OS Development
This may or may not be directly related to Linux Kernel, that’s the main reason it’s in a separate note.
The Bootloader
GRUB has to load the kernel at memory address equal or higher to 1MB, since lower values are reserved for GRUB, BIOS and I/O.
Basic Structure for ISO
iso
└── boot
├── grub
│ ├── menu.lst
│ └── stage2_eltorito
└── kernel.elf
ISO Generation Command
genisoimage -R \ # From `cdrtools` package in Arch
-b boot/grub/stage2_eltorito \
-no-emul-boot \
-boot-load-size 4 \
-A os \
-input-charset utf8 \
-quiet \
-boot-info-table \
-o os.iso \
iso
Using C to write the kernel
Since C uses a stack, we need to make the esp (stack pointer) point at the end of a free memory space. To make sure enough space is available, we reserve space in the bss (block starting symbol) segment. The result is reserved space with the esp pointing at the end of it.
cdecl calling convention states that arguments to a function should be passed via the stack, right-to-left, and the result stored in eax.
Adding __attribute__((packed)); tells the compiler not to adding any padding to structs, but this isn’t part of the C standard.
struct example {
unsigned char config;
unsigned short address;
unsigned char index;
} __attribute__((packed));
Hardware Interaction
Main two types of I/O:
- Memory-mapped I/O: works by changed values in specific addresses (i.e framebuffer).
- I/O Ports: uses out and in instructions for communication.
Framebuffer
Bit: | 15 14 13 12 11 10 9 8 | 7 6 5 4 | 3 2 1 0 | Content: | ASCII | FG | BG |
Compiling
Linking a loader into an ELF executable for the bootloader.
ld -T link.ld -melf_i386 loader.o -o kernel.elf
Flags needed for removing standard library and compiling for a kernel:
-m32 -nostdlib -nostdinc -fno-builtin -fno-stack-protector -nostartfiles -nodefaultlibs
# Recommended
-Wall -Wextra -Werror