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73 lines
3.2 KiB
Markdown
73 lines
3.2 KiB
Markdown
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**Goal: Clean the code a bit and parse user input**
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In this lesson we will do two things. First, we will clean up the code a bit, so it is ready
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for further lessons. During the previous ones I tried to put things in the most predictable places,
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but it is also a good exercise to know when the code base is growing and adapt it to current
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and further needs.
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Code cleaning
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-------------
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First of all, we will quickly start to need more utility functions
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for handling strings and so on. In a regular OS, this is called the C library,
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or libc for short.
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Right now we have a `utils.c` which we will split into `mem.c` and `string.c`, with their respective headers.
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Second, we will create a new function `irq_install()` so that the kernel
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only needs to perform one call to initialize all the IRQs. That function
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is akin to `isr_install()` and placed on the same `irq.c`.
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While we're here, we will disable the `kprint()` on `timer_callback()`
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to avoid filling the screen with junk, now that we know that it works
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properly.
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There is not a clear distinction between `cpu/` and `drivers/`.
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Keep in mind that I'm
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creating this tutorial while following many others, and each of them
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has a distinct folder structure. The only change we will do for now is to
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move `drivers/ports.*` into `cpu/` since it is clearly cpu-dependent code.
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`boot/` is also CPU-dependent code, but we will not mess with it until
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we implement the boot sequence for a different machine.
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There are more switches for the `CFLAGS` on the `Makefile`, since we will now
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start creating higher-level functions for our C library and we don't want
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the compiler to include any external code if we make a mistake with a declaration.
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We also added some flags to turn warnings into errors, since an apparently minor mistake
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converting pointers can blow up later on. This also forced us to modify some misc pointer
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declarations in our code.
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Finally, we'll add a macro to avoid warning-errors on unused parameters on `libc/function.h`
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Keyboard characters
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-------------------
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How to access the typed characters, then?
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- When a key is pressed, the callback gets the ASCII code via a new
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arrays which are defined at the beginning of `keyboard.c`
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- The callback then appends that character to a buffer, `key_buffer`
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- It is also printed on the screen
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- When the OS wants to read user input, it calls `libc/io.c:readline()`
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`keyboard.c` also parses backspace, by removing the last element
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of the key buffer, and deleting it from the screen, by calling
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`screen.c:kprint_backspace()`. For this we needed to modify a bit
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`print_char()` to not advance the offset when printing a backspace
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Responding to user input
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------------------------
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The keyboard callback checks for a newline, and then calls the kernel,
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telling it that the user has input something. Out final libc function
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is `strcmp()`, which compares two strings and returns 0 if they
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are equal. If the user inputs "END", we halt the CPU.
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This is the most basic shell ever, but you should be proud, because
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we implemented it from scratch. Do you realize how cool this is?
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If you want to, expand `kernel.c` to parse more stuff. In the future,
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when we have a filesystem, we will allow the user to run some basic commands.
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