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Operating System
a program that acts as an intermediate between a user and the computer hardware
controls all computers resource (memory, harddisk(s), ...)
create a base for user level application so that user-level programmer dont have to worry about the hardware facility, e.g. in using printf.
Components inside Operating System
(Four Basic Components)
Process Management (Scheduler)
Creation and deletion of both user and system processes
Scheduling of the processes
Keep track the status of the processes
Provide channel for interprocess communication
Memory Management
Memory allocation and deallocation
Program memory space protection
Paging
Swapping
Virtual Memory
File System
Disk space management
Maintain directory structure
Enforce security
Control read and write access
Device Drivers, I/O
User Level vs. System Level
When a user-level program wants to get kernel services, it will issues a system call. (kernel call or supervisor call)
Next, the instruction will switch the process from user level to kernel level. (system level)
Here, the operating system will take over the control flow.
After examining the parameter given to the system call, the system call will be run.
Finally, the control is passed back to the user-level program and i.e. from kernel mode back to user level
You may look at /usr/src/linux/include/asm/unistd.h to see the listings of system calls
What is Kernel? Monolithic vs microkernel?
Generally, the operating system, as a running program, is called the kernel and have the four basic components as shown above.
A trend in modern OS is to implement more function in user level and leave a minimal kernel. (microkernel)
In Linux, all the components are kept inside kernel and so, it is not a microkernel structure. However, it is now moving to modules hierarchy, e.g. fs, mm, ...
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