OS Structure
•  Topics
•  Hardware protection & privilege levels
•  Control transfer to and from the operating system
•  Learning Objectives:
•  Explain what hardware protection boundaries are.
•  Explain how applications interact with the operating system
and how control flows between them.
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What makes the kernel different?
Protection Boundary
Operating System
file system
device drivers
virtual memory
HW/SW Interface
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Protection Boundaries
•  Modern hardware multiple privilege levels.
•  Different software can run with different privileges.
•  Processor hardware typically provides (at least) two
different modes of operation:
•  User mode: how all “regular programs” run.
•  Kernel mode or supervisor mode: how the OS runs.
•  Most processors have only two modes; x86 has four; some older machines
had 8!
•  The mode in which a piece of software is running
•  What instructions may be executed.
•  How addresses are translated.
•  What memory locations may be accessed (enforced through
CS161 Spring 2015
Example: Intel
•  Four protection levels
•  Ring 0: Most privileged: OS runs here
•  Rings 1 & 2:Ignored in many environments, although, can run less
privileged code (e.g., third party device drivers; possibly some parts of
virtual machine monitors)
•  Ring 3: Application code
•  Memory is described in chunks called segments
•  Each segment also has a privilege level (0 through 3)
•  Processor maintains a “current protection level” (CPL) - usually the
protection level of the segment containing the currently executing
•  Program can read/write data in segments of less privilege than CPL
•  Program cannot directly call code in segments with more privilege.
•  Program cannot directly call code in segments with more privilege.
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Example: MIPS
•  Standard two mode processor
•  User mode: access to CPU/FPU registers and flat, uniform
virtual memory address space.
•  Kernel mode: can access memory mapping hardware and
special registers.
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Changing Protection Levels
•  Must answer two fundamental questions:
•  How do we transfer control between applications and the
•  When do we transfer control between applications and the
•  How: Fundamental mechanism that transfers control
from less privileged to more privileged is called a
•  There are different kinds of traps; this gets us to the
when …
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When does the OS get to run?
•  Sleeping Beauty Approach
•  Hope that something happens to wake you up.
•  What might happen?
•  System calls: An application might want the operating system to do
something on its behalf.
•  Exceptions: An application unintentionally does something that requires
OS assistance (e.g., divide by 0, read a page not in memory).
•  Interrupts: An asynchronous event (e.g., I/O completion).
•  This isn’t sufficient to achieve fairness.
•  Alarm Clock Approach
•  Set some kind of timer that will generate an interrupt when it
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Web Work Questions!
•  Please go to the Web Work for Tuesday and answer
the first 4 questions now.
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Transferring Control
•  Regardless of why and when control must transfer to the
operating system, the mechanism is the same.
•  First, we’ll talk about what must happen in the abstract
(i.e., not in the context of any particular processor).
•  Then, we’ll step through two different hardware platforms
and examine how they transfer control.
•  Key points:
•  We can invoke the operating system explicitly via a system call.
•  The operating system can be invoked implicitly via an exception
(sometimes called a software interrupt), such as a divide by
zero, or a bad memory reference.
•  The operating system can be invoked asynchronously via
(hardware) interrupts, such as a timer, an I/O device, etc.
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Trap Handling
Each type of trap is assigned a number. For example:
1 = system call
2 = timer interrupt
3 = disk interrupt
4 = interprocessor interrupt
The operating system sets up a table, indexed by trap number, that
contains the address of the code to be executed whenever that kind of trap
These pieces of code are called “trap handlers.”
Trap handler table
Trap handler
for trap 1
Trap handler
for trap 3
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MIPS (Sys161) Trap Handling
MIPS has only 5 distinct traps and those addresses are hardwired (no
software dispatch)
One each for: reset, NMI (non-maskable interrupt),fast-TLB loading and debug
Note: Sys/161 does not support NMI or debug
One for everything else (software must then do further dispatch).
Trap handling varies according to the type of trap.
The MIPS processor has special registers that get set with vital information
at trap time. For example:
The EPC (exception program counter) tells you the address that caused the exception.
The cause register is set to a value indicating the source of the trap -- interrupt,
exception, system call, and which kind of interrupt/exception/system it was.
The status register indicates:
Mode the processor was in when the interrupt happened.
The state of which kinds of interrupts/exceptions are enabled
Return from trap handlers using a combination of a JMP instruction and an
RFE (return from exception)
Later versions have ERET (exception return)
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X86 Trap Handling
Hardware register traditionally called PIC (Programmable Interrupt
Controller), then APIC (advanced PIC) and most recently LAPIC (local
advanced PIC, one per CPU in the system)
Has wires to up to 16 devices
Maps wires to particular locations in IDT (interrupt descriptor table).
PIC sends the appropriate value for the interrupt handler dispatch to the processor.
X86 has multiple protection levels
Cannot directly call code in a different level.
So, we need a special mechanism to facilitate the transfer.
IDT: contains special objects called gates.
Gates provide access from lower privileged segments to higher privileged segments.
When a low-privilege segment invokes a gate, it automatically raises the CPL to the higher level.
When returning from a gate, the CPL drops to its original level.
First 32 gates reserved for hardware defined traps.
Remaining entries are available to software using the INT (interrupt) instruction.
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X86 System Calls
•  There are multiple ways to handle system calls and
different operating systems use different ways:
•  Linux uses a single designated INT instruction (triggers a
software interrupt) and then dispatches again within a single
handler (like MIPS).
•  Solaris uses the LCALL instruction (goes through a gate).
•  Some new Linux systems use the newer SYSENTER/
SYSEXIT calls.
•  The IRET instruction returns from the trap
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•  The operating system is just a bunch of code that sits around
waiting for something to do (e.g., help out a user process,
respond to a hardware device, process a timer interrupt, etc).
•  The operating system runs in privileged mode.
•  Hardware provides some sort of mechanism to transfer control
from one privilege level to another.
•  We use the term trap to refer to any mechanism that transfers
control into the operating system.
•  There are different kinds of traps:
•  Interrupts (caused by hardware; asynchronous)
•  Exceptions (software interrupts; synchronous with respect to programs)
•  System calls: intentional requests of the operating system on behalf of
a program; synchronous with respect to the program)
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Web Work Questions!
•  Please go back to the web work and answer the next
2 questions.
CS161 Spring 2015