Download Kernel

제03강 : Introduction to Kernel
UNIX Kernel
Christian, The UNIX Operating System, 2nd Ed, Wiley.
Silberschatz, Galvin & Gagne, Applied Operating System Concepts,
John Wiley & Sons Inc.
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• Kernel
– memory resident part of UNIX
– majority written in C
rest
in assembler language
(HW dependent, speed)
– a.out (a plain C a.out program)
– consists of functions
• other programs can call (some of) these functions
• called “system call function”
– 3 parts
• process management
• file system
• I/O system
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1. Instruction
2. function
3. process (a.out)
Kernel is the first
kernel
a.out
a.out
3
to be loaded into memory
1. Instruction
2. function
3. process (a.out)
sh
a.out
Terminal Power-on
Kernel loads sh a.out
for this terminal
as its child
kernel
Now two processes are active
a.out
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Multi-user System -- multiple shells
User A:
sh
a.out
User B:
sh
a.out
Second terminal -- power on
Kernel creates sh for this terminal
------
kernel
Three processes active now
sh
sh
kernel
a.out
Only one CPU though.
Who’s running on CPU NOW?
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Shell creates a child process
and waits for it:
A:
sh
<sleep>
a.out
vi
a.out
User B:
User types a command (a.out)
on terminal B
csh
Sh creates command process
as its child
sh
<sleep>
a.out
creates
csh
a.out
<run>
kernel
a.out
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• Multi-user system -- Protection
P1
P2
P1’s
Data
P2’s
Data
– what if process PA (bug or virus, …)
illegally accesses PB ’s information? (read/write)
– Shall we detect it and recover from it (after 抹消)?
– No we should prevent it (豫防 before happening).
– Private Information -- stored in
• memory
• disk
Access to these should be 事前 統制
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Protection
Between Process Domain
“domain” : {memory, files, CRT, ...}
sh
a.out
csh
a.out
Allow sh
to do disk I/O?
Allow any a.out to do disk I/O?
How can you trust them
(Allow them to do disk I/O instruction?)
No I/O instructions allowed except kernel
Must ask kernel to do disk I/O via system call
kernel checks access right first
then do I/O for them
kernel
a.out
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1. Instruction
2. function
3. process (a.out)
--> “domain” : memory, files, CRT, ...
other
a.out
kernel
a.out
I/O not allowed here
So ask kernel
System
calls
(依賴)
Kernel has functions
To do I/O for you
(代行)
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CPU
Memory
1.
Address
PC
Control
Unit
ALU
unit
(instruction)
MBR
IR
R0
...
R7
MAR
2.
Return
Content
ALU
mode
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CPU
Memory
4. Address
(operand)
PC
Control
Unit
IR
MAR
MBR
3. Op-code
i
ALU
unit
R0
...
R7
mode bit
j
ALU
op-code
add
operands
i
j
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CPU “mode bit (CPU-現在役割-bit)”
CPU mode
kernel
memory
op-code
access any (全域)
execute any
mine only (局地)
restricted *
(全體 權限)
user
(部分 權限)
No I/O instruction
* privileged op-code No special-register access
(特殊 命令語?)
any thing that can harm others
(惡影響)
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instruction
memory
[Kernel mode]
Add
Sub
Disk write
Disk read
Tape write
Disable interrupt
Enable interrupt
[User mode]
a.out
Add
Sub
Disk write
Disk read
Tape write
Disable interrupt
Enable interrupt
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• CPU_mode_bit :
–
–
–
–
SW need HW’s help to “prevents” illegal action
one HW bit in CPU (usually part of PSW)
machine instruction (SW) can read/write this
Access to mode_bit is privileged op-code
Machine Instruction Cycle:
PC to memory
Instruction Fetch
CPU:
Decode (解讀)
Execute
“CPU-現在役割-bit”
< if mode_bit = user >
Monitor(檢閱) address to memory.
Stop this if address is outside a.out scope
while CPU is in user_mode (trap)
Monitor (檢閱) op-code
Stop this if privileged op-code is attempted
while CPU is in user_mode (trap)
Increment PC
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Control
Unit
ALU
unit
R0
...
R7
CPU
Memory
PC
MAR
IR
MBR
ALU
mode
“CPU-現在役割-bit”
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• When my program runs, CPU mode bit = user_mode
– cannot execute I/O instruction
– cannot access special registers
– cannot access memory outside current a.out
• When OS kernel runs, CPU mode bit = kernel_mode
– no restriction at all
…. Wait a minute. ….
My program runs in user_mode
read();
write();
How did my program handle it?
