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81 lines
3.2 KiB
81 lines
3.2 KiB
*Concepts you may want to Google beforehand: C types and structs, include guards, type attributes: packed, extern, volatile, exceptions*
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**Goal: Set up the Interrupt Descriptor Table to handle CPU interrupts**
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This lesson and the following ones have been heavily inspired
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by [JamesM's tutorial](https://web.archive.org/web/20160412174753/http://www.jamesmolloy.co.uk/tutorial_html/index.html)
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Data types
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----------
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First, we will define some special data types in `cpu/types.h`,
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which will help us uncouple data structures for raw bytes from chars and ints.
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It has been carefully placed on the `cpu/` folder, where we will
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put machine-dependent code from now on. Yes, the boot code
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is specifically x86 and is still on `boot/`, but let's leave
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that alone for now.
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Some of the already existing files have been changed to use
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the new `u8`, `u16` and `u32` data types.
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From now on, our C header files will also have include guards.
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Interrupts
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----------
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Interrupts are one of the main things that a kernel needs to
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handle. We will implement it now, as soon as possible, to be able
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to receive keyboard input in future lessons.
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Another examples of interrupts are: divisions by zero, out of bounds,
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invalid opcodes, page faults, etc.
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Interrupts are handled on a vector, with entries which are
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similar to those of the GDT (lesson 9). However, instead of
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programming the IDT in assembly, we'll do it in C.
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`cpu/idt.h` defines how an idt entry is stored `idt_gate` (there need to be
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256 of them, even if null, or the CPU may panic) and the actual
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idt structure that the BIOS will load, `idt_register` which is
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just a memory address and a size, similar to the GDT register.
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Finally, we define a couple variables to access those data structures
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from assembler code.
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`cpu/idt.c` just fills in every struct with a handler.
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As you can see, it is a matter
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of setting the struct values and calling the `lidt` assembler command.
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ISRs
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----
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The Interrupt Service Routines run every time the CPU detects an
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interrupt, which is usually fatal.
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We will write just enough code to handle them, print an error message,
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and halt the CPU.
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On `cpu/isr.h` we define 32 of them, manually. They are declared as
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`extern` because they will be implemented in assembler, in `cpu/interrupt.asm`
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Before jumping to the assembler code, check out `cpu/isr.c`. As you can see,
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we define a function to install all isrs at once and load the IDT, a list of error
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messages, and the high level handler, which kprints some information. You
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can customize `isr_handler` to print/do whatever you want.
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Now to the low level which glues every `idt_gate` with its low-level and
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high-level handler. Open `cpu/interrupt.asm`. Here we define a common
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low level ISR code, which basically saves/restores the state and calls
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the C code, and then the actual ISR assembler functions which are referenced
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on `cpu/isr.h`
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Note how the `registers_t` struct is a representation of all the registers
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we pushed in `interrupt.asm`
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That's basically it. Now we need to reference `cpu/interrupt.asm` from our
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Makefile, and make the kernel install the ISRs and launch one of them.
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Notice how the CPU doesn't halt even though it would be good practice
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to do it after some interrupts.
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