System call
There are certain tasks that can only be done if a process is running in kernel mode. Examples of these tasks can be interacting with hardware etc. So if a process wants to do such kind of task then it would require itself to be running in kernel mode which is made possible by system calls.
system call is not linked to a user program and executes in kernel space. system call execution time is counted as a part of system time. System calls cannot be debugged as they are executed by the kernel.
How does a C programmer actually issue a system call? There is no difference between a system call and any other function call. For example, the read system call might be issued like this:
amt = read(fd, buf, numbytes);
System calls are commands that are executed by the operating system. "System calls are the only way to access kernel facilities such as file system, multitasking mechanisms and the interprocess communication primitives."(Rochkind's book, Advanced Unix Programming)
Memory-mapped IO vs Port-mapped IO
Microprocessors normally use two methods to connect external devices: memory mapped or port mapped I/O. However, as far as the peripheral is concerned, both methods are really identical.