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93 lines
3.9 KiB
93 lines
3.9 KiB
*Concepts you may want to Google beforehand: freestanding, uint32_t, size_t*
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**Goal: Fix miscellaneous issues with our code**
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The OSDev wiki has a section [which describes some issues with
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JamesM's tutorial](http://wiki.osdev.org/James_Molloy%27s_Tutorial_Known_Bugs).
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Since we followed his tutorial for lessons 18-22 (interrupts through malloc), we'll
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need to make sure we fix any of the issues before moving on.
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1. Wrong CFLAGS
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---------------
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We add `-ffreestanding` when compiling `.o` files, which includes `kernel_entry.o` and thus
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`kernel.bin` and `os-image.bin`.
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Before, we disabled libgcc (not libc) through the use of `-nostdlib` and we didn't re-enable
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it for linking. Since this is tricky, we'll delete `-nostdlib`
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`-nostdinc` was also passed to gcc, but we will need it for step 3, so let's delete it.
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2. kernel.c `main()` function
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-----------------------------
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Modify `kernel/kernel.c` and change `main()` to `kernel_main()` since gcc recognizes "main" as
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a special keyword and we don't want to mess with that.
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Change `boot/kernel_entry.asm` to point to the new name accordingly.
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To fix the `i386-elf-ld: warning: cannot find entry symbol _start; defaulting to 0000000000001000`
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warning message, add a `global _start;` and define the `_start:` label in `boot/kernel_entry.asm`.
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3. Reinvented datatypes
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-----------------------
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It looks like it was a bad idea to define non-standard data types like `u32` and such, since
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C99 introduces standard fixed-width data types like `uint32_t`
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We need to include `<stdint.h>` which works even in `-ffreestanding` (but requires stdlibs)
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and use those data types instead of our own, then delete them on `type.h`
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We also delete the underscores around `__asm__` and `__volatile__` since they aren't needed.
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4. Improperly aligned `kmalloc`
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-------------------------------
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First, since `kmalloc` uses a size parameter, we'll use the correct data type `size_t` instead
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of `u32int_t`. `size_t` should be used for all parameters which "count" stuff and cannot be
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negative. Include `<stddef.h>`.
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We will fix our `kmalloc` in the future, making it a proper memory manager and aligning data types.
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For now, it will always return a new page-aligned memory block.
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5. Missing functions
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--------------------
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We will implement the missing `mem*` functions in following lessons
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6. Interrupt handlers
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---------------------
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`cli` is redundant, since we already established on the IDT entries if interrupts
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are enabled within a handler using the `idt_gate_t` flags.
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`sti` is also redundant, as `iret` loads the eflags value from the stack, which contains a
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bit telling whether interrupts are on or off.
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In other words the interrupt handler automatically restores interrupts whether or not
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interrupts were enabled before this interrupt
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On `cpu/isr.h`, `struct registers_t` has a couple issues.
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First, the alleged `esp` is renamed to `useless`.
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The value is useless because it has to do with the current stack context, not what was interrupted.
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Then, we rename `useresp` to `esp`
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We add `cld` just before `call isr_handler` on `cpu/interrupt.asm` as suggested
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by the osdev wiki.
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There is a final, important issue with `cpu/interrupt.asm`. The common stubs create an instance
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of `struct registers` on the stack and then call the C handler. But that breaks the ABI, since
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the stack belongs to the called function and they may change them as they please. It is needed
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to pass the struct as a pointer.
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To achieve this, edit `cpu/isr.h` and `cpu/isr.c` and change `registers_t r` into `registers_t *t`,
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then, instead of accessing the fields of the struct via `.`, access the fields of the pointer via `->`.
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Finally, in `cpu/interrupt.asm`, and add a `push esp` before calling both `isr_handler` and
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`irq_handler` -- remember to also `pop eax` to clear the pointer afterwards.
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Both current callbacks, the timer and the keyboard, also need to be changed to use a pointer to
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`registers_t`.
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