15. Address Translation - GitHub Pages › aos › 15.Address_Translation.pdf · Handle the trap Call switch() routine save regs(A) to proc-struct(A) (including base/bounds) restore

15. Address TranslationOperating System: Three Easy Pieces

1AOS@UC

Memory Virtualizing with Efficiency and Control

p Memory virtualizing takes a similar strategy known as limited direct

execution(LDE) for efficiency and control.

p In memory virtualizing, efficiency and control are attained by hardware

support.

w e.g., registers, TLB(Translation Look-aside Buffer)s, page-table

AOS@UC 2

Assumptions Made (just in this lecture)

p User address space is contiguous in memory

p User address space is smaller than the physical memory (up to 64KB)

p Each address space has the same size (up to 16KB)

AOS@UC 3

Address Translation

p Hardware transforms a virtual address to a physical address.

w The desired information is actually stored in a physical address.

p The OS must get involved at key points to set up the hardware.

w The OS must manage memory to judiciously intervene.

AOS@UC 4

Example: Address Translation

p C - Language code

w Load a value from memory

w Increment it by three

w Store the value back into memory

void func()int x;...x = x + 3; // this is the line of code we are interested in

AOS@UC 5

Example: Address Translation(Cont.)

p Assembly

w Presume that the address of ‘x’ has been place 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.

128 : movl 0x0(%ebx), %eax ; load 0+ebx into eax132 : addl $0x03, %eax ; add 3 to eax register135 : movl %eax, 0x0(%ebx) ; store eax back to mem

AOS@UC 6

Example: Address Translation(Cont.)

• 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)

3000

Stack

stack

heap

Heap

14KB

Program Code

16KB

15KB

0KB

1KB

2KB

3KB

4KB

128132135

movl 0x0(%ebx),%eaxAddl 0x03,%eaxmovl %eax,0x0(%ebx)

AOS@UC 7

Relocation Address Space

p The OS wants to place the process somewhere else in physical

memory, not at address 0.

w The address space start at address 0.

p But how make it transparently?

AOS@UC 8

A Single Relocated Process

(free)

Stack

stack

heap

Heap

Program Code

16KB

0KB

(not in use)

(not in use)

Operating System

0KB

16KB

32KB

48KB

64KB

Code

(allocated but not in use)

Heap

Stack

Relo

cate

d P

roce

ss

Address SpacePhysical Memory

AOS@UC 9

Base and Bounds Registers

(free)

Stack

stack

heap

Heap

Program Code

16KB

0KB

(not in use)

(not in use)

Operating System

0KB

16KB

32KB

48KB

64KB

Code


Heap

Stack


32KB

base register

16KB

bounds register

AOS@UC 10

Aside: Software-based Relocation

p If Harware-support is not present?

w Static-Relocation

p Loader should “transform” the executable

p Problems

w No protection

w Late relocation is hard

p Hardware support is mandatory!

AOS@UC 11

Dynamic (Hardware based) Relocation

p When a program starts running, the OS decides where in physical

memory a process should be loaded.

w Set the base register a value.

w Every virtual address must not be greater than bound and negative.

w ISA provide

¢ Privileged ins to handle those registers

¢ Specific exception to detect (and handle miss behaviors)

𝑝ℎ𝑦𝑐𝑎𝑙𝑎𝑑𝑑𝑟𝑒𝑠𝑠 = 𝑣𝑖𝑟𝑡𝑢𝑎𝑙𝑎𝑑𝑑𝑟𝑒𝑠𝑠 + 𝑏𝑎𝑠𝑒

0 ≤ 𝑣𝑖𝑟𝑡𝑢𝑎𝑙𝑎𝑑𝑑𝑟𝑒𝑠𝑠 < 𝑏𝑜𝑢𝑛𝑑𝑠

AOS@UC 12

Relocation and Address Translation

w Fetch instruction at address 128

w Execute this instruction

¢ Load from address 15KB

(free)

3000

Stack

stack

heap

Heap

14KB

Program Code

16KB

15KB

0KB

1KB

2KB

3KB

4KB

128132135

128 : movl 0x0(%ebx), %eax

32896 = 128 + 32𝐾𝐵(𝑏𝑎𝑠𝑒)

47𝐾𝐵 = 15𝐾𝐵 + 32𝐾𝐵(𝑏𝑎𝑠𝑒)

movl 0x0(%ebx),%eaxAddl 0x03,%eaxmovl %eax,0x0(%ebx)

AOS@UC 13

CPUMMU• base• bounds

Memory

Two ways of Bounds Register

(free)

Stack

Heap

Program Code

16KB

0KB

(not in use)

(not in use)

Operating System

0KB

16KB

32KB

48KB

64KB

Code


Heap

Stack


48KB16KB

bounds

𝒕𝒉𝒆𝒔𝒊𝒛𝒆𝒐𝒇𝒂𝒅𝒅𝒓𝒆𝒔𝒔𝒔𝒑𝒂𝒄𝒆

𝒑𝒉𝒚𝒔𝒊𝒄𝒂𝒍𝒂𝒅𝒅𝒓𝒆𝒔𝒔𝒐𝒇𝒕𝒉𝒆𝒆𝒏𝒅𝒐𝒇

𝒂𝒅𝒅𝒓𝒆𝒔𝒔𝒔𝒑𝒂𝒄𝒆

bounds

AOS@UC 14

OS Issues for Memory Virtualizing

p The OS must take action to implement base-and-bounds approach.

p Three critical junctures:

w When a process starts running:

¢ Finding space for address space in physical memory

w When a process is terminated:

¢ Reclaiming the memory for use

w When context switch occurs:

¢ Saving and storing the base-and-bounds pair

AOS@UC 15

OS Issues: When a Process Starts Running

p The OS must find a room for a new address space.

w free list : A list of the range of the physical memory which are not in use.

