Lesson 2 : System Calls and System Program
2.1. Learning Objectives
On completion of this lesson you will know:
iO system calls
On completion of this lesson you will know:
iO system calls
ii) categorized system calls and system programs
iii) discuss system program.
2.2. System Calls
User programs communicate with the operating system and request services from it by making system calls. Fundamental services are handled through the use of system calls. The interface between a running programs and the operating system is defined by what is referred to as systems calls. A system call is a special type of function call that allows user programs to access the services provided by the operation system. A system call will usually generate a trap, a form of software interrupt. The trap will force the machine to switch into the privileged kernel mode-that allows access to data structures and memory of the kernel. In other words, system calls establish a well defined boundary between a running object program and the operating system. When a system call appears in a program, the situation is equivalent to a conventional procedure call whereby control is transferred to operating system routine invoked during the run time along with change of mode from user to supervisor. These calls are generally available as assembly language instructions, and are usually listed in the manuals used by assembly language programmers.
System calls can be roughly grouped into three major categories: process or job control, device and file manipulation, and information maintenance. In the following discussion, the types of system calls provided by an operating system are presented.
User programs communicate with the operating system and request services from it by making system calls. Fundamental services are handled through the use of system calls. The interface between a running programs and the operating system is defined by what is referred to as systems calls. A system call is a special type of function call that allows user programs to access the services provided by the operation system. A system call will usually generate a trap, a form of software interrupt. The trap will force the machine to switch into the privileged kernel mode-that allows access to data structures and memory of the kernel. In other words, system calls establish a well defined boundary between a running object program and the operating system. When a system call appears in a program, the situation is equivalent to a conventional procedure call whereby control is transferred to operating system routine invoked during the run time along with change of mode from user to supervisor. These calls are generally available as assembly language instructions, and are usually listed in the manuals used by assembly language programmers.
System calls can be roughly grouped into three major categories: process or job control, device and file manipulation, and information maintenance. In the following discussion, the types of system calls provided by an operating system are presented.
2.2.1. Process and Job Control
A running program needs to be able to halt its execution either normally (end) or abnormally (abort). If the program discovers an error in its input and wants to terminate abnormally.
A process or job executing one program may want to load and execute another program. This allows the control card interpreter to execute a program as directed by the control cards of the user job.
If control returns to the existing program when the new program terminates, we must save the memory image of the existing program and effectively have created a mechanism for one program to call another program. If both programs continue concurrently, we have created a new job or process to be multi-programmed. Then system call (create process or submit job) are used.
If we create a new job or process, to control its execution, then control requires the ability to determine and reset the attributes of a job or process, including its priority, its maximum allowable execution time, and so on (get process attributes and set process attributes). We may also want to terminate a job or process that we created (terminate process) if we find that it is incorrect or on longer needed.
Having created new jobs or processes, we may need to wait for them to finish execution. We may want to wait for a certain amount of time (wait time), but more likely we want to wait for a specific event (wait event). The jobs or processes should then signal when that event has occurred (signal event).
A running program needs to be able to halt its execution either normally (end) or abnormally (abort). If the program discovers an error in its input and wants to terminate abnormally.
A process or job executing one program may want to load and execute another program. This allows the control card interpreter to execute a program as directed by the control cards of the user job.
If control returns to the existing program when the new program terminates, we must save the memory image of the existing program and effectively have created a mechanism for one program to call another program. If both programs continue concurrently, we have created a new job or process to be multi-programmed. Then system call (create process or submit job) are used.
If we create a new job or process, to control its execution, then control requires the ability to determine and reset the attributes of a job or process, including its priority, its maximum allowable execution time, and so on (get process attributes and set process attributes). We may also want to terminate a job or process that we created (terminate process) if we find that it is incorrect or on longer needed.
Having created new jobs or processes, we may need to wait for them to finish execution. We may want to wait for a certain amount of time (wait time), but more likely we want to wait for a specific event (wait event). The jobs or processes should then signal when that event has occurred (signal event).
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