Download Week 5 - Portland State University

OMSE 510: Computing
Foundations
5: The Operating System
Chris Gilmore <[email protected]>
Portland State University/OMSE
1
Today
The Operating System!
Back to talking about software!
Overview
Process/Kernel Abstraction
Timeslicing
Process Scheduling
2
Computer System (Idealized)
Disk
CPU
Memory
Disk
Controller
System Bus
3
What is an operating
system?
Operating system --“is a program that
controls the execution of application
programs and acts as an interface
between the user of a computer and the
computer hardware”

Narrow view


Traditional computer with applications running
on it (e.g. PCs, Workstations, Servers)
Broad view

Anything that needs to manage resources (e.g.
4
router OS, embedded devices, pagers, ...)
Two key OS functions
Abstract Machine



Hide details of the underlying hardware
Provide “common” API to applications and
services
Simplifies application writing
Resource Manager

Controls accesses to “shared” resources


CPU, memory, disks, network, ...
Allows for “global” policies to be implemented
5
Why is abstraction important?
Without OSs and abstract interfaces application
writers program all device access directly





load device command codes into device registers
handle initialization, recalibration, sensing, timing etc
for physical devices
understand physical characteristics and layout
control motors
interpret return codes … etc
Applications suffer severe code bloat!


very complicated maintenance and upgrading
writing this code once, and sharing it, is how OS
began!
6
Providing abstraction
via system calls
Application
Operating
System
CPU
Hardware
Video Card
Monitor
Memory
Network
Disk
Printer
7
Providing abstraction
via system calls
Application
System Calls: read(), open(), write(), mkdir(), kill() ...
Operating
System
Process Mgmt
Device Mgmt
File System
Network Comm.
Security
Protection
CPU
Hardware
Video Card
Monitor
Memory
Network
Disk
Printer
8
OS as a resource manager
Sharing resources among applications
across space and time
scheduling
 allocation

Making efficient use of limited resources
improving utilization
 minimizing overhead
 improving throughput/good put

Protecting applications from each other

enforcement of boundaries
9
Problems an OS must
solve
Time sharing the CPU among
applications
Space sharing the memory among
applications
Space sharing the disk among users
Time sharing access to the disk
Time sharing access to the network
10
More problems an OS must solve
Protection
of applications from each other
 of user data from other users
 of hardware/devices
 of the OS itself!

The OS needs help from the hardware
to accomplish these tasks!
11
Overview of computer
system layers
Hardware - CPU, memory, I/O devices - disk, network ...
12
The OS is just a program!
How can the OS cause application
programs to run?
How can applications programs cause
the OS to run?
How can the OS switch the CPU to run
a different application and later resume
the first one?
How can the OS maintain control?
In what ways can application code try to
cheat?
And how can the OS stop the cheating?13
How can the OS invoke an application?
The computer boots and begins running
the OS

fetch/decode/execute OS instructions
OS can request user input to identify an
application to run
OS loads the address of the application’s
starting instruction into the PC
 CPU fetches/decodes/executes the
application’s instructions

14
How can applications invoke
the OS?
Trap instruction changes PC to point to
an OS entry point instruction
application calls a library procedure that
includes the appropriate trap instruction
 fetch/decode/execute cycle begins at a
specified OS entry point called a system
call

15
How can the OS run a
new application?
To suspend execution of an application
simply capture its memory state and
processor state
copy values of all registers into a data
structure and save it to memory
 preserve the memory values of this
application so it can be restarted later

16
How can OS guarantee
to regain control?
What if a running application doesn’t make a
system call and hence hogs the CPU?
timer interrupts!
 OS must register a future timer interrupt before it
hands control of the CPU over to an application

How can the OS avoid trampling on the
processor state it wants to save?

carefully written interrupt handlers!
17
What if the application
tries to cheat?
What stops the running application from
disabling the future timer interrupt so
that the OS can not regain control?

the mode bit (in the PSW)!
Certain instructions can only be
executed when the mode bit is set
manipulating timer interrupts
 setting the mode bit!
 ...

18
What other ways are
there to cheat?
What stops the running application from
modifying the OS?
Memory protection!
 can only be set with mode bit set ….

The OS must clear the mode bit before
it hands control to an application!

interrupts and trap instructions set the
mode bit and transfer control to specific
locations (in the OS)
19
Why its not quite that
simple ...
Pipelined CPUs
Superscalar CPUs
Multi-level memory hierarchies
Virtual memory
Complexity of devices and buses
Heterogeneity of hardware
20
Pipelined CPUs
Fetch
unit
Decode
unit
Execute
unit
Execution of current instruction performed in parallel
with decode of next instruction and fetch of the one
after that
21
Superscalar CPUs
Fetch
unit
Execute
unit
Decode
unit
Holding
buffer
Fetch
unit
Decode
unit
Execute
unit
Execute
unit
22
What does this mean for the OS?
Pipelined CPUs
more complexity in capturing state of a running
application
 more expensive to suspend and resume
applications

Superscalar CPUs
even more complexity in capturing state of a
running application
 even more expensive to suspend and resume
applications

