From 16a538de1a643af7cb45133d1eb5fd28e58d25d1 Mon Sep 17 00:00:00 2001
From: Joursoir <chat@joursoir.net>
Date: Fri, 7 Oct 2022 19:04:00 +0300
Subject: x86: add GDT code to the kernel

---
 Makefile               |  4 +++-
 arch/x86/gdt.c         | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++
 arch/x86/include/gdt.h |  2 ++
 kernel/main.c          |  3 +++
 4 files changed, 71 insertions(+), 1 deletion(-)
 create mode 100644 arch/x86/gdt.c

diff --git a/Makefile b/Makefile
index aebce96..0db6ae4 100644
--- a/Makefile
+++ b/Makefile
@@ -5,6 +5,7 @@ KERNBIN = kernel.bin
 BOOT ?= bootloader
 
 ARCH = x86
+ARCH_PATH = arch/$(ARCH)
 ARCH_INCLUDE = $(PWD)/arch/$(ARCH)/include
 ARCH_BOOT = arch/$(ARCH)/boot/$(BOOT)
 
@@ -26,7 +27,8 @@ KERN_HEAD = $(ARCH_BOOT)/head.o
 C_SOURCES = \
 	kernel/main.c \
 	kernel/string.c \
-	drivers/video/console/vgacon.c
+	drivers/video/console/vgacon.c \
+	$(ARCH_PATH)/gdt.c
 OBJECTS = ${C_SOURCES:.c=.o}
 
 export CC LD AS OBJDUMP
diff --git a/arch/x86/gdt.c b/arch/x86/gdt.c
new file mode 100644
index 0000000..d7e449a
--- /dev/null
+++ b/arch/x86/gdt.c
@@ -0,0 +1,63 @@
+#include <gdt.h>
+
+struct gdt_asm_t {
+	uint16_t size_minus_one;
+	struct gdt_entry *gdt_table;
+} __attribute__((packed));
+
+static struct gdt_entry gdt[8];
+
+static void gdt_set_entry(struct gdt_entry *g, uint32_t base, uint32_t limit, uint8_t access, uint8_t flags)
+{
+	// TODO: ASSERT(limit <= GDT_LIMIT_MAX);
+	// TODO: ASSERT(flags <= 0xf); // flags is 4 bits
+
+	g->base_low = (base && 0xffff);
+	g->base_middle = (base >> 16) & 0xff;
+	g->base_high = (base >> 24) & 0xff;
+
+	g->limit_low = (limit & 0xffff);
+	g->limit_high = (limit >> 16) & 0x0f;
+
+	g->access = access;
+	g->flags = flags & 0x0f;
+}
+
+static void load_gdt(struct gdt_entry *g, uint32_t entries_count)
+{
+	// TODO: ASSERT(interrupts are disabled);
+
+	struct gdt_asm_t table = {
+		(uint16_t)(entries_count * sizeof(struct gdt_entry) - 1),
+		g,
+	};
+
+	asm volatile (
+		"lgdt (%0)"
+		: // no output
+		: "p" (&table)
+		: "memory"
+	);
+}
+
+void init_segmentation(void)
+{
+	// NULL descriptor
+	gdt_set_entry(&gdt[0], 0, 0, 0, 0);
+
+	// Kernel code segment
+	gdt_set_entry(&gdt[1],
+		0,
+		GDT_LIMIT_MAX,
+		GDT_ACCESS_PRESENT | GDT_ACCESS_S | GDT_ACCESS_PL0 | GDT_ACCESS_RW | GDT_ACCESS_EX,
+		GDT_GRAN_4KB | GDT_32BIT);
+
+	// Kernel data segment
+	gdt_set_entry(&gdt[2],
+		0,
+		GDT_LIMIT_MAX,
+		GDT_ACCESS_PRESENT | GDT_ACCESS_S | GDT_ACCESS_PL0 | GDT_ACCESS_RW,
+		GDT_GRAN_4KB | GDT_32BIT);
+
+	load_gdt(gdt, 3);
+}
diff --git a/arch/x86/include/gdt.h b/arch/x86/include/gdt.h
index b8dd2f0..c352f32 100644
--- a/arch/x86/include/gdt.h
+++ b/arch/x86/include/gdt.h
@@ -86,4 +86,6 @@ struct gdt_entry {
 	uint8_t base_high;
 } __attribute__((packed));
 
+void init_segmentation(void);
+
 #endif /* __X86_GDT_H */
diff --git a/kernel/main.c b/kernel/main.c
index 5e6491c..c7665a1 100644
--- a/kernel/main.c
+++ b/kernel/main.c
@@ -1,4 +1,5 @@
 #include "video-vga.h"
+#include "gdt.h"
 
 /* Check if the compiler thinks you are targeting the wrong operating system. */
 #if defined(__linux__)
@@ -10,6 +11,8 @@
 #endif
 void kernel_main(void) 
 {
+	init_segmentation();
+
 	/* Initialize VGA video hardware */
 	vga_init();
 
-- 
cgit v1.2.3-18-g5258