0KB

16KB

32KB

48KB

64KB

Code


Physical Memory

The OS lookup the free list

(not in use)

Heap

Stack

Operating System

(not in use)

Free list

16KB

48KB

AOS@UC 16

OS Issues: When a Process Is Terminated

p The OS must put the memory back on the free list.

Operating System

0KB

16KB

32KB

48KB

64KB

Physical Memory

(not in use)

(not in use)

Operating System

0KB

16KB

32KB

48KB

64KB

Physical Memory

(not in use)

(not in use)

(not in use)Process A

Free list

16KB

48KB

Free list

16KB

32KB

48KB

AOS@UC 17

OS Issues: When Context Switch Occurs

p The OS must save and restore the base-and-bounds pair.

w In process structure or process control block(PCB)

Operating System

0KB

16KB

32KB

48KB

64KB

Physical Memory

(not in use)

Process ACurrently Running

Process B

48KB

bounds

32KB

base

Context SwitchingOperating System

0KB

16KB

32KB

48KB

64KB

Physical Memory

(not in use)

Process A

Process BCurrently Running

64KB

48KB

Process A PCB

…base : 32KBbounds : 48KB …

bounds

base

AOS@UC 18

Summary: Dynamic Relocation (strawman version)MECHANISM: ADDRESS TRANSLATION 11

OS @ boot Hardware(kernel mode)initialize trap table

remember addresses of...system call handlertimer handlerillegal mem-access handlerillegal instruction handler

start interrupt timerstart timer; interrupt after X ms

initialize process tableinitialize free list

OS @ run Hardware Program(kernel mode) (user mode)To start process A:

allocate entry in process tableallocate memory for processset base/bounds registersreturn-from-trap (into A)

restore registers of Amove to user modejump to A’s (initial) PC

Process A runsFetch instruction

Translate virtual addressand perform fetch

Execute instructionIf explicit load/store:

Ensure address is in-bounds;Translate virtual address

and perform load/store...

Timer interruptmove to kernel modeJump to interrupt handler

Handle the trapCall switch() routine

save regs(A) to proc-struct(A)(including base/bounds)restore regs(B) from proc-struct(B)(including base/bounds)

return-from-trap (into B)restore registers of Bmove to user modejump to B’s PC

Process B runsExecute bad load

Load is out-of-bounds;move to kernel modejump to trap handler

Handle the trapDecide to terminate process Bde-allocate B’s memoryfree B’s entry in process table

Figure 15.5: Limited Direct Execution Protocol (Dynamic Relocation)

c⃝ 2014, ARPACI-DUSSEAUTHREE

EASYPIECES

AOS@UC 19

Summary: Dynamic Relocation (strawman version)

MECHANISM: ADDRESS TRANSLATION 11

OS @ boot Hardware(kernel mode)initialize trap table

remember addresses of...system call handlertimer handlerillegal mem-access handlerillegal instruction handler

start interrupt timerstart timer; interrupt after X ms

initialize process tableinitialize free list

OS @ run Hardware Program(kernel mode) (user mode)To start process A:

allocate entry in process tableallocate memory for processset base/bounds registersreturn-from-trap (into A)

restore registers of Amove to user modejump to A’s (initial) PC

Process A runsFetch instruction

Translate virtual addressand perform fetch

Execute instructionIf explicit load/store:

Ensure address is in-bounds;Translate virtual address

and perform load/store...

Timer interruptmove to kernel modeJump to interrupt handler

Handle the trapCall switch() routine

save regs(A) to proc-struct(A)(including base/bounds)restore regs(B) from proc-struct(B)(including base/bounds)

return-from-trap (into B)restore registers of Bmove to user modejump to B’s PC

Process B runsExecute bad load

Load is out-of-bounds;move to kernel modejump to trap handler

Handle the trapDecide to terminate process Bde-allocate B’s memoryfree B’s entry in process table

Figure 15.5: Limited Direct Execution Protocol (Dynamic Relocation)

c⃝ 2014, ARPACI-DUSSEAUTHREE

EASYPIECES

AOS@UC 20

p Disclaimer: Disclaimer: This lecture slide set is used in AOS course at University of Cantabria.

Was initially developed for Operating System course in Computer Science Dept. at Hanyang

University. This lecture slide set is for OSTEP book written by Remzi and Andrea Arpaci-

Dusseau (at University of Wisconsin)

AOS@UC 21

Documents

15. Address Translation - GitHub Pages › aos › 15.Address_Translation.pdf · Handle the trap Call switch() routine save regs(A) to proc-struct(A) (including base/bounds) restore