[gdt] Global Descriptor Table driver

Makefile

@@ -7,8 +7,8 @@ install: kernel
 	cp kernel /mnt/kernel
 	umount /mnt
 
-kernel: start.o link.ld main.o vga.o
-	ld -m elf_i386 -T link.ld -o kernel start.o main.o vga.o
+kernel: start.o link.ld main.o vga.o gdt.o
+	ld -m elf_i386 -T link.ld -o kernel *.o
 
 %.o: %.c
 	gcc -Wall -m32 -O -fstrength-reduce -fomit-frame-pointer -finline-functions -nostdinc -fno-builtin -I./include -c -o $@ $<

gdt.c

@@ -0,0 +1,74 @@
+#include <system.h>
+
+/*
+ * Global Descriptor Tablet Entry
+ */
+struct gdt_entry {
+	unsigned short limit_low;
+	unsigned short base_low;
+	unsigned char base_middle;
+	unsigned char access;
+	unsigned char granularity;
+	unsigned char base_high;
+} __attribute__((packed));
+
+/*
+ * GDT pointer
+ */
+struct gdt_ptr {
+	unsigned short limit;
+	unsigned int base;
+} __attribute__((packed));
+
+struct gdt_entry	gdt[3];
+struct gdt_ptr		gp;
+
+/*
+ * (ASM) gdt_flush
+ * Reloads the segment registers
+ */
+extern void gdt_flush();
+
+/*
+ * gdt_set_gate
+ * Set a GDT descriptor
+ */
+void
+gdt_set_gate(
+		int num,
+		unsigned long base,
+		unsigned long limit,
+		unsigned char access,
+		unsigned char gran
+		) {
+	/* Base Address */
+	gdt[num].base_low =		(base & 0xFFFF);
+	gdt[num].base_middle =	(base >> 16) & 0xFF;
+	gdt[num].base_high =	(base >> 24) & 0xFF;
+	/* Limits */
+	gdt[num].limit_low =	(limit & 0xFFFF);
+	gdt[num].granularity =	(limit >> 16) & 0X0F;
+	/* Granularity */
+	gdt[num].granularity |= (gran & 0xF0);
+	/* Access flags */
+	gdt[num].access = access;
+}
+
+/*
+ * gdt_install
+ * Install the kernel's GDTs
+ */
+void
+gdt_install() {
+	/* GDT pointer and limits */
+	gp.limit = (sizeof(struct gdt_entry) * 3) - 1;
+	gp.base = &gdt;
+	/* NULL */
+	gdt_set_gate(0, 0, 0, 0, 0);
+	/* Code segment */
+	gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
+	/* Data segment */
+	gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
+	/* Go go go */
+	gdt_flush();
+}

include/system.h

@@ -16,4 +16,8 @@ extern void puts(unsigned char *str);
 extern void settextcolor(unsigned char forecolor, unsigned char backcolor);
 extern void init_video();
 
+/* GDT */
+extern void gdt_install();
+extern void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran);
+
 #endif

main.c

@@ -101,6 +101,7 @@ outportb(
  */
 int
 main() {
+	gdt_install();
 	init_video();
 	puts("Hello world!\n");
 	for (;;);

start.asm

@@ -32,6 +32,19 @@ stublet:
 	jmp		$
 
 ; GDT
+global gdt_flush
+extern gp
+gdt_flush:
+	lgdt [gp]
+	mov ax, 0x10
+	mov ds, ax
+	mov es, ax
+	mov fs, ax
+	mov gs, ax
+	mov ss, ax
+	jmp 0x08:flush2
+flush2:
+	ret
 
 ; Interrupt Service Routines