Processes, signals and shells Virtual memory

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Processes, signals and shells Virtual memory

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Title: Processes, signals and shells Virtual memory

1
Processes, signals and shellsVirtual memory

Processes and signals
Creating and destroying processes
Process Hierarchy
Shells
Signals.
Virtual memory
Motivation for VM
Address translation
Accelerating translation with TLBs
All source code is posted at http//reed.cs.depaul
.edu/lperkovic/csc374/lectures/lecture4/

2
System calls

Unix systems provide many different types of
systems calls to be used by application programs
when they need a service from the kernel
Reading a file
Creating a new process
To get the complete list, type
man syscalls

3
fork Creating new processes

int fork(void)
creates a new process (child process) that is
identical to the calling process (parent process)
returns 0 to the child process
returns childs pid to the parent process

if (fork() 0) printf("hello from
child\n") else printf("hello from
parent\n")
Fork is interesting (and often confusing) because
it is called once but returns twice
4
Fork Example 1

Key Points
Parent and child both run same code
Distinguish parent from child by return value
from fork
Start with same state, but each has private copy
Including shared output file descriptor
Relative ordering of their print statements
undefined

void fork1() int x 1 pid_t pid
fork() if (pid 0) printf("Child has x
d\n", x) else printf("Parent has x
d\n", --x) printf("Bye from process
d with x d\n", getpid(), x)
5
Fork Example 2

Key Points
Both parent and child can continue forking

void fork2() printf("L0\n") fork()
printf("L1\n") fork()
printf("Bye\n")
6
Fork Example 3

Key Points
Both parent and child can continue forking

void fork3() printf("L0\n") fork()
printf("L1\n") fork()
printf("L2\n") fork()
printf("Bye\n")
7
Fork Example 4

Key Points
Both parent and child can continue forking

void fork4() printf("L0\n") if (fork()
! 0) printf("L1\n") if (fork() ! 0)
printf("L2\n") fork()
printf("Bye\n")
8
Fork Example 5

Key Points
Both parent and child can continue forking

void fork5() printf("L0\n") if (fork()
0) printf("L1\n") if (fork() 0)
printf("L2\n") fork()
printf("Bye\n")
9
exit Destroying Process

void exit(int status)
exits a process
Normally return with status 0
atexit() registers functions to be executed upon
exit

void cleanup(void) printf("cleaning
up\n") void fork6() atexit(cleanup)
fork() exit(0)
10
Zombies

Idea
When process terminates, it still consumes system
resources
Various tables maintained by OS
Called a zombie
Living corpse, half alive and half dead
Reaping
Performed by parent on terminated child
Parent is given exit status information
Kernel discards process
What if Parent Doesnt Reap?
If any parent terminates without reaping a child,
then child will be reaped by init process
Only need explicit reaping for long-running
processes
E.g., shells and servers

11
ZombieExample
void fork7() if (fork() 0) / Child
/ printf("Terminating Child, PID d\n",
getpid()) exit(0) else
printf("Running Parent, PID d\n",
getpid()) while (1) / Infinite loop /

linuxgt ./forks 7 1 6639 Running Parent, PID
6639 Terminating Child, PID 6640 linuxgt ps
PID TTY TIME CMD 6585 ttyp9 000000
tcsh 6639 ttyp9 000003 forks 6640 ttyp9
000000 forks ltdefunctgt 6641 ttyp9 000000
ps linuxgt kill 6639 1 Terminated linuxgt ps
PID TTY TIME CMD 6585 ttyp9 000000
tcsh 6642 ttyp9 000000 ps

ps shows child process as defunct
Killing parent allows child to be reaped

12
NonterminatingChildExample
void fork8() if (fork() 0) / Child
/ printf("Running Child, PID d\n",
getpid()) while (1) / Infinite loop /
else printf("Terminating Parent, PID
d\n", getpid()) exit(0)
linuxgt ./forks 8 Terminating Parent, PID
6675 Running Child, PID 6676 linuxgt ps PID
TTY TIME CMD 6585 ttyp9 000000
tcsh 6676 ttyp9 000006 forks 6677 ttyp9
000000 ps linuxgt kill 6676 linuxgt ps PID TTY
TIME CMD 6585 ttyp9 000000 tcsh
6678 ttyp9 000000 ps

Child process still active even though parent has
terminated
Must kill explicitly, or else will keep running
indefinitely

