4 files changed, 312 insertions, 0 deletions
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..31f1bbb
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,33 @@
+OSNAME = mfsos
+OSBIN = $(OSNAME).bin
+
+OBJECTS = kernel/main.o
+LINKER = kernel/linker.ld
+BOOT_SRC = kernel/boot.s
+BOOT_OBJ = ${BOOT_SRC:.s=.o}
+
+TARGET_TOOLS = $(HOME)/path/to/cross/compiler/i686-elf-
+CC = $(TARGET_TOOLS)gcc
+AS = $(TARGET_TOOLS)as
+
+.PHONY: all qemu clean
+
+all: $(OSBIN)
+
+$(OSBIN): $(BOOT_OBJ) $(OBJECTS)
+	$(CC) -T $(LINKER) -o $(OSBIN) -ffreestanding -O2 -nostdlib \
+		$(OBJECTS) $(BOOT_OBJ) -lgcc
+
+$(BOOT_OBJ): $(BOOT_SRC)
+	$(AS) $< -o $@
+
+%.o: %.c
+	$(CC) -c $< -o $@ \
+		-std=gnu89 -ffreestanding -O2 -Wall \
+		-Wpedantic -Wextra
+
+qemu: $(OSBIN)
+	qemu-system-i386 -kernel $(OSBIN)
+
+clean:
+	rm -rf $(BOOT_OBJ) $(OBJECTS) $(OSBIN)
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 <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
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 <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
+
+#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!");
+}