Transcript Fundamental Concepts

Module 1.2: Fundamental Concepts
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Interrupts
System Calls
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Operating Systems
Dual-Mode Operation
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Sharing system resources requires operating system to ensure
that an incorrect program cannot cause other programs to
execute incorrectly.
Provide hardware support to differentiate between at least two
modes of operations.
1. User mode – execution done on behalf of a user.
2. Monitor mode (also supervisor mode or system mode) –
execution done on behalf of operating system.
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Operating Systems
Dual-Mode Operation (Cont.)
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Mode bit added to computer hardware to indicate the current mode:
monitor (0) or user (1).
– Part of EFLAG in x86 architecture
– Part of PSW in Motorola architecture
When an interrupt or fault occurs hardware switches to monitor mode.
Interrupt/fault
monitor
user
set user mode
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Privileged instructions can be issued only in monitor mode.
All I/O instructions are privileged instructions.
Must ensure that a user program could never gain control of the
computer in monitor mode (I.e., a user program that, as part of its
execution, traps if it executes a privileged instruction).
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Operating Systems
Interrupts (fundamental concept)
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An interruption of the normal processing of processor.
Interrupts are a mechanism for causing the CPU to suspend its current
computation and take up some new task. Control may be returned to
the original task at some time later.
Reasons for interrupts (or traps):
– control of asynchronous I/O devices
– CPU scheduling
– exceptional conditions (e.g., div. by zero, page fault, illegal
instruction) arising during execution
– user process requests for OS services
Interrupts are essentially what drives an OS. We can view the OS as
an event-driven system, where an interrupt is an event.
By their very nature, interrupts need to be serviced carefully and quickly
by the OS.
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Operating Systems
Interrupt Handling
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The servicing of an interrupt is known as interrupt handling.
An integer is associated with each type of interrupt. When an interrupt
occurs, the corresponding integer is supplied to the OS usually by the
hardware (in a register).
The OS maintains a table, known as the interrupt vector, that associates
each interrupt's id with the starting address of its service routine.
Example interrupt vector:
Interrupt No.
0
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Interrupt Handler
clock
disk
tty
dev
soft
other
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Operating Systems
Typical interrupt handling sequence
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Interrupt initiated by: I/O device signaling CPU, exceptional
condition arising, execution of special instruction, etc.
CPU suspends execution of current instruction stream and
saves the state of the interrupted process (on hardware stack).
State typically refers to contents of registers: PC, PSW, SP,
general-purpose registers.
The cause of the interrupt is determined (and the unit no. of the
interrupt, if applicable) and the interrupt vector is consulted in
order to transfer control to the appropriate interrupt handler.
Interrupt handler performs whatever processing is necessary to
deal with the interrupt.
Previous CPU state is restored (popped) from system stack,
and CPU returns control to interrupted task.
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Operating Systems
EXAMPLE: Servicing a Timer Interrupt
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Timer device is used in CPU scheduling to make sure control is
returned to system every so often (e.g., 1/60 sec.)
Typically, timer has a single register that can be loaded with an
integer indicating a particular time delay (# of ticks).
Once loaded, timer counts down and when 0 is reached, an
interrupt is generated.
Interrupt handler might do the following:
– update time-of-day information
– signal any processes that are "asleep" and awaiting this
alarm
– call the CPU scheduler
Control returns to user mode, possibly to a different process
than the one executing when the interrupt occurred.
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Operating Systems
EXAMPLE: Servicing a Disk Interrupt
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When disk controller completes previous transfer, it generates an
interrupt.
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Interrupt handler changes the state of a process that was waiting
for just-completed transfer from wait-state to ready-state.
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It also examines queue of I/O requests to obtain next request.
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I/O is initiated on next request.
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CPU scheduler called.
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Control returned to user mode.
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Operating Systems
Priority Interrupts
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Some fast devices (e.g. disk, timer) must be serviced with stringent real-time
constraints. Other, slower devices (e.g. TTY) need not be serviced as quickly.
Failure to service fast devices soon enough may result in lost interrupts.
The priority interrupt mechanism allows the interrupt handler of a slow device to
be interrupted by a faster device, while blocking out interrupts from slower
devices during execution of interrupt handler of a fast device.
Machine Errors
Clock
Higher priority
Disk
Network Devices
Terminals
Lower Priority
Software interrupts
Typical Interrupt Levels
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Operating Systems
Priority Interrupt Mechanism
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How it works:
– CPU status register (PSW) contains bits specifying
processor priority (or execution) level.
– Each device has an associated device priority level. A
device may cause an interrupt only when its priority level is
higher than the current processor priority level.
– Interrupt handler for a device executes at processor priority
equal to device priority.
– Effect: An interrupt handler can only be interrupted by
devices of higher priority.
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Operating Systems
System Calls
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Provide "direct access" to operating system services (e.g., file system,
I/O routines, memory allocate & free routines) by user programs.
System calls execute instructions that control the resources of the
computer system, e.g., I/O instructions for devices.
We want such privileged instructions to be executed only by a system
routine, under the control of the OS!
As we will see, system calls are special, and in fact, are treated as a
special case of interrupts.
Programs that make system calls were traditionally called "system
programs" and were traditionally implemented in assembly language.
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Operating Systems
System Calls (cont.)
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Now, system calls can be made from high-level languages, such as C and
Modula-2 (to a degree).
Unix has about 32 system calls: read(), write(), open(), close(), fork(), …
using trap instructions:
– i = read(fd,80, buffer)
push buffer
push 80
push fd
trap read
pop i
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Each system call had a particular number. Instruction set has a special
instruction for making system calls:
SVC
(IBM 360/370)
trap
(PDP 11)
tw
(PowerPC) - trap word
tcc
(Sparc)
break
(MIPS)
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Operating Systems
User vs. System Mode
case
“trap”
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O.S.
i-call
l:
System
(or kernel)
memory
n:
code for read
trap n
User Program
(text)
Special mode-bit set in PSW register:
mode-bit = 0 => user program executing
mode-bit = 1 => system routine executing
Privileged instructions possible only when mode-bit = 1!
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Operating Systems
System Call Scenario
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User program executing (mode-bit = 0)
User makes a system call
 hardware sets mode-bit to 1
 system saves state of user process
 branch to case statement in system code
 branch to code for system routine based on system call
number
 copy parameters from user stack
 execute system call (using privileged instructions)
 restore state of user program
 hardware resets mode-bit
 return to user process
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Operating Systems
System Call Scenario (cont.)
File system
User
program
I/O
devices
memory
Operating System
User program is confined!
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Operating Systems