13
wait Synchronizing with children

int wait(int child_status)
suspends current process until one of its
children terminates
return value is the pid of the child process that
terminated
if child_status ! NULL, then it points to a
status indicating why the child process terminated

14
wait Synchronizing with children
void fork9() int child_status if
(fork() 0) printf("HC hello from
child\n") else printf("HP hello
from parent\n") wait(child_status)
printf("CT child has terminated\n")
printf("Bye\n") exit()
15
Wait Example

If multiple children completed, will take in
arbitrary order
Can use macros WIFEXITED and WEXITSTATUS to get
information about exit status

void fork10() pid_t pidN int i
int child_status for (i 0 i lt N i) if
((pidi fork()) 0) exit(100i) /
Child / for (i 0 i lt N i) pid_t
wpid wait(child_status) if
(WIFEXITED(child_status)) printf("Child d
terminated with exit status d\n", wpid,
WEXITSTATUS(child_status)) else
printf("Child d terminate abnormally\n", wpid)

16
Waitpid

waitpid(pid, status, options)
Can wait for specific process
Various options

void fork11() pid_t pidN int i
int child_status for (i 0 i lt N i) if
((pidi fork()) 0) exit(100i) /
Child / for (i 0 i lt N i) pid_t
wpid waitpid(pidi, child_status, 0) if
(WIFEXITED(child_status)) printf("Child d
terminated with exit status d\n", wpid,
WEXITSTATUS(child_status)) else
printf("Child d terminated abnormally\n",
wpid)
17
Wait/Waitpid Example Outputs
Using wait (fork10)
Child 3565 terminated with exit status 103 Child
3564 terminated with exit status 102 Child 3563
terminated with exit status 101 Child 3562
terminated with exit status 100 Child 3566
terminated with exit status 104
Using waitpid (fork11)
Child 3568 terminated with exit status 100 Child
3569 terminated with exit status 101 Child 3570
terminated with exit status 102 Child 3571
terminated with exit status 103 Child 3572
terminated with exit status 104
18
Command line arguments in C/C

Command line arguments to C/C programs are
passed through the argv arrayint main (int
argc, char argv)argc is the number of
command line arguments, including the name of the
program or command being executed ( passed as
argv0)Example 1 printArgs.cExample 2
printN.c

19
exec Running new programs

int execl(char path, char arg0, char arg1, ,
0)
loads and runs executable at path with args arg0,
arg1,
path is the complete path of an executable
arg0 becomes the name of the process
typically arg0 is either identical to path, or
else it contains only the executable filename
from path
real arguments to the executable start with
arg1, etc.
list of args is terminated by a (char )0
argument
returns -1 if error, otherwise doesnt return!

main() if (fork() 0)
execl("/usr/bin/cp", "cp", "foo", "bar", 0)
wait(NULL) printf("copy completed\n")
exit()
20
Running printArgs from a program

Instead of running the printArgs program from
the Unix shell, we can run it from a program,
using execlp.
Example 1 runls.c
Example 2 prog.c
Example 3 prog2.c

21
Creating new processes in UNIX

A process creates a new process that executes a
given program or command as followsCall fork(
) to create a new processCall exec( ) within
the new process to execute the program or command
Example progExec.c

22
Shell Programs

A shell is an application program that runs
programs on behalf of the user.
sh Original Unix Bourne Shell
csh BSD Unix C Shell, tcsh Enhanced C Shell
bash Bourne-Again Shell

23
Writing a Unix Shell

Pseudo Code for a shell
print a prompt.
while( EOF is not signaled and an input line is
read )
create a child process (using fork)
have the child process replace its program (this
shell program) with the program specified in the
input line. (using execlp)
wait for the child to finish executing its
program. (using wait)
print a prompt

24
Shell Programs

A shell is an application program that runs
programs on behalf of the user.
sh Original Unix Bourne Shell
csh BSD Unix C Shell, tcsh Enhanced C Shell
bash Bourne-Again Shell

int main() char cmdlineMAXLINE
while (1) / read / printf("gt ")
Fgets(cmdline, MAXLINE, stdin) if
(feof(stdin)) exit(0) / evaluate
/ eval(cmdline)
Execution is a sequence of read/evaluate
steps See shellex.c
25
Simple Shell eval Function
void eval(char cmdline) char
argvMAXARGS / argv for execve() / int
bg / should the job run in bg or
fg? / pid_t pid / process id
/ bg parseline(cmdline, argv) if
(!builtin_command(argv)) if ((pid Fork())
0) / child runs user job / if
(execve(argv0, argv, environ) lt 0)
printf("s Command not found.\n",
argv0) exit(0) if (!bg) /
parent waits for fg job to terminate /
int status if (waitpid(pid, status, 0) lt
0) unix_error("waitfg waitpid
error") else / otherwise, dont
wait for bg job / printf("d s", pid,
cmdline)
26
Problem with Simple Shell Example

