Add kernel. Link everything

nob.c

@@ -61,6 +61,40 @@ bool build_c(Nob_Cmd *cmd) {
     return true;
 }
 
+bool link_everything(Nob_Cmd *cmd) {
+    Nob_File_Paths objectFiles = {0};
+    nob_da_append(&objectFiles, BUILD_FOLDER "kernel.o");
+    nob_da_append(&objectFiles, BUILD_FOLDER "boot.o");
+    nob_cmd_append(cmd,
+                   "i686-elf-gcc",
+                   "-T", SRC_FOLDER "linker.ld",
+                   "-o", BUILD_FOLDER "kernel",
+                   "-ffreestanding",
+                   "-O2",
+                   "-nostdlib");
+
+    for (size_t i = 0; i < objectFiles.count; ++i) {
+        nob_cmd_append(cmd, objectFiles.items[i]);
+    }
+
+    nob_cmd_append(cmd, "-lgcc");
+    if (!nob_cmd_run(cmd))
+        return false;
+
+    return true;
+}
+
+bool check_kernel(Nob_Cmd *cmd) {
+    nob_cmd_append(cmd, "grub-file", "--is-x86-multiboot", BUILD_FOLDER "kernel");
+    if (!nob_cmd_run(cmd))
+        return false;
+
+    return true;
+}
+
+bool clean_build() {
+}
+
 int main(int argc, char **argv) {
     NOB_GO_REBUILD_URSELF(argc, argv);
 
@@ -71,10 +105,15 @@ int main(int argc, char **argv) {
 
     if (!render_clangd_config())
         return 1;
+
     if (!build_assembly(&cmd))
         return 1;
     if (!build_c(&cmd))
         return 1;
+    if (!link_everything(&cmd))
+        return 1;
+    if (!check_kernel(&cmd))
+        return 1;
 
     return 0;
 }

shell.nix

@@ -6,6 +6,7 @@ pkgs.mkShell {
       pkgsCross.i686-embedded.buildPackages.gcc
       pkgsCross.i686-embedded.buildPackages.binutils
       gdb
+      grub2
     ];
 
   shellHook = ''

src/kernel.c

@@ -2,15 +2,104 @@
 #include <stddef.h>
 #include <stdint.h>
 
+/* Check if the compiler thinks you are targeting the wrong operating system. */
 #if defined(__linux__)
-#error "Not using a cross compiler"
+#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 "Not using the right compiler"
+#error "This tutorial needs to be compiled with a ix86-elf compiler"
 #endif
 
-int main() {
+/* Hardware text mode color constants. */
-    return 1;
+enum vga_color {
-    return false;
+    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;
+}
+
+#define VGA_WIDTH 80
+#define VGA_HEIGHT 25
+#define VGA_MEMORY 0xB8000
+
+size_t terminal_row;
+size_t terminal_column;
+uint8_t terminal_color;
+uint16_t *terminal_buffer = (uint16_t *)VGA_MEMORY;
+
+void terminal_initialize(void) {
+    terminal_row = 0;
+    terminal_column = 0;
+    terminal_color = vga_entry_color(VGA_COLOR_LIGHT_GREY, VGA_COLOR_BLACK);
+
+    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");
 }

src/linker.ld

@@ -0,0 +1,48 @@
+ENTRY(_start)
+
+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. */
+}