Lesson 3 : Operating System Structure
3.1. Learning Objectives
On completion of this lesson you will know :
i) different types of OS system structure
ii) how a system call can be made
On completion of this lesson you will know :
i) different types of OS system structure
ii) how a system call can be made
iii) micro kernel.
3.2. Operating System Structure
A number of approaches can be taken for configuring the components of an operating system, ranging from a monolithic to a virtual machines. To conclude the introduction, we identify several of the approaches that have been used to build OS. There are four different structures of operating system, but in this lesson we will discuss only three of them.
3.2.1. Monolithic System
The monolithic organization does not attempt to implement the various functions process, file, device and memory management in distinct modules. Instead, all function are implemented within a single module that contains all system routines or process and all operating system data structure.
The operating system is written as a collection of procedures, each can call any of the other ones whenever it needs to. When this technique is used, each procedure in the system has a well defined interface in terms of parameters and results, and each one is free to call any other one, if the latter provides some useful computation that the former needs.
In monolithic systems, it is possible to have at least a little structure. The services (system calls) provided by the operating system are requested by putting the parameters in well-defined places, such as in registers or on the stack, and then executing a special trap instruction known as a kernel call or supervisor call.
This instruction switches the machine from user mode to kernel mode (also known as supervisor mode), and transfers control to the operating system, shown as event 1 in Fig. 2.3. Most CPUs have two modes; kernel mode, for the operating system, in which all instructions are allowed; and user mode, for user programs, in which I/O and certain other instructions are not allowed.
The operating system then examines the parameters of the call to determine which system call is to be carried out, shown as 2 in Fig. 2.3. Next the operating system indexes into a table that contains in slot x a pointer to the procedure that carries out system call x. This operation, shown as 3 in Fig..2.3, identifies the service procedure which is then called. Finally, the system call is finished and control is given back to the user program.
The monolithic organization does not attempt to implement the various functions process, file, device and memory management in distinct modules. Instead, all function are implemented within a single module that contains all system routines or process and all operating system data structure.
The operating system is written as a collection of procedures, each can call any of the other ones whenever it needs to. When this technique is used, each procedure in the system has a well defined interface in terms of parameters and results, and each one is free to call any other one, if the latter provides some useful computation that the former needs.
In monolithic systems, it is possible to have at least a little structure. The services (system calls) provided by the operating system are requested by putting the parameters in well-defined places, such as in registers or on the stack, and then executing a special trap instruction known as a kernel call or supervisor call.
This instruction switches the machine from user mode to kernel mode (also known as supervisor mode), and transfers control to the operating system, shown as event 1 in Fig. 2.3. Most CPUs have two modes; kernel mode, for the operating system, in which all instructions are allowed; and user mode, for user programs, in which I/O and certain other instructions are not allowed.
The operating system then examines the parameters of the call to determine which system call is to be carried out, shown as 2 in Fig. 2.3. Next the operating system indexes into a table that contains in slot x a pointer to the procedure that carries out system call x. This operation, shown as 3 in Fig..2.3, identifies the service procedure which is then called. Finally, the system call is finished and control is given back to the user program.
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