Shell correctly waits for and reaps foreground
jobs.
But what about background jobs?
Will become zombies when they terminate.
Will never be reaped because shell (typically)
will not terminate.
Creates a memory leak that will eventually crash
the kernel when it runs out of memory.
Solution Reaping background jobs requires a
mechanism called a signal.
Another problem Ctrl-C kills the shell, not the
foreground job
Solution Ctrl-C requests the kernel to send a
signal to the shell, which must redirect it to
the foreground processes

27
Signals

A signal is a small message that notifies a
process that an event of some type has occurred
in the system.
Kernel abstraction for exceptions and interrupts.
Sent from the kernel (sometimes at the request of
another process) to a process.
Different signals are identified by small integer
IDs
The only information in a signal is its ID and
the fact that it arrived.

28
Signal Concepts

Sending a signal
Kernel sends (delivers) a signal to a destination
process by updating some state in the context of
the destination process.
Kernel sends a signal for one of the following
reasons
Kernel has detected a system event such as
divide-by-zero (SIGFPE) or the termination of a
child process (SIGCHLD)
Another process has invoked the kill system call
to explicitly request the kernel to send a signal
to the destination process.
A user can also do this using the kill program

29
Signal Concepts (cont)

Receiving a signal
A destination process receives a signal when it
is forced by the kernel to react in some way to
the delivery of the signal.
Three possible ways to react
Ignore the signal (do nothing)
Terminate the process.
Catch the signal by executing a user-level
function called a signal handler.
Akin to a hardware exception handler being called
in response to an asynchronous interrupt.

30
Signal Concepts (cont)

A signal is pending if it has been sent but not
yet received.
There can be at most one pending signal of any
particular type.
Important Signals are not queued
If a process has a pending signal of type k, then
subsequent signals of type k that are sent to
that process are discarded.
A process can block the receipt of certain
signals.
Blocked signals can be delivered, but will not be
received until the signal is unblocked.
A pending signal is received at most once.

31
Signal Concepts

Kernel maintains pending and blocked bit vectors
in the context of each process.
pending represents the set of pending signals
Kernel sets bit k in pending whenever a signal of
type k is delivered.
Kernel clears bit k in pending whenever a signal
of type k is received
blocked represents the set of blocked signals
Can be set and cleared by the application using
the sigprocmask function.

32
Process Groups

Every process belongs to exactly one process group

Shell
pid10 pgid10
Fore- ground job
Back- ground job 1
Back- ground job 2
pid20 pgid20
pid32 pgid32
pid40 pgid40
Background process group 32
Background process group 40
Child
Child
getpgrp() Return process group of current
process setpgid() Change process group of a
process
pid21 pgid20
pid22 pgid20
Foreground process group 20
33
Sending Signals with kill Program

kill program sends arbitrary signal to a process
or process group
Examples
kill 9 24818
Send SIGKILL to process 24818
kill 9 24817
Send SIGKILL to every process in process group
24817.

linuxgt ./forks 16 linuxgt Child1 pid24818
pgrp24817 Child2 pid24819 pgrp24817
linuxgt ps PID TTY TIME CMD 24788
pts/2 000000 tcsh 24818 pts/2 000002
forks 24819 pts/2 000002 forks 24820 pts/2
000000 ps linuxgt kill -9 -24817 linuxgt ps
PID TTY TIME CMD 24788 pts/2
000000 tcsh 24823 pts/2 000000 ps linuxgt
34
Sending Signals from the Keyboard

Typing ctrl-c (ctrl-z) sends a SIGTERM (SIGTSTP)
to every job in the foreground process group.
SIGTERM default action is to terminate each
process
SIGTSTP default action is to stop (suspend)
each process