More details, but fundamentally the same task
23
Terminology review metric units
The metric prefixes
24
The memory hierarchy
2GHz processor  0.5 ns
Data/inst. cache  0.5ns – 10 ns, 64 kB1MB
(this is where the CPU looks first!)
Main memory  60 ns,
512 MB – 1GB
Magnetic disk 
10 ms, 160 Gbytes
Tape  Longer than you want,
less than magnetic disk!
25
Who manages the memory hierarchy?
Movement of data from main memory to
cache is under hardware control


cache lines loaded on demand automatically
replacement policy fixed by hardware
Movement of data from cache to main
memory can be affected by OS


instructions for “flushing” the cache
can be used to maintain consistency of main
memory
Movement of data among lower levels of the
memory hierarchy is under direct control of
the OS


virtual memory page faults
file system calls
26
OS implications of a memory hierarchy?
How do you keep the contents of memory
consistent across layers of the hierarchy?
How do you allocate space at layers of the
memory hierarchy “fairly” across different
applications?
How do you hide the latency of the slower
subsystems?



Main memory… yikes!
Disk
Tape
How do you protect one application’s area of
memory from other applications?
How do you relocate an application in
memory?
27
Summary/Quiz
How does the OS solve these problems:
Time sharing the CPU among
applications?
 Space sharing the memory among
applications?
 Space sharing the disk among users?
 Time sharing access to the disk?
 Time sharing access to the network?
 Protection of applications from each other?
 Protection of user data from other users?
 Protection of hardware/devices?
 Protection of the OS itself?
28

Section overview
OS-Related Hardware & Software
Memory protection and relocation
Virtual memory & MMUs
I/O & Interrupts
Processes
Process scheduling
Process states
Process hierarchies
Process system calls in Unix
29
Memory protection and relocation ...
Memory protection (basic ideas)

virtual vs physical addresses



address range in each application starts at 0
“base register” used to convert each virtual
address to a physical address before main
memory is accessed
address is compared to a “limit register” to keep
memory references within bounds
Relocation

by changing the base register value
Paged virtual memory

same basic concept, but more powerful (and
complex)
30
Base & Limit Registers
(single & multiple)
31
Virtual memory and MMUs
Memory management unit (MMU)




hardware provided equivalent of base registers
at the granularity of “pages” of memory, say
2kB, i.e., lots of them!
supports relocation at page granularity
applications need not occupy contiguous
physical memory
Memory protection


limit registers don’t work in this context
per-page and per-application protection registers
Relocation and protection occur at CPU
speeds!
32
What about I/O devices?
Monitor
Bus
A simplified view of a computer system
33
Structure of a large
Pentium system
34
How do programs interact with
devices?
Devices vs device controllers vs device
drivers


device drivers are part of the OS
programs call the OS which calls the device
driver
Device drivers interact with device controllers


either using special IO instructions
or by reading/writing controller registers that
appear as memory locations
Why protect access to devices by accessing
them indirectly via the OS?
35
How do devices interact
with programs?
Interrupts
36
Different types of interrupts
Timer interrupts
Allows OS to maintain control
 One way to keep track of time

I/O interrupts

Keyboard, mouse, disks, etc…
Hardware failures
Program generated (traps)
Programming errors: seg. faults, divide by zero, etc.
 System calls like read(), write(), gettimeofday()

37
Timer interrupts
OS can ask timer device to interrupt after a
specified time period has elapse
Interrupt invokes timer interrupt handler which
invokes OS “scheduler”
OS can take the opportunity to save the
current application and restore a different one

context switch
38
Why use traps for system calls?
The Operating System is just a program!
It must have the privilege to manipulate the
hardware
 set base and limit registers for memory
protection
 access devices
 set and clear mode bit to enable privilege
If user programs execute with the mode bit
clear, and do not have privilege to set it, how
can they invoke the OS so that it can run with
the mode bit set?
 That’s what traps do … set the mode bit
and begin execution at a specific point in
memory (in the OS!)
39
System calls
System calls are the mechanism by
which programs communicate with the
O.S.
Implemented via a TRAP instruction
Example UNIX system calls:
open(), read(), write(), close()
kill(), signal()
fork(), wait(), exec(), getpid()
link(), unlink(), mount(), chdir()
setuid(), getuid(), chown()
40
The inner workings of a
system call
User-level code
Process usercode
{
...
read (file, buffer, n);
...
}
Library code
Procedure read(file, buff, n)
{
...
read(file, buff, n)
...
}
_read:
LOAD
LOAD
LOAD
TRAP
r1, @SP+2
r2, @SP+4
r3, @SP+6
Read_Call
41
Steps in making a
system call
42
What about disks and
file storage?
Structure of a disk drive
43
Disks and file storage
Manipulating the disk device is complicated

hide some of the complexity behind disk controller,
disk device driver
Disk blocks are not a very user-friendly
abstraction for storage
contiguous allocation may be difficult for large data
items
 how do you manage administrative information?

