1 files changed, 127 insertions, 0 deletions
diff --git a/src/kernel/arch/i386/gdt/gdt.c b/src/kernel/arch/i386/gdt/gdt.c
new file mode 100644
index 0000000..2935e6e
--- /dev/null
+++ b/src/kernel/arch/i386/gdt/gdt.c
@@ -0,0 +1,127 @@
+#include <kernel/arch/generic.h>
+#include <kernel/arch/i386/gdt.h>
+#include <kernel/util.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+
+struct gdt_entry {
+	uint64_t limit_low  : 16;
+	uint64_t base_low   : 24;
+	uint64_t accessed   :  1; // set by the processor
+	                          // CODE        | DATA
+	uint64_t rw         :  1; // readable?   | writeable?
+	uint64_t conforming :  1; // conforming? | expands down?
+	uint64_t code       :  1; // 1           | 0
+
+	uint64_t codeordata :  1; // 1 for everything other than TSS and LDT
+	uint64_t ring       :  2;
+	uint64_t present    :  1; // always 1
+	uint64_t limit_high :  4;
+	uint64_t available  :  1; // ???
+	uint64_t long_mode  :  1;
+	uint64_t x32        :  1;
+	uint64_t gran       :  1; // 1 - 4kb, 0 - 1b
+	uint64_t base_high  :  8;
+} __attribute__((packed));
+
+struct tss_entry {
+	uint32_t _unused0;
+	uint32_t esp0; // kernel mode stack pointer
+	uint32_t ss0;  // kernel mode stack segment
+	uint8_t  _unused1[0x5c];
+} __attribute__((packed));
+
+struct lgdt_arg {
+	uint16_t limit;
+	uint32_t base;
+} __attribute__((packed));
+
+static struct gdt_entry GDT[SEG_end];
+static struct tss_entry TSS;
+static struct lgdt_arg lgdt_arg; // probably doesn't need to be global
+
+static void gdt_fillout(struct gdt_entry* entry, uint8_t ring, bool code);
+static void gdt_prepare();
+static void gdt_load();
+
+
+static void gdt_fillout(struct gdt_entry* entry, uint8_t ring, bool code) {
+	*entry = (struct gdt_entry) {
+		// set up the identity mapping
+		.limit_low  = 0xFFFF,
+		.limit_high = 0xF,
+		.gran       = 1, // 4KB * 0xFFFFF = (almost) 4GB
+		.base_low   = 0,
+		.base_high  = 0,
+
+		.ring       = ring,
+		.code       = code,
+
+		.accessed   = 0,
+		.rw         = 1,
+		.conforming = 0,
+		.codeordata = 1,
+		.present    = 1,
+		.long_mode  = 0, // ???
+		.available  = 1, // ???
+		.x32        = 1,
+	};
+}
+
+static void gdt_prepare() {
+	GDT[SEG_null].present = 0;
+
+	gdt_fillout(&GDT[SEG_r0code], 0, true);
+	gdt_fillout(&GDT[SEG_r0data], 0, false);
+	gdt_fillout(&GDT[SEG_r3code], 3, true);
+	gdt_fillout(&GDT[SEG_r3data], 3, false);
+
+	// tss
+	memset(&TSS, 0, sizeof(TSS));
+	TSS.ss0 = SEG_r0data << 3; // kernel data segment
+	TSS.esp0 = (uintptr_t) &stack_top;
+
+	GDT[SEG_TSS] = (struct gdt_entry) {
+		.limit_low  = sizeof(TSS),
+		.limit_high = sizeof(TSS) >> 16,
+		.gran       = 0,
+		.base_low   =  (uintptr_t) &TSS,
+		.base_high  = ((uintptr_t) &TSS) >> 24,
+
+		.accessed   = 1, // 1 for TSS
+		.rw         = 0, // 1 busy / 0 not busy
+		.conforming = 0, // 0 for TSS
+		.code       = 1, // 32bit
+		.codeordata = 0, // is a system entry
+		.ring       = 3,
+		.present    = 1,
+		.available  = 0, // 0 for TSS
+		.long_mode  = 0,
+		.x32        = 0, // idk
+	};
+}
+
+static void gdt_load() {
+	lgdt_arg.limit = sizeof(GDT) - 1;
+	lgdt_arg.base = (uintptr_t) &GDT;
+	asm("lgdt (%0)" 
+	    : : "r" (&lgdt_arg) : "memory");
+
+	asm("ltr %%ax"
+	    : : "a" (SEG_TSS << 3 | 3) : "memory");
+
+	// update all segment registers
+	gdt_farjump(SEG_r0code << 3);
+	asm("mov %0, %%ds;"
+	    "mov %0, %%ss;"
+	    "mov %0, %%es;"
+	    "mov %0, %%fs;"
+	    "mov %0, %%gs;"
+	    : : "r" (SEG_r0data << 3) : "memory");
+}
+
+void gdt_init() {
+	gdt_prepare();
+	gdt_load();
+}