cfenollosa_os-tutorial/23-fixes

Concepts you may want to Google beforehand: freestanding, uint32_t, size_t

Goal: Fix miscellaneous issues with our code

The OSDev wiki has a section which describes some issues with JamesM's tutorial. Since we followed his tutorial for lessons 18-22 (interrupts through malloc), we'll need to make sure we fix any of the issues before moving on.

1. Wrong CFLAGS

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We add -ffreestanding when compiling .o files, which includes kernel_entry.o and thus kernel.bin and os-image.bin.

Before, we disabled libgcc (not libc) through the use of -nostdlib and we didn't re-enable it for linking. Since this is tricky, we'll delete -nostdlib

-nostdinc was also passed to gcc, but we will need it for step 3, so let's delete it.

2. kernel.c main() function

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Modify kernel/kernel.c and change main() to kernel_main() since gcc recognizes "main" as a special keyword and we don't want to mess with that.

Change boot/kernel_entry.asm to point to the new name accordingly.

To fix the i386-elf-ld: warning: cannot find entry symbol _start; defaulting to 0000000000001000 warning message, add a global _start; and define the _start: label in boot/kernel_entry.asm.

3. Reinvented datatypes

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It looks like it was a bad idea to define non-standard data types like u32 and such, since C99 introduces standard fixed-width data types like uint32_t

We need to include <stdint.h> which works even in -ffreestanding (but requires stdlibs) and use those data types instead of our own, then delete them on type.h

We also delete the underscores around __asm__ and __volatile__ since they aren't needed.

4. Improperly aligned kmalloc

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First, since kmalloc uses a size parameter, we'll use the correct data type size_t instead of u32int_t. size_t should be used for all parameters which "count" stuff and cannot be negative. Include <stddef.h>.

We will fix our kmalloc in the future, making it a proper memory manager and aligning data types. For now, it will always return a new page-aligned memory block.

5. Missing functions

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We will implement the missing mem* functions in following lessons

6. Interrupt handlers

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cli is redundant, since we already established on the IDT entries if interrupts are enabled within a handler using the idt_gate_t flags.

sti is also redundant, as iret loads the eflags value from the stack, which contains a bit telling whether interrupts are on or off. In other words the interrupt handler automatically restores interrupts whether or not interrupts were enabled before this interrupt

On cpu/isr.h, struct registers_t has a couple issues. First, the alleged esp is renamed to useless. The value is useless because it has to do with the current stack context, not what was interrupted. Then, we rename useresp to esp

We add cld just before call isr_handler on cpu/interrupt.asm as suggested by the osdev wiki.

There is a final, important issue with cpu/interrupt.asm. The common stubs create an instance of struct registers on the stack and then call the C handler. But that breaks the ABI, since the stack belongs to the called function and they may change them as they please. It is needed to pass the struct as a pointer.

To achieve this, edit cpu/isr.h and cpu/isr.c and change registers_t r into registers_t *t, then, instead of accessing the fields of the struct via ., access the fields of the pointer via ->. Finally, in cpu/interrupt.asm, and add a push esp before calling both isr_handler and irq_handler -- remember to also pop eax to clear the pointer afterwards.

Both current callbacks, the timer and the keyboard, also need to be changed to use a pointer to registers_t.