1 files changed, 114 insertions, 0 deletions
diff --git a/kernel/kernel.c b/kernel/kernel.c
new file mode 100644
index 0000000..5054231
--- /dev/null
+++ b/kernel/kernel.c
@@ -0,0 +1,114 @@
+#if !defined(__cplusplus)
+#include <stdbool.h> /* C doesn't have booleans by default. */
+#endif
+#include <stddef.h>
+#include <stdint.h>
+
+/* Check if the compiler thinks we are targeting the wrong operating system. */
+#if defined(__linux__)
+/* TODO how does this check for a cross compiler? It was failing. */
+/*#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 {
+    COLOR_BLACK         = 0,
+    COLOR_BLUE          = 1,
+    COLOR_GREEN         = 2,
+    COLOR_CYAN          = 3,
+    COLOR_RED           = 4,
+    COLOR_MAGENTA       = 5,
+    COLOR_BROWN         = 6,
+    COLOR_LIGHT_GREY    = 7,
+    COLOR_DARK_GREY     = 8,
+    COLOR_LIGHT_BLUE    = 9,
+    COLOR_LIGHT_GREEN   = 10,
+    COLOR_LIGHT_CYAN    = 11,
+    COLOR_LIGHT_RED     = 12,
+    COLOR_LIGHT_MAGENTA = 13,
+    COLOR_LIGHT_BROWN   = 14,
+    COLOR_WHITE         = 15,
+};
+
+static uint8_t make_color(enum vga_color fg, enum vga_color bg)
+{
+    return fg | bg << 4;
+}
+
+static uint16_t make_vgaentry(char c, uint8_t color)
+{
+    uint16_t c16 = c;
+    uint16_t color16 = color;
+    return c16 | color16 << 8;
+}
+
+/* static size_t strlen(const char* str) */
+/* { */
+/*     size_t ret = 0; */
+/*     for (const char* p = str; *p != 0; p++, ret++); */
+/*     return ret; */
+/* } */
+
+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 void terminal_initialize()
+{
+    terminal_row = 0;
+    terminal_column = 0;
+    terminal_color = make_color(COLOR_LIGHT_GREY, 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] = make_vgaentry(' ', terminal_color);
+        }
+    }
+}
+
+/* static void terminal_setcolor(uint8_t color) */
+/* { */
+/*     terminal_color = color; */
+/* } */
+
+static 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] = make_vgaentry(c, color);
+}
+
+static 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;
+        }
+    }
+}
+
+static void terminal_writestring(const char* data)
+{
+    for (const char* p = data; *p != 0; p++)
+        terminal_putchar(*p);
+}
+
+#if defined(__cplusplus)
+extern "C" /* Use C linkage for kernel_main. */
+#endif
+void kernel_main()
+{
+    terminal_initialize();
+    terminal_writestring("Hello, kernel World!\n");
+}