Transcript Operating Systems - The College of Saint Rose

Operating
Systems
CIS 432
David Goldschmidt, Ph.D.
Computer Science
The College of Saint Rose
Operating System Services
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An operating system provides services to its users:
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Program execution
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I/O operations
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Load programs into memory, run/suspend/halt programs,
handle/display errors
Seamlessly interact with I/O devices, including
disks, networks connection, etc.
Filesystem manipulation
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Read/write/traverse filesystem directories,
read/write files, enforce permissions, search for files, etc.
Operating System Services
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Other services an operating system provides:
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Inter-Process Communications (IPC)
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Processes exchange information via shared memory,
message passing, sockets, pipes, files, etc.
Might span multiple computer systems
Error detection and recovery
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Errors arise in CPU, memory, I/O devices,
processes, etc.
Operating system should detect and recover from errors,
ensuring correct and consistent operations
Operating System Structure
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Using a layered approach, the operating system is
divided into N levels or layers
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Layer 0 is the hardware
Layer N is the top-level
user interface
Each layer uses functions
and services of lower layers
Layer 1 often is the kernel
Operating System Structure
Operating System Kernel
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The core program running in an
operating system is the kernel
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When a computer is switched on,
a bootstrap program executes from ROM
Bootstrap program initializes all aspects of the system,
then loads the operating system kernel and starts its
execution
User and Kernel Modes
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Dual-mode operation provides a user mode and
a kernel mode of operation
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Allows the operating system to protect itself and other
system components
User and Kernel Modes
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Kernel relinquishes control to a user process,
but may set a timer to ensure a process does not
run beyond its allotted time
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Avoid infinite loops, memory leaks, memory hogs, etc.
Not entirely effective in practice?
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How many times have you fought to stop a runaway process?
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Kernel also provides security
via privileged instructions
System Calls via APIs
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Operating system services available to users
via GUI and CLI, which are just software programs
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System calls are made via programming interface
called an Application Program Interface (API)
Common operating system APIs:
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Win32 API for Windows
POSIX API for POSIX-based systems,
including UNIX, Linux, Mac OS X
Java API for Java Virtual Machine
C++ Standard Library
System Calls via APIs
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Types of system calls include:
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Process control (e.g. start/suspend/stop a process)
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File management
Device management
Information retrieval and maintenance
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Debugging information, too
Current date/time, number of current users, OS version,
amount of free memory, process information, etc.
Communications (e.g. IPC, network)
System Calls via APIs
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System call sequence for a file copy program:
System Calls via APIs
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An API successfully hides the implementation details
of the operating system
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Programmers just need to abide by
the API specifications
Change the operating system,
but maintain the API such that
it is backwards compatible
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Deprecate old behavior over
long period of time
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Why?
hey, the dude
abides, man
System Calls via APIs
System Calls via APIs
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Example using the
printf() function
from C
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One API may call
another, which may
call another, etc.
System Calls via APIs
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Pass parameters to the operating system
via registers or a block of data in memory