cowos attempted gdt, but at least we have vga drivers now

kernel/x86/gdt.c

@@ -0,0 +1,73 @@
+#include <gdt.h>
+
+static GDTEntry descriptors[5];
+static GDTPtr gdtptr;
+
+void
+gdt_reload()
+{
+	asm volatile("lgdt %0\n\t"
+		"push $0x28\n\t"
+		"lea 1f(%%rip), %%rax\n\t"
+             	"push %%rax\n\t"
+               	"lretq\n\t"
+               	"1:\n\t"
+               	"mov $0x30, %%eax\n\t"
+               	"mov %%eax, %%ds\n\t"
+               	"mov %%eax, %%es\n\t"
+               	"mov %%eax, %%fs\n\t"
+               	"mov %%eax, %%gs\n\t"
+               	"mov %%eax, %%ss\n\t"
+               	:
+               	: "m"(gdtptr)
+               	: "rax", "memory");	
+}
+
+void
+initGDT()
+{
+	/* null descriptor (0) */
+	descriptors[0].limit = 0;
+	descriptors[0].base_low = 0;
+  	descriptors[0].base_mid = 0;
+  	descriptors[0].access = 0;
+  	descriptors[0].granularity = 0;
+  	descriptors[0].base_high = 0;
+	
+	 /* kernel code 64 (40) */
+  	descriptors[1].limit = 0;
+  	descriptors[1].base_low = 0;
+  	descriptors[1].base_mid = 0;
+  	descriptors[1].access = 0b10011010;
+  	descriptors[1].granularity = 0b00100000;
+  	descriptors[1].base_high = 0;
+
+  	/* kernel data 64 (48) */
+  	descriptors[2].limit = 0;
+  	descriptors[2].base_low = 0;
+  	descriptors[2].base_mid = 0;
+  	descriptors[2].access = 0b10010010;
+  	descriptors[2].granularity = 0;
+  	descriptors[2].base_high = 0;
+
+	// user code 64 (72)
+  	descriptors[3].limit = 0;
+  	descriptors[3].base_low = 0;
+  	descriptors[3].base_mid = 0;
+  	descriptors[3].access = 0b11111010;
+  	descriptors[3].granularity = 0b00100000;
+  	descriptors[3].base_high = 0;
+
+  	// user data 64. (80)
+  	descriptors[4].limit = 0;
+  	descriptors[4].base_low = 0;
+  	descriptors[4].base_mid = 0;
+  	descriptors[4].access = 0b11110010;
+  	descriptors[4].granularity = 0;
+  	descriptors[4].base_high = 0;
+	
+	gdtptr.limit = sizeof(descriptors) - 1;
+	gdtptr.base = (uint64_t)&descriptors;
+
+	gdt_reload();
+}