How does instructions like I/O work in user mode?

Question

I am curious because I am reading this OS book which mentions

"User programs always run in user mode, which permits only a subset of the instructions [...]. Generally, all instructions involving I/O and memory protection are disallowed in user mode. To obtain services from the OS, a user program must make a system call, which traps into the kernel and invokes the OS."

If I/O is not allowed generally in user mode, but let's say I have a program in C++ or Java which asks for input, or let's say something else like a search bar in any program. Whenever I select the search bar (meaning I will write something) then a TRAP instruction is called to invoke the OS (since the OS runs in kernel) to be able to have access to I/O, that is, the keyboard? I am not sure if I follow correctly or what am I getting wrong.

The I/O is not allowed in user mode, but you use Input for applications in the OS, or even with the OS itself there are keyboard commands. If you can use keyboard commands that means the OS is ready for I/O at any time. Then the original statement about I/O instructions being disallowed in user mode.

I am sorry for my ignorance but I am just a little bit confused with these terms and difference between user and kernel. I know the OS runs in kernel mode, and the applications run in the OS, so in the end the applications do have access to I/O.

Answer 1

Toss your book in the garbage or use it to line a cat box.

Your apparent paradox is that you think you have to be in kernel mode to do I/O but your book says:

"User programs always run in user mode, which permits only a subset of the instructions"

The resolution to your paradox is that your book is spouting nonsense.

Use programs do not always run in user mode. They frequently run in kernel mode. One of the basic functions of an operating system is to provide a set of kernel mode system services that provide controlled access to kernel mode.

In other words, your instincts here are better than your confusing book's text.

Answer 2

Don't application need to have the OS deal with there I/O for them? Meaning, only the OS has the authority to do those type of things...

Am I wrong?

Answer 3

It is important to understand what "user-mode" and "kernel-mode" mean. A process is mapped to memory regions which are user-priviliged, depending on the memory layout.
Kernel-mode is basically routines that are in supervisor priviliged memory regions, which are invoked by your program to do the desired work (I/O).