Shell
pid10 pgid10
Fore- ground job
Back- ground job 1
Back- ground job 2
pid20 pgid20
pid32 pgid32
pid40 pgid40
Background process group 32
Background process group 40
Child
Child
pid21 pgid20
pid22 pgid20
Foreground process group 20
35
Example of ctrl-c and ctrl-z
linuxgt ./forks 17 Child pid24868 pgrp24867
Parent pid24867 pgrp24867 lttyped
ctrl-zgt Suspended linuxgt ps a PID TTY
STAT TIME COMMAND 24788 pts/2 S 000
-usr/local/bin/tcsh -i 24867 pts/2 T
001 ./forks 17 24868 pts/2 T 001
./forks 17 24869 pts/2 R 000 ps a
bassgt fg ./forks 17 lttyped ctrl-cgt linuxgt ps
a PID TTY STAT TIME COMMAND 24788
pts/2 S 000 -usr/local/bin/tcsh -i
24870 pts/2 R 000 ps a
36
Sending Signals with kill Function
void fork12() pid_t pidN int i,
child_status for (i 0 i lt N i) if
((pidi fork()) 0) while(1) / Child
infinite loop / / Parent terminates the
child processes / for (i 0 i lt N i)
printf("Killing process d\n",
pidi) kill(pidi, SIGINT) /
Parent reaps terminated children / for (i
0 i lt N i) pid_t wpid wait(child_status)
if (WIFEXITED(child_status))
printf("Child d terminated with exit status
d\n", wpid, WEXITSTATUS(child_status)) els
e printf("Child d terminated abnormally\n",
wpid)
37
Receiving Signals

Suppose kernel is returning from exception
handler and is ready to pass control to process
p.
Kernel computes pnb pending blocked
The set of pending nonblocked signals for process
p
If (pnb 0)
Pass control to next instruction in the logical
flow for p.
Else
Choose least nonzero bit k in pnb and force
process p to receive signal k.
The receipt of the signal triggers some action by
p
Repeat for all nonzero k in pnb.
Pass control to next instruction in logical flow
for p.

38
Default Actions

Each signal type has a predefined default action,
which is one of
The process terminates
The process terminates and dumps core.
The process stops until restarted by a SIGCONT
signal.
The process ignores the signal.

39
Installing Signal Handlers

The signal function modifies the default action
associated with the receipt of signal signum
handler_t signal(int signum, handler_t handler)
Different values for handler
SIG_IGN ignore signals of type signum
SIG_DFL revert to the default action on receipt
of signals of type signum.
Otherwise, handler is the address of a signal
handler
Called when process receives signal of type
signum
Referred to as installing the handler.
Executing handler is called catching or
handling the signal.
When the handler executes its return statement,
control passes back to instruction in the control
flow of the process that was interrupted by
receipt of the signal.

40
Signal Handling Example
void int_handler(int sig) printf("Process
d received signal d\n", getpid(),
sig) exit(0) void fork13() pid_t
pidN int i, child_status
signal(SIGINT, int_handler) . . .
linuxgt ./forks 13 Killing process 24973 Killing
process 24974 Killing process 24975 Killing
process 24976 Killing process 24977 Process
24977 received signal 2 Child 24977 terminated
with exit status 0 Process 24976 received signal
2 Child 24976 terminated with exit status 0
Process 24975 received signal 2 Child 24975
terminated with exit status 0 Process 24974
received signal 2 Child 24974 terminated with
exit status 0 Process 24973 received signal 2
Child 24973 terminated with exit status 0
linuxgt
41
Signal Handler Funkiness

Pending signals are not queued
For each signal type, just have single bit
indicating whether or not signal is pending
Even if multiple processes have sent this signal
See signal1.c

void handler1(int sig) pid_t pid if ((pid
waitpid(-1, NULL, 0)) lt 0)
unix_error("waitpid error") printf("Handler
reaped child d\n", (int)pid)
Sleep(2) return int main() int i, n
char bufMAXBUF if (signal(SIGCHLD, handler1)
SIG_ERR) unix_error("signal error")
for (i 0 i lt 3 i) if (Fork() 0)
printf("Hello from child d\n",
(int)getpid()) Sleep(1) exit(0)
if ((n read(STDIN_FILENO, buf,
sizeof(buf))) lt 0) unix_error("read")
printf("Parent processing input\n") while (1)
exit(0)
42
Living With Nonqueuing Signals

