From 39f626a5d0cf0f5db95e695d013b70a04c29348e Mon Sep 17 00:00:00 2001 From: Joursoir Date: Mon, 2 Aug 2021 12:45:38 +0000 Subject: init project --- kernel/boot.s | 114 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ kernel/linker.ld | 51 +++++++++++++++++++++++++ kernel/main.c | 114 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 279 insertions(+) create mode 100644 kernel/boot.s create mode 100644 kernel/linker.ld create mode 100644 kernel/main.c (limited to 'kernel') diff --git a/kernel/boot.s b/kernel/boot.s new file mode 100644 index 0000000..33e54f1 --- /dev/null +++ b/kernel/boot.s @@ -0,0 +1,114 @@ +# https://www.gnu.org/software/grub/manual/multiboot/multiboot.html + +/* +Declare constants for the multiboot header. +*/ +.set ALIGN, 1 << 0 # align loaded modules on page boundaries +.set MEMINFO, 1 << 1 # provide memory map +.set FLAGS, ALIGN | MEMINFO # this is the Multiboot 'flag' field +.set MAGIC, 0x1BADB002 # 'magic number' lets bootloader find the header +.set CHECKSUM, -(MAGIC + FLAGS) # checksum of above, to prove we are multiboot + +/* +Declare a multiboot header that marks the program as a kernel. +These are magic values that are documented in the multiboot standard. +The bootloader will search for this signature in the first 8 KiB of +the kernel file, aligned at a 32-bit boundary. The signature is in +its own section so the header can be forced to be within the first +8 KiB of the kernel file. +*/ +.section .multiboot +.align 4 +.long MAGIC +.long FLAGS +.long CHECKSUM + +/* +The multiboot standard does not define the value of the +stack pointer register (esp) and it is up to the kernel to provide a stack. +This allocates room for a small stack by creating a symbol at the bottom +of it, then allocating 16384 bytes for it, and finally creating a symbol +at the top. The stack grows downwards on x86. The stack is in its own +section so it can be marked nobits, which means the kernel file is smaller +because it does not contain an uninitialized stack. The stack on x86 +must be 16-byte aligned according to the System V ABI standard and +de-facto extensions. The compiler will assume the stack is properly +aligned and failure to align the stack will result in undefined behavior. +*/ +.section .bss +.align 16 +stack_bottom: +.skip 16384 # 16 KiB +stack_top: + +/* +The linker script specifies _start as the entry point to the kernel and the +bootloader will jump to this position once the kernel has been loaded. It +doesn't make sense to return from this function as the bootloader is gone. +*/ +.section .text +.global _start +.type _start, @function +_start: + /* + The bootloader has loaded us into 32-bit protected mode on a x86 + machine. Interrupts are disabled. Paging is disabled. The processor + state is as defined in the multiboot standard. The kernel has full + control of the CPU. The kernel can only make use of hardware features + and any code it provides as part of itself. There's no printf + function, unless the kernel provides its own header and a + printf implementation. There are no security restrictions, no + safeguards, no debugging mechanisms, only what the kernel provides + itself. It has absolute and complete power over the + machine. + */ + + /* + To set up a stack, we set the esp register to point to the top of the + stack (as it grows downwards on x86 systems). This is necessarily done + in assembly as languages such as C cannot function without a stack. + */ + mov $stack_top, %esp + + /* + This is a good place to initialize crucial processor state before the + high-level kernel is entered. It's best to minimize the early + environment where crucial features are offline. Note that the + processor is not fully initialized yet: Features such as floating + point instructions and instruction set extensions are not initialized + yet. The GDT should be loaded here. Paging should be enabled here. + C++ features such as global constructors and exceptions will require + runtime support to work as well. + */ + + /* + Enter the high-level kernel. The ABI requires the stack is 16-byte + aligned at the time of the call instruction (which afterwards pushes + the return pointer of size 4 bytes). The stack was originally 16-byte + aligned above and we've pushed a multiple of 16 bytes to the + stack since (pushed 0 bytes so far), so the alignment has thus been + preserved and the call is well defined. + */ + call kernel_main + + /* + If the system has nothing more to do, put the computer into an + infinite loop. To do that: + 1) Disable interrupts with cli (clear interrupt