gdt: add simple gdt implementation

Add a simple Global Descriptor Table implementation based on [0].

[0] http://www.lowlevel.eu/wiki/Global_Descriptor_Table

2 files changed, 80 insertions, 0 deletions

diff --git a/core/gdt.c b/core/gdt.c
new file mode 100644
index 0000000..0085e2f
--- /dev/null
+++ b/core/gdt.c
@@ -0,0 +1,77 @@
+/*
+ * Copyright (C) 2017 Robin Krahl <robin.krahl@ireas.org>
+ * 
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation, either version 3 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ */
+
+#include <garmos/gdt.h>
+#include <garmos/types.h>
+
+struct gdt_sgmt {
+	uint16_t limit_lo;
+	uint16_t base_lo;
+	uint8_t base_hi;
+	uint8_t access;
+	uint8_t flags_limit_h;
+	uint8_t base_vhi;
+} __attribute__((packed));
+
+enum gdt_sgmts {
+	GDT_SGMT_NULL,
+	GDT_SGMT_CODE,
+	GDT_SGMT_DATA,
+	GDT_SGMT_COUNT
+};
+
+static struct gdt_sgmt gdt[GDT_SGMT_COUNT];
+
+struct gdtp {
+	uint16_t limit;
+	void *pointer;
+} __attribute__((packed));
+
+static struct gdtp gdtp = {
+	.limit = GDT_SGMT_COUNT * sizeof(*gdt) - 1,
+	.pointer = gdt,
+};
+
+static void gdt_sgmt_set(uint32_t base, uint32_t limit, uint8_t access,
+		struct gdt_sgmt *s)
+{
+	if (limit <= 0xFFFF) {
+		s->flags_limit_h = 0x40;
+	} else {
+		if ((limit & 0xFFF) != 0xFFF) {
+			limit = (limit >> 12) - 1;
+		} else {
+			limit >>= 12;
+		}
+		s->flags_limit_h = 0xC0;
+	}
+
+	s->limit_lo = limit & 0xFFFF;
+	s->flags_limit_h |= (limit >> 16) & 0xF;
+
+	s->base_lo = base & 0xFFFF;
+	s->base_hi = (base >> 16) & 0xFF;
+	s->base_vhi = (base >> 24) & 0xFF;
+
+	s->access = access;
+}
+
+void gdt_init(void)
+{
+	gdt_sgmt_set(0, 0, 0, &gdt[GDT_SGMT_NULL]);
+	gdt_sgmt_set(0, 64 * 1024 * 1024, 0x9A, &gdt[GDT_SGMT_CODE]);
+	gdt_sgmt_set(0, 64 * 1024 * 1024, 0x92, &gdt[GDT_SGMT_NULL]);
+
+	asm volatile("lgdt %0" : : "m" (gdtp));
+}
diff --git a/core/kernel.c b/core/kernel.c
index be9782d..586fe20 100644
--- a/core/kernel.c
+++ b/core/kernel.c
@@ -12,11 +12,14 @@
  * more details.
  */
 
+#include <garmos/gdt.h>
 #include <garmos/term.h>
 #include <garmos/types.h>
 
 void kernel_main(uint32_t multiboot, unsigned int *magicno)
 {
+	gdt_init();
+
 	term_clear();
 	term_print("garmos 0.1.0\n\n");