Must check for all terminated jobs
Typically loop with wait
See signal2.c

void handler2(int sig) pid_t pid while
((pid waitpid(-1, NULL, 0)) gt 0)
printf("Handler reaped child d\n", (int)pid)
if (errno ! ECHILD) unix_error(waitpid
error) Sleep(2) return int main()
. . if (signal(SIGCHLD, handler2) SIG_ERR)
. .
43
A Program That Reacts toExternally Generated
Events (ctrl-c)
include ltstdlib.hgt include ltstdio.hgt include
ltsignal.hgt void handler(int sig)
printf("You think hitting ctrl-c will stop the
bomb?\n") sleep(2) printf("Well...")
fflush(stdout) sleep(1) printf("OK\n")
exit(0) main() signal(SIGINT,
handler) / installs ctl-c handler / while(1)

44
Virtual Memory

Virtual memory
Motivation for VM
Address translation
Accelerating translation with TLBs

45
Motivations for Virtual Memory

Use Physical DRAM as a Cache for the Disk
Address space of a process can exceed physical
memory size
Sum of address spaces of multiple processes can
exceed physical memory
Simplify Memory Management
Multiple processes resident in main memory.
Each process with its own address space
Only active code and data is actually in memory
Allocate more memory to process as needed.
Provide Protection
One process cant interfere with another.
because they operate in different address spaces.
User process cannot access privileged information
different sections of address spaces have
different permissions.

46
Motivation 1 DRAM a Cache for Disk

Full address space is quite large
32-bit addresses
4,000,000,000 (4 billion) bytes
64-bit addresses 16,000,000,000,000,000,000 (16
quintillion) bytes
Disk storage is 400X cheaper than DRAM storage
200 GB of DRAM 40,000
200 GB of disk 110
To access large amounts of data in a
cost-effective manner, the bulk of the data must
be stored on disk

47
Levels in Memory Hierarchy
cache
virtual memory
Memory
disk
8 B
32 B
4 KB
Register
Cache
Memory
Disk Memory
size speed /Mbyte line size
32 B 1 ns 8 B
32 KB-4MB 2 ns 125/MB 32 B
1024 MB 30 ns 0.20/MB 4 KB
100 GB 8 ms 0.001/MB
larger, slower, cheaper
48
DRAM vs. SRAM as a Cache

DRAM vs. disk is more extreme than SRAM vs. DRAM
Access latencies
DRAM 10X slower than SRAM
Disk 100,000X slower than DRAM
Importance of exploiting spatial locality
First byte is 100,000X slower than successive
bytes on disk
Bottom line
Design decisions made for DRAM caches driven by
enormous cost of misses

DRAM
Disk
SRAM
49
Impact of Properties on Design

If DRAM was to be organized similar to an SRAM
cache, how would we set the following design
parameters?
Line size?
Large, since disk better at transferring large
blocks
Associativity?
High, to mimimize miss rate
Write through or write back?
Write back, since cant afford to perform small
writes to disk
What would the impact of these choices be on
miss rate
Extremely low. ltlt 1
hit time
Must match cache/DRAM performance
miss latency
Very high. 20ms
tag storage overhead
Low, relative to block size

50
Locating an Object in a Cache

SRAM Cache
Tag stored with cache line
No tag for block not in cache
Hardware retrieves information
can quickly match against multiple tags

51
Locating an Object in Cache (cont.)

DRAM Cache
Each allocated page of virtual memory has entry
in page table
Mapping from virtual pages to physical pages
Page table entry even if page not in memory
Specifies disk address
Only way to indicate where to find page
OS retrieves information

Cache
Page Table
Location
0
On Disk

1
52
A System with Physical Memory Only

Examples
most Cray machines, early PCs, nearly all
embedded systems, etc.

Addresses generated by the CPU correspond
directly to bytes in physical memory

53
A System with Virtual Memory

Examples
workstations, servers, modern PCs, etc.

Memory
Page Table
Virtual Addresses
Physical Addresses
0
1
P-1
Disk

Address Translation Hardware converts virtual
addresses to physical addresses via OS-managed
lookup table (page table)

54
Page Faults (like Cache Misses)

What if an object is on disk rather than in
memory?
Page table entry indicates virtual address not in
memory
OS exception handler invoked to move data from
disk into memory
current process suspends, others can resume
OS has full control over placement, etc.