enable in eflags). + They are already disabled by the bootloader, so this is not needed. + Mind that you might later enable interrupts and return from + kernel_main (which is sort of nonsensical to do). + 2) Wait for the next interrupt to arrive with hlt (halt instruction). + Since they are disabled, this will lock up the computer. + 3) Jump to the hlt instruction if it ever wakes up due to a + non-maskable interrupt occurring or due to system management mode. + */ + cli +1: hlt + jmp 1b + +/* +Set the size of the _start symbol to the current location '.' minus its +start. This is useful when debugging or when you implement call tracing. +*/ +.size _start, . - _start diff --git a/kernel/linker.ld b/kernel/linker.ld new file mode 100644 index 0000000..245f4a7 --- /dev/null +++ b/kernel/linker.ld @@ -0,0 +1,51 @@ +/* +The bootloader will look at this image and start execution at the +symbol designated as the entry point. +*/ +ENTRY(_start) + +/* Tell where the various sections of the object files will be put +in the final kernel image. */ +SECTIONS +{ + /* + Begin putting sections at 1 MiB, a conventional place for kernels to be + loaded at by the bootloader. + */ + . = 1M; + + /* + First put the multiboot header, as it is required to be put very early + in the image or the bootloader won't recognize the file format. + Next we'll put the .text section. + */ + .text BLOCK(4K) : ALIGN(4K) + { + *(.multiboot) + *(.text) + } + + /* Read-only data. */ + .rodata BLOCK(4K) : ALIGN(4K) + { + *(.rodata) + } + + /* Read-write data (initialized) */ + .data BLOCK(4K) : ALIGN(4K) + { + *(.data) + } + + /* Read-write data (uninitialized) and stack */ + .bss BLOCK(4K) : ALIGN(4K) + { + *(COMMON) + *(.bss) + } + + /* + The compiler may produce other sections, by default it will put them in + a segment with the same name. Simply add stuff here as needed. + */ +} diff --git a/kernel/main.c b/kernel/main.c new file mode 100644 index 0000000..47fcecd --- /dev/null +++ b/kernel/main.c @@ -0,0 +1,114 @@ +#include +#include +#include + +/* Check if the compiler thinks you are targeting the wrong operating system. */ +#if defined(__linux__) +#error "You are not using a cross-compiler, you will most certainly run into trouble" +#endif + +#if !defined(__i386__) +#error "This kernel needs to be compiled with a ix86-elf compiler" +#endif + +/* Hardware text mode color constants. */ +enum vga_color { + VGA_COLOR_BLACK = 0, + VGA_COLOR_BLUE = 1, + VGA_COLOR_GREEN = 2, + VGA_COLOR_CYAN = 3, + VGA_COLOR_RED = 4, + VGA_COLOR_MAGENTA = 5, + VGA_COLOR_BROWN = 6, + VGA_COLOR_LIGHT_GREY = 7, + VGA_COLOR_DARK_GREY = 8, + VGA_COLOR_LIGHT_BLUE = 9, + VGA_COLOR_LIGHT_GREEN = 10, + VGA_COLOR_LIGHT_CYAN = 11, + VGA_COLOR_LIGHT_RED = 12, + VGA_COLOR_LIGHT_MAGENTA = 13, + VGA_COLOR_LIGHT_BROWN = 14, + VGA_COLOR_WHITE = 15 +}; + +static const size_t VGA_WIDTH = 80; +static const size_t VGA_HEIGHT = 25; + +size_t terminal_row; +size_t terminal_column; +uint8_t terminal_color; +uint16_t *terminal_buffer; + +static uint8_t vga_entry_color(enum vga_color fg, enum vga_color bg) +{ + return fg | bg << 4; +} + +static uint16_t vga_entry(unsigned char uc, uint8_t color) +{ + return (uint16_t) uc | (uint16_t) color << 8; +} + +size_t strlen(const char* str) +{ + size_t len = 0; + while(str[len]) + len++; + return len; +} + +void terminal_initialize(void) +{ + size_t x, y; + terminal_row = 0; + terminal_column = 0; + terminal_color = vga_entry_color(VGA_COLOR_LIGHT_GREY, VGA_COLOR_BLACK); + terminal_buffer = (uint16_t*) 0xB8000; + for(y = 0; y < VGA_HEIGHT; y++) { + for(x = 0; x < VGA_WIDTH; x++) { + const size_t index = y * VGA_WIDTH + x; + terminal_buffer[index] = vga_entry(' ', terminal_color); + } + } +} + +void terminal_setcolor(uint8_t color) +{ + terminal_color = color; +} + +void terminal_putentryat(char c, uint8_t color, size_t x, size_t y) +{ + const size_t index = y * VGA_WIDTH + x; + terminal_buffer[index] = vga_entry(c, color); +} + +void terminal_putchar(char c) +{ + terminal_putentryat(c, terminal_color, terminal_column, terminal_row); + if(++terminal_column == VGA_WIDTH) { + terminal_column = 0; + if(++terminal_row == VGA_HEIGHT) + terminal_row = 0; + } +} + +void terminal_write(const char* data, size_t size) +{ + size_t i; + for(i = 0; i < size; i++) + terminal_putchar(data[i]); +} + +void terminal_writestring(const char* data) +{ + terminal_write(data, strlen(data)); +} + +void kernel_main(void) +{ + /* Initialize terminal interface */ + terminal_initialize(); + + terminal_writestring("Welcome to MFSOS!"); +} -- cgit v1.2.3-18-g5258