One application should not (automatically) be
able to access another application’s storage

OS needs to provide a “file system”
44
File systems
File system - an abstraction above disk
blocks
45
What about networks?
Network interfaces are just another kind
of shared device/resource
Need to hide complexity
send and receive primitives, packets,
interupts etc
 protocol layers

Need to protect the device

access via the OS
Need to allocate resources fairly

packet scheduling
46
The Process Concept
Process – a program in execution

Program



description of how to perform an activity
instructions and static data values
Process




a snapshot of a program in execution
memory (program instructions, static and dynamic
data values)
CPU state (registers, PC, SP, etc)
operating system state (open files, accounting
statistics etc)
47
Process address space
Each process runs in its own virtual memory address space
that consists of:



Stack space – used for function and system calls
Data space – variables (both initialized and uninitialized)
Text – the program code (usually read only)
stack
Address
space
data
text
Invoking the same program multiple times results in the
creation of multiple distinct address spaces
48
Running a process on a CPU
In its simplest form, a computer
performs instructions on operands.
Registers are used to hold values
temporarily to speed things up
CPU
Memory
ADD R1, R2
Program
Code
Program
Data
ALU
SP PC
49
Switching among multiple processes
Saving all the information about a process
allows a process to be temporarily
suspended
CPU
Memory
Program
Code
ALU
Program
Data
SP PC
Program 1 has CPU
Program
State
50
Switching among multiple processes
Saving all the information about a process
allows a process to be temporarily
suspended
CPU
Memory
Program
Code
Program
Data
Program
Code
SUB R1, R2
ALU
Program
Data
SP PC
Program 2 has CPU
Program
State
51
Switching among multiple processes
Saving all the information about a process
allows a process to be temporarily
suspended
CPU
Memory
Program
Code
Program
Data
Program
Code
ALU
Program
Data
SP PC
Program 2 has CPU
Program Program
State
State
52
Switching among multiple processes
Saving all the information about a process
allows a process to be temporarily
suspended
CPU
Memory
Program
Code
Program
Data
Program
Code
ALU
Program
Data
SP PC
Program 1 has CPU
Program Program
State
State
53
Switching among multiple processes
Saving all the information about a process
allows a process to be temporarily
suspended
CPU
Memory
Program
Code
Program
Data
Program
Code
ADD R1, R3
ALU
Program
Data
SP PC
Program 1 has CPU
Program Program
State
State
54
Why use the process
abstraction?
Multiprogramming of four programs in the same
address space
Conceptual model of 4 independent, sequential
processes
Only one program active at any instant
55
The role of the scheduler
Lowest layer of process-structured OS

handles interrupts & scheduling of
processes
Above that layer are sequential
processes
56
Process states
Possible process states
running
 blocked
 ready

57
Implementation of
process switching
Skeleton of what the lowest levels of the OS
do when an interrupt occurs
58
How do processes get
created?
Principal events that cause process
creation
System initialization
Initiation of a batch job
User request to create a new process
Execution of a process creation
system call from another process
59
Process hierarchies
Parent creates a child process,
special system calls for communicating with
and waiting for child processes
 each process is assigned a unique
identifying number or process ID (PID)

Child processes can create their own
child processes
Forms a hierarchy
 UNIX calls this a "process group"

Windows has no concept of process
hierarchy

all processes are created equal
60
How do processes
terminate?
Conditions which terminate processes
Normal exit (voluntary)
Error exit (voluntary)
Fatal error (involuntary)
Killed by another process (involuntary)
61
Process creation in UNIX
All processes have a unique process id

getpid(), getppid() allow processes to get their
information
Process creation



fork() creates a copy of a process with the lone
exception of the return value of fork()
exec() replaces an address space with a new
program
system() like CreateProcess()
Process termination, signaling

signal(), kill() allows a process to be terminated
or have specific signals sent to it
62
Example: process
creation in UNIX
csh (pid = 22)
…
pid = fork()
if (pid == 0) {
// child…
…
exec();
}
else {
// parent
wait();
}
…
63
Process creation in
UNIX example
csh (pid = 22)
csh (pid = 24)
…
…
pid = fork()
if (pid == 0) {
// child…
…
exec();
}
else {
// parent
wait();
}
…
pid = fork()
if (pid == 0) {
// child…
…
exec();
}
else {
// parent
wait();
}
…
64
Process creation in
UNIX example
csh (pid = 22)
csh (pid = 24)
…
…
pid = fork()
if (pid == 0) {
// child…
…
exec();
}
else {
// parent
wait();
}
…
pid = fork()
if (pid == 0) {
// child…
…
exec();
}
else {
// parent
wait();
}
…
65
Process creation in
UNIX example
csh (pid = 22)
csh (pid = 24)
…
…
pid = fork()
if (pid == 0) {
// child…
…
exec();
}
else {
// parent
wait();
}
…
pid = fork()
if (pid == 0) {
// child…
…
exec();
}
else {
// parent
wait();
}
…
66
Process creation in
UNIX example
csh (pid = 22)
ls (pid = 24)
…
//ls program
pid = fork()
if (pid == 0) {
// child…
…
exec();
}
else {
// parent
wait();
}
…
main(){
//look up dir
…
}
67
What other process state
does the OS manage?
Fields of a process table entry
68
What about the OS?
Is the OS a process?
It is a program in execution, after all …
Does it need a process control block?
Who manages its state when its not running?
69