Before fault
After fault
Memory
Memory
Page Table
Page Table
Virtual Addresses
Physical Addresses
Virtual Addresses
Physical Addresses
CPU
CPU
Disk
Disk
55
Servicing a Page Fault
(1) Initiate Block Read

Processor Signals Controller
Read block of length P starting at disk address X
and store starting at memory address Y
Read Occurs
Direct Memory Access (DMA)
Under control of I/O controller
I / O Controller Signals Completion
Interrupt processor
OS resumes suspended process

Processor
Reg
(3) Read Done
Cache
Memory-I/O bus
(2) DMA Transfer
I/O controller
Memory
disk
Disk
56
Motivation 2 Memory Management

Multiple processes can reside in physical memory.
How do we resolve address conflicts?
what if two processes access something at the
same address?

memory invisible to user code
kernel virtual memory
stack
esp
Memory mapped region For shared libraries
Linux/x86 process memory image
the brk ptr
runtime heap (via malloc)
uninitialized data (.bss)
initialized data (.data)
program text (.text)
forbidden
0
57
Solution Separate Virt. Addr. Spaces

Virtual and physical address spaces divided into
equal-sized blocks
blocks are called pages (both virtual and
physical)
Each process has its own virtual address space
operating system controls how virtual pages as
assigned to physical memory

0
Physical Address Space (DRAM)
Address Translation
Virtual Address Space for Process 1
0
VP 1
PP 2
VP 2
...
N-1
(e.g., read/only library code)
PP 7
Virtual Address Space for Process 2
0
VP 1
PP 10
VP 2
...
M-1
N-1
58
Motivation 3 Protection

Page table entry contains access rights
information
hardware enforces this protection (trap into OS
if violation occurs)

Page Tables
Memory
Process i
Process j
59
VM Address Translation

Virtual Address Space
V 0, 1, , N1
Physical Address Space
P 0, 1, , M1
M lt N
Address Translation
MAP V ? P U ?
For virtual address a
MAP(a) a if data at virtual address a at
physical address a in P
MAP(a) ? if data at virtual address a not in
physical memory
Either invalid or stored on disk

60
VM Address Translation Hit
Processor
Hardware Addr Trans Mechanism
Main Memory
a
a'
physical address
virtual address
part of the on-chip memory mgmt unit (MMU)
61
VM Address Translation Miss
page fault
fault handler
Processor
?
Hardware Addr Trans Mechanism
Main Memory
Secondary memory
a
a'
OS performs this transfer (only if miss)
physical address
virtual address
part of the on-chip memory mgmt unit (MMU)
62
VM Address Translation

Parameters
P 2p page size (bytes).
N 2n Virtual address limit
M 2m Physical address limit

n1
0
p1
p
virtual address
virtual page number
page offset
address translation
0
p1
p
m1
physical address
physical page number
page offset
Page offset bits dont change as a result of
translation
63
Page Tables
Memory resident page table (physical page or
disk address)
Virtual Page Number
Physical Memory
Valid
1
1
0
1
1
1
0
1
Disk Storage (swap file or regular file system
file)
0
1
64
Address Translation via Page Table
65
Page Table Operation

Translation
Separate (set of) page table(s) per process
VPN forms index into page table (points to a page
table entry)

66
Page Table Operation

Computing Physical Address
Page Table Entry (PTE) provides information about
page
if (valid bit 1) then the page is in memory.
Use physical page number (PPN) to construct
address
if (valid bit 0) then the page is on disk
Page fault

67
Page Table Operation

Checking Protection
Access rights field indicate allowable access
e.g., read-only, read-write, execute-only
typically support multiple protection modes
(e.g., kernel vs. user)
Protection violation fault if user doesnt have
necessary permission

68
Integrating VM and Cache

Most Caches Physically Addressed
Accessed by physical addresses
Allows multiple processes to have blocks in cache
at same time
Allows multiple processes to share pages
Cache doesnt need to be concerned with
protection issues
Access rights checked as part of address
translation
Perform Address Translation Before Cache Lookup
But this could involve a memory access itself (of
the PTE)
Of course, page table entries can also become
cached

69
Speeding up Translation with a TLB

Translation Lookaside Buffer (TLB)
Small hardware cache in MMU
Maps virtual page numbers to physical page
numbers
Contains complete page table entries for small
number of pages

70
Address Translation with a TLB
n1
0
p1
p
virtual address
virtual page number
page offset
valid
physical page number
tag
TLB
.
.
.

TLB hit
physical address
tag
byte offset
index
valid
tag
data
Cache

data
cache hit
71
Simple Memory System Example