Transcript UNIT 2 - WordPress.com

UNIT -II
Introduction to the Kernel
1. Kernel:
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kernel is the central component of operating system.
It acts as an interface between the user applications and the hardware.
kernel is a computer program that manages input/output requests from
software and translates them into data processing instructions for the CPU
and other electronic components of a computer.
kernel can provide the lowest-level abstraction layer for the resources.
Applications and processes use these resources through
IPC mechanisms and System calls.
The main tasks of the kernel are :
Process management
Device management
Memory management
Interrupt handling
I/O communication
Shell
Kernel
Hardware;
CPU, I/O, Memory
and Storage Devices
Types Of Kernels
Monolithic
Kernel
Microkernel Exokernel Hybrid Kernel
Fig. Monolithic Kernel
• Exokernel is an operating system kernel developed by the MIT Parallel and
Distributed Operating Systems group, and also a class of similar operating
systems.
• The idea behind exokernels is to force as few abstractions as possible on
developers, enabling them to make as many decisions as possible about hardware
abstractions.
Fig. Exokernel
- A hybrid kernel is a kernel architecture based on combining aspects of
microkernel and monolithic kernel architectures used in computer operating
Systems
- The idea behind this category is to have a kernel structure similar to a
microkernel, but implemented in terms of a monolithic kernel
- No performance overhead for message passing and context switching between
kernel and user mode, as in monolithic kernels, there are no reliability benefits of
having services in user space, as in microkernel.
Fig. Hybrid kernel
Types Of kernels
2. Unix Kernel
• The Unix Kernel is monolithic in design.
• The Kernel can be split into two sections :• machine dependent
• and machine independent
User Program
Libraries
trap
trap
User level
Kernel level
System call Interface
Process Control subsystem
File Subsystem
IPC
Scheduler
Memory Management
Buffer cache
Character/block
device drivers
Hardware control
Kernel level
Hardware
Fig. Block diagram of the system kernel
Hardware level
Modern Unix Kernel
Examples of modern Unix
Systems
• System V Release 4[SVR4]
• Solaris 9
• BSD[Berkeley Software Distribution]
User and Kernel Stack
Process Table entry Fields
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PID - unique process identifiers
PPID –PID of parent
User ID
state -running, sleeping,..
event descriptor - address of event to wakeup a
sleeping process
Pointers to page tables
size and location of u area & pregion entry - for
swapping & switching
scheduling parameters determine access to CPU system priority : user priority
array of signals pending
execution and kernel utilisation times
u area (user area) fields:
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pointer to process table entry of the currently executing process
UIDs to permit access rights
time spent executing in user and kernel modes
an array containing signal handlers
login terminal associated with the process
error from a system call
return value from a system call
I/O parameters: amount of data to transfer, the address of the
source (or target) data array in user space, file offsets for I/O,
etc.
current and root directory
user file descriptor table records the files the process has open
size limits of a process and of a file it can write
permission of files the process creates
Data Structure for Process
per process
region table
u area
Kernel
process table
Kernel region table
main memory
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Context of a process
User-level Context Register Context
- Process text
- Program counter
- Process Data
- Processor status
- User stack
register
- Shared memory
- Stack pointer
- Shared memory
System-level
Context
-Process table
entry
- U area
- Per process
region table
- Kernel stack
Process States
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5
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6. Sleep And Wakeup
• When process is executing in the kernel it may need to wait for some event.
• To do this it executes the sleep routine.
• Process state changed to blocked and scheduler selects other process.
• When the event occurs the interrupt routine executes the wakeup routine.
I.e. process state changed from blocked to runnable.
• Several processes may be waiting for the same event - all are made runnable.
• Interrupts are disabled while sleep and wakeup are executing.
• Kernel sets the priority of the sleeping process according to the reason
for sleeping.
process A
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Buffer locked
sleeps
process B
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Buffer locked
sleeps
process C
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Buffer locked sleeps
Buffer unlocked. wakeup all sleeping processes A,B,C
Ready to run
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Runs
buffer locked
sleeps
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Ready to run
Ready to run
ready to run
Runs buffer
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unlocked
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Lock buffer
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sleeps for
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some reason
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Runs
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Buffer locked – sleeps
Wakes up buffer
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Unlocks Buffer
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Wakeup all
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sleeping proc.
Ready to run