init project structure

src/boot.s

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+/* 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 <stdio.h> 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
+

src/kernel.c

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+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+/* 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
+
+/* This tutorial will only work for the 32-bit ix86 targets. */
+#if !defined(__i386__)
+#error "This tutorial 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 inline uint8_t vga_entry_color(enum vga_color fg, enum vga_color bg) {
+  return fg | bg << 4;
+}
+
+static inline uint16_t vga_entry(unsigned char uc, uint8_t color) {
+  return (uint16_t)uc | (uint16_t)color << 8;
+}
+
+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;
+
+void terminal_initialize(void) {
+  terminal_row = 0;
+  terminal_column = 0;
+  terminal_color = vga_entry_color(VGA_COLOR_LIGHT_GREY, VGA_COLOR_BLACK);
+  terminal_buffer = (uint16_t *)0xB8000;
+  for (size_t y = 0; y < VGA_HEIGHT; y++) {
+    for (size_t 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) {
+  for (size_t 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();
+
+  /* Newline support is left as an exercise. */
+  terminal_writestring("Hello, kernel World!\n");
+}