Give kernel image a header.

Kernel image now has a header with version, magic number, and a
pointer to its actual entrypoint. Entry point is now _start in
boot.s, and we now generate versions.s in the build tree for the
version macros.

src/modules/main/boot.s

@@ -0,0 +1,26 @@
+MAGIC	equ  0x600db007			; Popcorn OS header magic number
+
+section .header
+align 4
+global _header
+_header:
+	dd MAGIC
+	db VERSION_MAJOR
+	db VERSION_MINOR
+	dw VERSION_PATCH
+	dd VERSION_GITSHA
+	dq _start
+
+section .text
+align 16
+global _start:function (_start.end - _start)
+_start:
+	extern kernel_main
+	call kernel_main
+
+	cli
+
+.hang:
+	hlt
+	jmp .hang
+.end:

src/modules/main/debug.s

@@ -0,0 +1,13 @@
+
+section .text
+global do_the_set_registers
+do_the_set_registers:
+	mov rax, 0xdeadbeef0badc0de
+	mov rbx, 0xdeadbeef0badc0de
+	mov rcx, 0xdeadbeef0badc0de
+	mov rdx, 0xdeadbeef0badc0de
+
+global _halt
+_halt:
+	hlt
+	jmp _halt

src/modules/main/main.c

@@ -1,8 +1,13 @@
-__attribute__((section(".text.entry")))
+#include "vga.h"
+
+void do_the_set_registers();
+
 void kernel_main() {
 	volatile register int foo = 0x1a1b1c10;
 	volatile register int bar = 0;
 
-	while(1)
-		foo = foo | 0xfffffff0 + bar++ | 0xf;
+	terminal_initialize(5);
+	terminal_writestring("YES HELLO THIS IS KERNEL");
+
+	do_the_set_registers();
 }

src/modules/main/vga.c

@@ -0,0 +1,54 @@
+#include "vga.h"
+
+static const size_t VGA_WIDTH = 80;
+static const size_t VGA_HEIGHT = 25;
+
+static size_t terminal_row;
+static size_t terminal_column;
+static uint8_t terminal_color;
+
+/* Note the use of the volatile keyword to prevent the compiler from eliminating dead stores. */
+static volatile uint16_t* terminal_buffer;
+
+uint8_t vga_entry_color(enum vga_color fg, enum vga_color bg);
+uint16_t vga_entry(unsigned char uc, uint8_t color);
+
+static size_t strlen(const char* str) {
+	size_t len = 0;
+	while (str[len++]);
+	return len;
+}
+
+void terminal_initialize(size_t startrow) {
+	terminal_row = startrow;
+	terminal_column = 0;
+	terminal_color = vga_entry_color(VGA_COLOR_LIGHT_GREY, VGA_COLOR_BLACK);
+	terminal_buffer = (uint16_t*) 0xB8000;
+}
+
+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));
+}

src/modules/main/vga.h

@@ -0,0 +1,38 @@
+#pragma once
+#include <stddef.h>
+#include <stdint.h>
+
+/* 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,
+};
+
+inline uint8_t vga_entry_color(enum vga_color fg, enum vga_color bg) {
+	return fg | bg << 4;
+}
+
+inline uint16_t vga_entry(unsigned char uc, uint8_t color) {
+	return (uint16_t) uc | (uint16_t) color << 8;
+}
+
+void terminal_initialize(size_t rowstart);
+void terminal_setcolor(uint8_t color);
+void terminal_putentryat(char c, uint8_t color, size_t x, size_t y);
+void terminal_putchar(char c);
+void terminal_write(const char* data, size_t size);
+void terminal_writestring(const char* data);