cfenollosa_os-tutorial/18-interrupts

Concepts you may want to Google beforehand: C types and structs, include guards, type attributes: packed, extern, volatile, exceptions

Goal: Set up the Interrupt Descriptor Table to handle CPU interrupts

This lesson and the following ones have been heavily inspired by JamesM's tutorial

Data types

First, we will define some special data types in cpu/types.h, which will help us uncouple data structures for raw bytes from chars and ints. It has been carefully placed on the cpu/ folder, where we will put machine-dependent code from now on. Yes, the boot code is specifically x86 and is still on boot/, but let's leave that alone for now.

Some of the already existing files have been changed to use the new u8, u16 and u32 data types.

From now on, our C header files will also have include guards.

Interrupts

Interrupts are one of the main things that a kernel needs to handle. We will implement it now, as soon as possible, to be able to receive keyboard input in future lessons.

Another examples of interrupts are: divisions by zero, out of bounds, invalid opcodes, page faults, etc.

Interrupts are handled on a vector, with entries which are similar to those of the GDT (lesson 9). However, instead of programming the IDT in assembly, we'll do it in C.

cpu/idt.h defines how an idt entry is stored idt_gate (there need to be 256 of them, even if null, or the CPU may panic) and the actual idt structure that the BIOS will load, idt_register which is just a memory address and a size, similar to the GDT register.

Finally, we define a couple variables to access those data structures from assembler code.

cpu/idt.c just fills in every struct with a handler. As you can see, it is a matter of setting the struct values and calling the lidt assembler command.

ISRs

The Interrupt Service Routines run every time the CPU detects an interrupt, which is usually fatal.

We will write just enough code to handle them, print an error message, and halt the CPU.

On cpu/isr.h we define 32 of them, manually. They are declared as extern because they will be implemented in assembler, in cpu/interrupt.asm

Before jumping to the assembler code, check out cpu/isr.c. As you can see, we define a function to install all isrs at once and load the IDT, a list of error messages, and the high level handler, which kprints some information. You can customize isr_handler to print/do whatever you want.

Now to the low level which glues every idt_gate with its low-level and high-level handler. Open cpu/interrupt.asm. Here we define a common low level ISR code, which basically saves/restores the state and calls the C code, and then the actual ISR assembler functions which are referenced on cpu/isr.h

Note how the registers_t struct is a representation of all the registers we pushed in interrupt.asm

That's basically it. Now we need to reference cpu/interrupt.asm from our Makefile, and make the kernel install the ISRs and launch one of them. Notice how the CPU doesn't halt even though it would be good practice to do it after some interrupts.