15. Address Translation

Transcription

1 15. Address Translation Oerating System: Three Easy Pieces 1

2 Memory Virtualizing with Efficiency and Control Memory virtualizing takes a similar strategy known as limited direct execution(lde) for efficiency and control. In memory virtualizing, efficiency and control are attained by hardware suort. w e.g., registers, TLB(Translation Look-aside Buffer)s, age-table AOS@UC 2

3 Assumtions Made (just in this lecture) User address sace is contiguous in memory User address sace is smaller than the hysical memory (u to 64KB) Each address sace has the same size (u to ) AOS@UC 3

4 Address Translation Hardware transforms a virtual address to a hysical address. w The desired information is actually stored in a hysical address. The OS must get involved at key oints to set u the hardware. w The OS must manage memory to judiciously intervene. AOS@UC 4

5 Examle: Address Translation C - Language code void func() int x;... x = x + 3; // this is the line of code we are interested in w Load a value from memory w Increment it by three w Store the value back into memory AOS@UC 5

6 Examle: Address Translation(Cont.) Assembly 128 : movl 0x0(%ebx), %eax ; load 0+ebx into eax 132 : addl $0x03, %eax ; add 3 to eax register 135 : movl %eax, 0x0(%ebx) ; store eax back to mem w Presume that the address of x has been lace in ebx register. w Load the value at that address into eax register. w Add 3 to eax register. w Store the value in eax back into memory. AOS@UC 6

7 Examle: Address Translation(Cont.) 1KB 2KB 3KB 4KB movl 0x0(%ebx),%eax Addl 0x03,%eax movl %eax,0x0(%ebx) Program Code Hea hea Fetch instruction at address 128 Execute this instruction (load from address 15KB) Fetch instruction at address 132 Execute this instruction (no memory reference) Fetch the instruction at address 135 Execute this instruction (store to address 15 KB) (free) stack 14KB 15KB 3000 Stack 7

8 Relocation Address Sace The OS wants to lace the rocess somewhere else in hysical memory, not at address 0. w The address sace start at address 0. But how make it transarently? AOS@UC 8

9 A Single Relocated Process Program Code Oerating System Hea hea (free) stack Code Hea (allocated but not in use) Stack Relocated Process Stack Address Sace 64KB Physical Memory 9

10 Base and Bounds Registers Program Code Oerating System Hea base register hea (free) Code Hea (allocated but not in use) stack Stack bounds register Stack Address Sace 64KB Physical Memory 10

11 Aside: Software-based Relocation If Harware-suort is not resent? w Static-Relocation Loader should transform the executable Problems w No rotection w Late relocation is hard Hardware suort is mandatory! AOS@UC 11

12 Dynamic (Hardware based) Relocation When a rogram starts running, the OS decides where in hysical memory a rocess should be loaded. w Set the base register a value. hycal address = virtual address + base w Every virtual address must not be greater than bound and negative. 0 virtual address < bounds w ISA rovide Privileged ins to handle those registers Secific excetion to detect (and handle miss behaviors) AOS@UC 12

13 Relocation and Address Translation 128 : movl 0x0(%ebx), %eax w Fetch instruction at address = (base) w Execute this instruction Load from address 15KB 47KB = 15KB + (base) 1KB 2KB 3KB 4KB movl 0x0(%ebx),%eax Addl 0x03,%eax movl %eax,0x0(%ebx) Program Code Hea hea (free) stack CPU MMU base bounds Memory 14KB 15KB 3000 Stack AOS@UC 13

14 Two ways of Bounds Register Program Code Oerating System Hea the size of address sace bounds (free) Code Hea (allocated but not in use) Stack hysical address of the end of address sace bounds Stack Address Sace 64KB Physical Memory 14

15 OS Issues for Memory Virtualizing The OS must take action to imlement base-and-bounds aroach. Three critical junctures: w When a rocess starts running: Finding sace for address sace in hysical memory w When a rocess is terminated: Reclaiming the memory for use w When context switch occurs: Saving and storing the base-and-bounds air AOS@UC 15

16 OS Issues: When a Process Starts Running The OS must find a room for a new address sace. w free list : A list of the range of the hysical memory which are not in use. The OS looku the free list Free list Oerating System 64KB Code Hea (allocated but not in use) Stack Physical Memory AOS@UC 16

17 OS Issues: When a Process Is Terminated The OS must ut the memory back on the free list. Free list Oerating System Free list Oerating System Process A 64KB Physical Memory 64KB Physical Memory AOS@UC 17

18 OS Issues: When Context Switch Occurs The OS must save and restore the base-and-bounds air. w In rocess structure or rocess control block(pcb) Process A PCB Oerating System Context Switching Oerating System base : bounds : base base 64KB Process A Currently Running Process B bounds 64KB Process A Process B Currently Running bounds 64KB Physical Memory Physical Memory AOS@UC 18

19 Summary: Dynamic Relocation (strawman version) boot (kernel mode) initialize tra table start interrut timer initialize rocess table initialize free list Hardware remember addresses of... system call handler timer handler illegal mem-access handler illegal instruction handler start timer; interrut after X ms run Hardware Program (kernel mode) (user mode) To start rocess A: allocate entry in rocess table allocate memory for rocess set base/bounds registers return-from-tra (into A) restore registers of A move to user mode jum to A s (initial) PC Translate virtual address and erform fetch If exlicit load/store: Process A runs Fetch instruction Execute instruction AOS@UC 19

20 Summary: Dynamic Relocation (strawman version) Handle the tra Call switch() routine save regs(a) to roc-struct(a) (including base/bounds) restore regs(b) from roc-struct(b) (including base/bounds) return-from-tra (into B) Handle the tra Decide to terminate rocess B de-allocate B s memory free B s entry in rocess table Translate virtual address and erform fetch If exlicit load/store: Ensure address is in-bounds; Translate virtual address and erform load/store Timer interrut move to kernel mode Jum to interrut handler restore registers of B move to user mode jum to B s PC Load is out-of-bounds; move to kernel mode jum to tra handler... Fetch instruction Execute instruction Process B runs Execute bad load AOS@UC 20

21 Disclaimer: Disclaimer: This lecture slide set is used in AOS course at University of Cantabria. Was initially develoed for Oerating System course in Comuter Science Det. at Hanyang University. This lecture slide set is for OSTEP book written by Remzi and Andrea Araci- Dusseau (at University of Wisconsin) 21