WIP GDT code

src/kernel/kernel.c

@@ -5,11 +5,106 @@
 #include "libc.h"
 #include "tty.h"
 
+#define STR_(x) #x
+#define STR(x) STR_(x)
+
+/**
+ * GDT Setup
+ * =========
+ */
+enum gdt_segment_descriptor_index : size_t {
+	GDT_SDESC_BASE_0 = 2,
+	GDT_SDESC_BASE_1 = 3,
+	GDT_SDESC_BASE_2 = 4,
+	GDT_SDESC_BASE_3 = 7,
+
+	GDT_SDESC_LIMIT_0 = 0,
+	GDT_SDESC_LIMIT_1 = 1,
+	GDT_SDESC_LIMIT_2 = 6,
+
+	GDT_SDESC_ACCESSBYTE = 5,
+
+	GDT_SDESC_FLAGS = 6,
+};
+
+enum gdt_access_flag : uint8_t {
+	GDT_ACCESS_ACCESSED   = (1<<0),
+	GDT_ACCESS_RW 		  = (1<<1),
+	GDT_ACCESS_DC 		  = (1<<2),
+	GDT_ACCESS_EXEC       = (1<<3),
+	GDT_ACCESS_DESCRIPTOR = (1<<4),
+	GDT_ACCESS_DPL        = (1<<5),
+	GDT_ACCESS_PRESENT    = (1<<6),
+};
+
+enum gdt_flags : uint8_t {
+	GDT_RESERVED    = (1<<0),
+	GDT_LONG        = (1<<1),
+	GDT_SIZE        = (1<<2),
+	GDT_GRANULARITY = (1<<3),
+}
+
+struct gdt {
+	uint32_t base;
+	uint32_t limit;
+	uint8_t  access_byte;
+   	uint8_t  flags;
+};
+
+static void gdt_encode_entry(uint8_t *target, struct GDT source)
+{
+#define GDT_LIMIT_MAX 0xFFFFF
+	if (source.limit > GDT_LIMIT_MAX)
+		panic(str_attach("GDT cannot encode limits larger than " STR(GDT_LIMIT_MAX)));
+
+	// encode the limit
+	target[GDT_SDESC_LIMIT_0] = (source.limit >>  0) & 0xFF;
+	target[GDT_SDESC_LIMIT_1] = (source.limit >>  8) & 0xFF;
+	target[GDT_SDESC_LIMIT_2] = (source.limit >> 16) & 0xFF;
+
+	// encode the base
+	target[GDT_SDESC_BASE_0] = (source.base >>  0) & 0xFF;
+	target[GDT_SDESC_BASE_1] = (source.base >>  8) & 0xFF;
+	target[GDT_SDESC_BASE_2] = (source.base >> 16) & 0xFF;
+	target[GDT_SDESC_BASE_3] = (source.base >> 24) & 0xFF;
+
+	// encode the access byte
+	target[GDT_SDESC_ACCESSBYTE] = source.access_byte;
+
+	// encode the flags
+	target[GDT_SDESC_FLAGS] |= (source.flags << 4);
+}
+
+void gdt_set(uint16_t limit, uint32_t base)
+{
+	/* the lgdt instruction requires a packed alignment */
+    struct __attribute__((packed)) gdtr  {
+        uint16_t limit;
+        uint32_t base;
+    };
+
+    struct gdtr gdtr = {
+        .limit = limit,
+        .base = base,
+    };
+
+    __asm__ volatile (
+        "lgdt %0"
+        :
+        : "m" (gdtr));
+}
+
+void gdt_segment_reload()
+
+/**
+ * Kernel entrypoint
+ * =================
+ */
 void kernel_main(void)
 {
 	terminal_clear();
 
-    for (size_t i = 0; i < 15; i++) {
+    for (size_t i = 0; i < VGA_HEIGHT / 2 + 1; i++) {
         printf(str_attach("hello {u32}!\n"), i);
     }
 }