Download System Calls

System Calls, Interrupts and
Exceptions
What is an operating system
• The first program
• A program that lets you run other programs
• A program that provides controlled access to resources:
– CPU
– Memory
– Display, keyboard, mouse
– Persistent storage
– Network
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
Operating System Structure
www.cs rutgers.edu/~pxk
kernel
• Core component of the operating
system: the central program that
manages resources and scheduling
– Controls execution of programs
– Schedules
– Allocates memory
– Allows programs to controlled access to
devices
Privilege Levels
• Some processor functionality cannot be made
accessible to untrusted user applications
– e.g. HALT, Read from disk, set clock, reset devices,
manipulate device settings, …
• Need to have a designated mediator between
untrusted/untrusting applications
– The operating system (OS)
• Need to delineate between untrusted applications
and OS code
– Use a “privilege mode” bit in the processor
– 0 = Untrusted = user, 1 = Trusted = OS
Privilege Mode
• Privilege mode bit indicates if the current program
can perform privileged operations
– On system startup, privilege mode is set to 1, and the
processor jumps to a well-known address
– The operating system (OS) boot code resides at this
address
– The OS sets up the devices, loads applications, and resets
the privilege bit before invoking the application
• Applications must transfer control back to OS for
privileged operations
Hardware Support:
Dual-Mode Operation
• Kernel mode
– Execution with the full privileges of the hardware
– Read/write to any memory, access any I/O device,
read/write any disk sector, send/read any packet
• User mode
– Limited privileges
– Only those granted by the operating system kernel
Am I in user mode?
• Processor instructions available only to
privileged programs (like OS)
• Status register usually contains
information about privilege
–
Execution: User Mode vs Kernel
Mode
• Kernel mode
= privileged, system, supervisor
mode
–
–
–
–
–
Access restricted regions of memory
Modify the memory management unit
Set timers
Define interrupt vectors
Halt the processor
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
Context switch between usermode and kernel
Mode Switch
• From user-mode to kernel
– Interrupts
• Triggered by timer and I/O devices
– Exceptions
• Triggered by unexpected program behavior
• Or malicious behavior!
– System calls (aka protected procedure call)
• Request by program for kernel to do some operation on
its behalf
• Only limited # of very carefully coded entry points
Mode Switch
• From kernel-mode to user
– New process/new thread start
• Jump to first instruction in program/thread
– Return from interrupt, exception, system call
• Resume suspended execution
– Process/thread context switch
• Resume some other process
– User-level upcall
• Asynchronous notification to user program
How do I get to kernel mode
Interrupt Vector Table
• Configured by kernel at boot time
• Depending on architecture
– Code entry points
• Control jumps to an entry in the table based on trap number
• Table will contain a set of JMP instructions to different Handlers in
the kernel
– List of addresses
• Each entry contains a structure that defines the target address &
privilege level
• –Table will contain a set of addresses for different handlers in the
kernel
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
Interrupt Vector
• Table set up by OS kernel; pointers to code to
run on different events
Flow of control
• Each interrupt has a number associated
that is an index into the interrupt
vector table
• Table also has address of the code to
handle that specific interrupt
Flow of control
• Save processors registers
• Set up for execution in the kernel
– Choose a place the for the kernel to start
executing
• Retrieve information about the event
• Transfer the control back to the user
Flow of Control
• User-programmed interrupt instruction
• The instruction forces the program to
jump to a well-known address based on
the number of the interrupt.
Interrupt Management
• Interrupt controllers manage interrupts
– Maskable interrupts: can be turned off by the CPU for
critical processing
– Nonmaskable interrupts: signifies serious errors (e.g.
unrecoverable memory error, power out warning, etc)
• Interrupts contain a descriptor of the interrupting
device
– A priority selector circuit examines all interrupting devices,
reports highest level to the CPU
• Interrupt controller implements interrupt priorities
– Can optionally remap priority levels
Interrupt Masking
• Interrupt handler runs with interrupts off
– Reenabled when interrupt completes
• OS kernel can also turn interrupts off
– Eg., when determining the next process/thread to
run
– If defer interrupts too long, can drop I/O events
Interrupt Handlers
• Non-blocking, run to completion
– Minimum necessary to allow device to take next
interrupt
– Any waiting must be limited duration
– Wake up other threads to do any real work
• Pintos: semaphore_up
• Rest of device driver runs as a kernel thread
– Queues work for interrupt handler
– (Sometimes) wait for interrupt to occur
At end of handler
• Handler restores saved registers
• Atomically return to interrupted
process/thread
– Restore program counter
– Restore program stack
– Restore processor status word/condition codes
– Switch to user mode
Exceptional Situations
•
System calls are control transfers to the OS, performed under the control of the
user application
•
Sometimes, need to transfer control to the OS at a time when the user program
least expects it
–
–
–
–
Division by zero,
Alert from the power supply that electricity is about to go out,
Alert from the network device that a packet just arrived,
Clock notifying the processor that the clock just ticked,
•
Some of these causes for interruption of execution have nothing to do with the
user application
•
Need a (slightly) different mechanism, that allows resuming the user application
Before
After
Interrupt Stack
• Per-processor, located in kernel (not user)
memory
– Usually a thread has both: kernel and user stack
• Why can’t interrupt handler run on the stack
of the interrupted user process?