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• My program Source:
“read next byte from disk file X into my variable Y”
• Binary:
“prepare all parameters (for disk read)”
“execute chmodk” instruction”
/* Yes, disk I/O instructions (read, write functions)
are not included in my a.out
Instead they are kept in kernel
My a.out has to call those functions at run time
This is all done by (compiler, OS and HW) */
My a.out
invoke
read()
write()
invoke
Kernel a.out
read:
I/O
write:
I/O
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• At run time (part I: hardware)
–
–
–
–
my program executes “chmodk”
this is privileged instruction
CPU cannot continue (in user_mode)
HW trap
• HW saves CPU state vector (including return address)
• HW sets CPU_mode_bit <== kernel mode
• HW jumps to trap handling routine (in kernel a.out)
My a.out
(read para)
chmodk
Kernel a.out
trap
To kernel_mode
trap:
read:
I/O
“CPU-現在役割-bit”
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• Run time (part I: software)
– Now trap handler (in kernel a.out) starts
– inspects what caused trap
• system call, divide by zero, memory bound, ….?
– invoke appropriate kernel function (system call)
– All done? …. CPU_mode <== user_mode
–
restore state vector
– return to interrupted location in “my a.out”
My a.out
Kernel a.out
(read para)
chmodk
trap
trap:
read:
user_mode
“CPU-現在役割-bit”
I/O
Kernel read( ) checks
permission first
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Main Memory
P1
a.out
(1) P1 is executing in user mode”
P1 invokes a kernel function
(system call)
P2
a.out
P3
a.out
(2)
kernel
a.out
causes HW trap (in kernel mode)
call appropriate kernel function
kernel returns to P1’s a.out
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Main Memory
P1
a.out
P2
We say
“P1 is running in user mode”
calling functions in P1 a.out
using the stack in P1.a.out
a.out
P3
a.out
kernel
a.out
We say
“P1 is running in kernel mode”
calling functions in kernel a.out
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using the stack in kernel a.out
Main Memory
User
stack
Kernel stack
User
stack
Kernel stack
P1
a.out
P2
a.out
P3
a.out
kernel
a.out
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Program execution
CPU mode
現在役割-bit
P1 a.out
Kernel a.out
P1 a.out
user
mode
kernel
mode
user
mode
Kernel
Service
My own
code
My own
code
System
call
Program
begins
Return
from
Kernel
To my
a.out
kernel
mode
System
call
Program
ends
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OS Kernel
(plain C program with variables and functions)
Process 1
Process 2
Process 3
PCB
PCB
PCB
CPU
mem
disk
tty
CPU
mem
disk
tty
: Table (Data Structure)
: Object (hardware or software)
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User a.out
Kernel a.out
Process 1
PCB
CPU
Process 2
Process 3
PCB
PCB
mem
disk
tty
Hardware
CPU
mem
disk
tty
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•
•
•
•
•
•
•
Kernel a.out can access any memory location
That includes your program’s main( ), stack.
Kernel can push any values into your stack
They become parameters to main( )
Examples --- main(argv, envp)
Kernel can call any function in memory
Kernel sets CPU user mode  calls your main( )
memory
[User mode]
[Kernel mode]
a.out
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Second shell runs:
User A:
sh
a.out
cat ch1 ch2 > ch12
cat
User B:
sh
kernel
sh
sh
a.out
vi
cat
kernel
a.out
<sleep>
<run>
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sh (B)
when
child
finish
main()
{scanf( )
$PATH, search files
load cat (a.out) file
initialize it
put it into ready queue
wait system call(sleep)
printf(prompt)
go to top
Read from the standard input file
cat ch1 ch2 > ch12
printf() -- output to screen
scanf() – read from keyboard
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sh (B)
when
child
finish
main()
{scanf( )
$PATH, search files
load cat (a.out) file
initialize it
put it into ready queue
wait system call(sleep)
printf(prompt)
go to top
}
Read from the standard input file
cat ch1 ch2 > ch12
printf() -- output to screen
scanf() – read from keyboard
takes 1st word (which is cat)
from disk (command - /bin)
push (env, arg) to stack of cat
cat is now in ready-queue
sh goes to sleep – cat runs
child
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OS Kernel
(plain C program with variables and functions)
Process 1
Process 2
Process 3
Process 4
Process 5
PCB
PCB
PCB
PCB
PCB
ready queue
disk I/O queue
CPU
mem
disk
tty
CPU
mem
disk
tty
: Table (Data Structure)
: Object (hardware or software)
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cat ch1 ch2 > ch12
Passed via Stack
sh
child
main()
{scanf(eg cat ch1 ch2 > ch12)
$PATH, search files
load a.out file
main(argv, argc, envp)
{
any program
intialize stack
(argv, argc, envp)
put it into ready queue
sleep (wait system call)
printf(prompt)
go to top
}
}
/* CPU goes back to kernel
kernel wakes up parent
parent proceeds
*/
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