first

Makefile

@@ -0,0 +1,33 @@
+CC = i686-elf-gcc
+ASMBLR = i686-elf-as
+
+CFLAGS = -std=gnu99 -ffreestanding 
+
+
+LDFLAGS = -ffreestanding -nostdlib 
+LDLIBS = -lgcc
+LDFILE = -T linker.ld
+
+c_objects = kernel.o
+asm_objects = boot.o
+
+ISODIR = isodir
+
+myos.iso: myos.bin grub.cfg
+	mkdir -p isodir/boot/grub
+	cp myos.bin isodir/boot/myos.bin
+	cp grub.cfg isodir/boot/grub/grub.cfg
+	grub-mkrescue -o myos.iso isodir
+
+
+myos.bin: $(c_objects) $(asm_objects) 
+	$(CC) $(LDFILE) -o $@  $(LDFLAGS) $(c_objects) $(asm_objects) $(LDLIBS)  
+
+$(c_objects): %.o: %.c
+
+$(asm_objects): %.o: %.s
+	$(ASMBLR) $< -o $@
+
+clean:
+	rm -f *.o myos.bin
+	rm -rf isodir

boot.s

@@ -0,0 +1,109 @@
+/* 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

grub.cfg

@@ -0,0 +1,3 @@
+menuentry "myos" {
+	multiboot /boot/myos.bin
+}

kernel.c

@@ -0,0 +1,155 @@
+#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;
+}
+ 
+size_t strlen(const char* str) {
+	size_t len = 0;
+	while (str[len])
+		len++;
+	return len;
+}
+ 
+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_shiftup(void) {
+	for(size_t y = 0; y < VGA_HEIGHT-1; y++) {
+		for (size_t x = 0; x < VGA_WIDTH; x++) {
+			size_t index_current = y * VGA_WIDTH  + x;
+			size_t index_next = (y+1) * VGA_WIDTH  + x;
+			terminal_buffer[index_current] =  terminal_buffer[index_next];
+		}
+	}
+}
+ 
+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) {
+	if (c == '\n'){
+		terminal_row = terminal_row + 1;
+		terminal_column = 0;
+		return;
+	}
+	terminal_putentryat(c, terminal_color, terminal_column, terminal_row);
+	terminal_column = terminal_column + 1;
+	if (terminal_column >= VGA_WIDTH) {
+		terminal_column = 0;
+		terminal_row = terminal_row + 1;
+	}
+	if (terminal_row >= VGA_HEIGHT) {
+	terminal_shiftup();
+	//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 int_to_string(char * str, size_t strlen, int conv) {
+	char buffer[100];
+	for (int i = 0; i < 100; i++) {
+		buffer[i] = -1;
+	}
+
+	size_t startpos = 0;
+	while (conv != 0) {
+		buffer[startpos] = (conv % 10) + 48;
+		conv = conv / 10;
+		startpos = startpos + 1;
+	}
+	size_t maxindex = startpos - 1;
+
+	int j = 0;
+	for (int i = maxindex; i >= 0; i--) {
+		if (i >= strlen) {
+			
+		} else {
+		str[i] = buffer[j];
+		j = j + 1;
+		}
+	}
+
+}
+ 
+void kernel_main(void) {
+	/* Initialize terminal interface */
+	terminal_initialize();
+ 
+	/* Newline support is left as an exercise. */
+
+	char x[50];
+	int_to_string(x, 50, 12345);
+	terminal_writestring(x);
+	terminal_writestring("Hello, kernel World!\n");
+	terminal_writestring("Hello, kernel World!\n");
+	terminal_writestring("Hello, kernel World!\n");
+}

linker.ld

@@ -0,0 +1,52 @@
+/* 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
+{
+	/* It used to be universally recommended to use 1M as a start offset,
+	   as it was effectively guaranteed to be available under BIOS systems.
+	   However, UEFI has made things more complicated, and experimental data
+	   strongly suggests that 2M is a safer place to load. In 2016, a new
+	   feature was introduced to the multiboot2 spec to inform bootloaders
+	   that a kernel can be loaded anywhere within a range of addresses and
+	   will be able to relocate itself to run from such a loader-selected
+	   address, in order to give the loader freedom in selecting a span of
+	   memory which is verified to be available by the firmware, in order to
+	   work around this issue. This does not use that feature, so 2M was
+	   chosen as a safer option than the traditional 1M. */
+	. = 2M;
+ 
+	/* 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. */
+}