Interrupt Stack
Context switch
System Calls
System Calls
• Sole interface between user and kernel
• Implemented as library routines that
execute trap instructions to enter kernel
• Errors indicated by returns of –1; error
code is in errno
if (write(fd, buffer, bufsize) == –1) {
// error!
printf("error %d\n", errno);
// see perror
}
System Calls: Interacting with the
OS
• A system call is a way for a user program to request services from the
operating system
– The operating system remains in control of devices
– Enforces policies
• Use trap mechanism to switch to the kernel
– User ↔ Kernel mode switch: Mode switch
– Note: most architectures support an optimized trap for system calls
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
System Calls: Interacting with the
OS
• Use trap mechanism to switch to the kernel
• Pass a number that represents the OS service (e.g., read)
– System call number; usually set in a register
• A system call does the following:
– Set the system call number
– Save parameters
– Issue the trap (jump to kernel mode)
• OS gets control
• Saves registers, does the requested work
• Return from exception (back to user mode)
– Retrieve results and return them to the calling function
• System call interfaces are encapsulated as library functions
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
System Calls
other stuff
Kernel portion
of address space
kernel stack
kernel text
trap into kernel
User portion of
address space
write(fd, buf, len)
System Calls (1)
Figure 1-17. The 11 steps in making the system call
read(fd, buffer, nbytes).
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
System Calls (2)
Figure 1-18. Some of the major POSIX system calls. The return code s
is −1 if an error has occurred. The return codes are as follows: pid is a
process id, fd is a file descriptor, n is a byte count, position is an offset
within the file, and seconds is the elapsed time.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
System Calls (3)
Figure 1-18. Some of the major POSIX system calls. The return code s
is −1 if an error has occurred. The return codes are as follows: pid is a
process id, fd is a file descriptor, n is a byte count, position is an offset
within the file, and seconds is the elapsed time.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
System Calls (4)
Figure 1-18. Some of the major POSIX system calls. The return code s
is −1 if an error has occurred. The return codes are as follows: pid is a
process id, fd is a file descriptor, n is a byte count, position is an offset
within the file, and seconds is the elapsed time.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
System Calls (5)
Figure 1-18. Some of the major POSIX system calls. The return code s
is −1 if an error has occurred. The return codes are as follows: pid is a
process id, fd is a file descriptor, n is a byte count, position is an offset
within the file, and seconds is the elapsed time.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
System Calls for Process
Management
Figure 1-19. A stripped-down shell. Throughout this book,
TRUE is assumed to be defined as 1.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
The Windows Win32 API (1)
Figure 1-23. The Win32 API calls that roughly correspond to
the UNIX calls of Fig. 1-18.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
The Windows Win32 API (2)
Figure 1-23. The Win32 API calls that roughly correspond to
the UNIX calls of Fig. 1-18.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
Operating System Structure
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
Monolithic Systems (1)
Basic structure of OS
1. A main program that invokes the requested
service procedure.
2. A set of service procedures that carry out the
system calls.
3. A set of utility procedures that help the
service procedures.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
Microkernels
Figure 1-26. Simplified structure of the
MINIX 3 system.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
Client-Server Model
Figure 1-27. The client-server model over a network.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
Virtual Machines
Figure 1-28. The structure of VM/370 with CMS.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
Virtual Machines Rediscovered
Figure 1-29. (a) A type 1 hypervisor. (b) A pure type 2
hypervisor. (c) A practical type 2 hypervisor.
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.
Virtual Machines
Virtual Machines
• A virtual machine takes the layered approach
to its logical conclusion. It treats hardware
and the operating system kernel as though
they were all hardware
• A virtual machine provides an interface
identical to the underlying bare hardware
• The operating system host creates the
illusion that a process has its own processor
and (virtual memory)
• Each guest provided with a (virtual) copy of
underlying computer
Virtual Machine
User-Level Virtual Machine
• How does VM Player work?
– Runs as a user-level application
– How does it catch privileged instructions, interrupts,
device I/O, …
• Installs kernel driver, transparent to host kernel
–
–
–
–
Requires administrator privileges!
Modifies interrupt table to redirect to kernel VM code
If interrupt is for VM, upcall
If interrupt is for another process, reinstalls interrupt
table and resumes kernel
End
Chapter 1
Tanenbaum & Bo, Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved.