ss0 ss0 ESP ss0 ss0 EBP ss0 ss0 ESI ss0 ss0 EDI ss0 ES CS SS = field is ‘static’ DS FS GS = field is ‘volatile’ LDTR IOMAP TRAP = field is ‘reserved’ I/O permission bitmap

21. Extensions to ‘paging’ scheme • In the Pentium Intel has provided several enhancements to the original 386 paging • These enhancements are ‘optional’ and must be selectively enabled by software • Control Register CR4 implements bits to “turn on” the desired ‘paging-extension’ and some other enhancements that are unrelated to the ‘paging’ architectures

22. Control Register CR4 31 13 10 9 8 7 6 5 4 3 2 1 0 V M X E P C E P G E M C E P A E P S E D E T S D P V I V M E Legend (for paging-related extensions): PSE = Page-Size Extension is enabled (1 = yes, 0 = no) PAE = Page-Address Extension is enabled (1 = yes, 0 = no) PGE = Page-Global Extension is enabled (1 = yes, 0 = no)

23. Efficiency? • When paging is enabled, every reference to memory requires the CPU to ‘translate’ the virtual address into a physical-address • That ‘translation’ is based on table-lookups • These lookups must be done ‘sequentially’ • So ‘address-translation’ could be costly in terms of CPU speed – a high percentage of instructions typically refer to memory

24. The ‘TLB’ solution • When the CPU has performed the table lookups that map a virtual-address to a physical-address, it “remembers” that relationship by saving the pair of page-addresses (virtual-page  physical page) in a special CPU cache known as the TLB (“Translation Look-aside Buffer”) • So future references to that virtual page can be quickly resolved via that cache

25. 4-way set-associative • The TLB is implemented as a ‘4-way set-associative’ cache -- it’s like a parallelized version of a Hash Table (with ‘evictions’) • Due to the ‘locality of reference’ principle, the TLB concept generally works well in most common programming contexts as an efficient ‘speedup’ of the page-address table-lookup translation mechanism • Modifying CR3 will invalidate the TLB

26. In-class Exercise • Modify the ‘trypages.s’ demo so it does a little bit of ‘non-identical’ page-mapping (i.e., so some virtual-address is different from its physical-address) • For example, try mapping the page-frame at virtual address 0x00008000 to physical address 0x000B8000 (by changing the Page-Table entry at table-index 8), then write a message at address 0x00008000

27. The exercise illustrated . . . . . . 0x000B8000 video memory video memory video memory 0x00008000 physical page-frames virtual page-frames