cfenollosa_os-tutorial/21-shell

Goal: Clean the code a bit and parse user input

In this lesson we will do tho things. First, we will clean up the code a bit, so it is ready for further lessons. During the previous ones I tried to put things in the most predictable places, but it is also a good exercise to know when the code base is growing and adapt it to current and further needs.

Code cleaning

First of all, we will quickly start to need more utility functions for handling strings and so on. In a regular OS, this is called the C library, or libc for short.

Right now we have a utils.c which we will split into mem.c and string.c, with their respective headers.

Second, we will create a new function irq_install() so that the kernel only needs to perform one call to initialize all the IRQs. That function is akin to isr_install() and placed on the same irq.c. While we're here, we will disable the kprint() on timer_callback() to avoid filling the screen with junk, now that we know that it works properly.

There is not a clear distinction between cpu/ and drivers/. Keep in mind that I'm creating this tutorial while following many others, and each of them has a distinct folder structure. The only change we will do for now is to move drivers/ports.* into cpu/ since it is clearly cpu-dependent code. boot/ is also CPU-dependent code, but we will not mess with it until we implement the boot sequence for a different machine.

There are more switches for the CFLAGS on the Makefile, since we will now start creating higher-level functions for our C library and we don't want the compiler to include any external code if we make a mistake with a declaration. We also added some flags to turn warnings into errors, since an apparantly minor mistake converting pointers can blow up later on. This also forced us to modify some misc pointer declarations in our code.

Finally, we'll add a macro to avoid warning-errors on unused parameters on libc/function.h

Keyboard characters

How to access the typed characters, then?

• When a key is pressed, the callback gets the ASCII code via a new arrays which are defined at the beginning of keyboard.c
• The callback then appends that character to a buffer, key_buffer
• It is also printed on the screen
• When the OS wants to read user input, it calls libc/io.c:readline()

keyboard.c also parses backspace, by removing the last element of the key buffer, and deleting it from the screen, by calling screen.c:kprint_backspace(). For this we needed to modify a bit print_char() to not advance the offset when printing a backspace

Responding to user input

The keyboard callback checks for a newline, and then calls the kernel, telling it that the user has input something. Out final libc function is strcmp(), which compares two strings and returns 0 if they are equal. If the user inputs "END", we halt the CPU.

This is the most basic shell ever, but you should be proud, because we implemented it from scratch. Do you realize how cool this is?

If you want to, expand kernel.c to parse more stuff. In the future, when we have a filesystem, we will allow the user to run some basic commands.