lesson 21, keyboard input and shell

21-shell/README.md

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+
+**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.
+
+
+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.