Answer the following questions about system call:

$($a$)$ What is system call?

$($b$)$ Give two examples in which your user programs must use system call to perform some tasks? Describe the problems you would encounter if the system calls were not available and your programs had to carry out the tasks by themselves $($in terms of convenience and$/$or protection$).$

$($c$)$ However, programmers usually don't directly initiate system calls. In this case, how do they access the system calls?

OS provides various abstractions to allow user programs to conveniently use hardware resources with protection. One example is the abstraction for the disk $($SSD or hard disk$)$ that is enabled by the file system. What are included in this abstraction $($for programmers to conveniently access$)?$

What are two most essential components $($among CPU, memory, disk, keyboard, mouse, monitor, etc$)$ that form a computer? Why?

Why must data and instructions that are directly accessed by a program be in memory $($rather than on the disk$)?$ If a program needs to access data on the disk, how does it do$?$

How does interrupt help make timesharing $($in the concurrent use of CPU by multiple processes$)$ possible?

How does the distinction between kernel mode and user mode function as a rudimentary form of protection $($security$)?$

What's the difference between $($hardware$)$ irterrupt and exception$/$trap$?$ "Both of them cause the OS code to run $($or control of the CPU to be transferred to the OS$)".$ Is this statement correct?