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System Calls
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System Calls
In computing, a system call is how a program requestsa service
from an operating system's kernel that it doesnot normally have
permission to run.
System calls provide the interface between a processand the
operating system.
Most operations interacting with the system requirepermissions
not available to a user level process, e.g.
I/O performed with a device present on the system, orany form of
communication with other processesrequires the use of system
calls.
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Privileges
The design of the microprocessor architecture practically on
all modern systems (except some embedded systems) offers aseries
ofCPU modes
The (low) privilege level in which normal applications
execute
limits the address space of the program so that it cannot
access or modify other running applications nor the
operating
system itself.
It also prevents the application from directly using devices
(e.g. the frame buffer or network devices). But obviously
many normal applications need these abilities; therefore,
pre-
defined system calls are made available by the operating
system.
The operating system executes at the highest level of
privilege, and allows applications to request services via
system calls, which are often implemented through
interrupts.
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Privileges
If allowed, the system enters a higher privilege level,executes
a specific set of instructions over which theinterrupting program
has no direct control, returns tothe calling application's
privilege level, then returns
control to the calling application. This concept alsoserves as a
way to implement security.
With the development of separate operating modeswith varying
levels of privilege, a mechanism was
needed for transferring control safely from lesserprivileged
modes to higher privileged modes.
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Library as an Intermediary
Generally, systems provide a library or API that sitsbetween
normal programs and the operating system,that provides functions
for the system calls,
The library's wrapper functions expose an ordinaryfunction
calling convention (a subroutine call onthe assembly level) for
using the system call, as well asmaking the use of the system call
more modular.
In this way the library, which exists between the OS
and the application, increases portability.
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Groups of System Calls
Process Control.
load
execute
create process
terminate process
get/set process attributes
Wait for time, wait event, signal event
allocate, free memory
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Groups of System Calls
File management.
create file, delete file
open, close
read, write, reposition
get/set file attributes
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Groups of System Calls
Device Management.
request device, release device
read, write, reposition
get/set device attributes
logically attach or detach devices
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Groups of System Calls
Information Maintenance.
get/set time or date
get/set system data
get/set process, file, or device attributes
Communication.
create, delete communication connection
send, receive messages transfer status information
attach or detach remote devices
