1 files changed, 121 insertions, 0 deletions
diff --git a/src/kernel/gdt.c b/src/kernel/gdt.c
new file mode 100644
index 0000000..b0d6320
--- /dev/null
+++ b/src/kernel/gdt.c
@@ -0,0 +1,121 @@
+#include <kernel/gdt.h>
+#include <kernel/util.h>
+#include <stdint.h>
+
+extern void stack_top; // platform/boot.s
+
+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 prev_tss; // unused
+	uint32_t esp0; // kernel mode stack pointer
+	uint32_t ss0;  // kernel mode stack segment
+	// total size = 0x68 (?) - 3 * sizeof(uint32_t) = 5c
+	uint8_t  _unused[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_prepare();
+static void gdt_load();
+
+
+static void gdt_prepare() {
+	GDT[SEG_null].present = 0;
+
+	GDT[SEG_r0data].limit_low  = 0xFFFF;
+	GDT[SEG_r0data].limit_high = 0xF;
+	GDT[SEG_r0data].gran       = 1; // 4KB * 0xFFFFF = (almost) 4GB
+	GDT[SEG_r0data].base_low   = 0;
+	GDT[SEG_r0data].base_high  = 0;
+	GDT[SEG_r0data].accessed   = 0;
+	GDT[SEG_r0data].rw         = 1;
+	GDT[SEG_r0data].conforming = 0;
+	GDT[SEG_r0data].code       = 0;
+	GDT[SEG_r0data].codeordata = 1;
+	GDT[SEG_r0data].ring       = 0;
+	GDT[SEG_r0data].present    = 1;
+	GDT[SEG_r0data].long_mode  = 0; // ???
+	GDT[SEG_r0data].available  = 1; // ???
+	GDT[SEG_r0data].x32        = 1;
+
+	// copy to r0 code
+	GDT[SEG_r0code] = GDT[SEG_r0data];
+	GDT[SEG_r0code].code = 1;
+
+	GDT[SEG_r3data] = GDT[SEG_r0data];
+	GDT[SEG_r3data].ring = 3;
+	GDT[SEG_r3code] = GDT[SEG_r0code];
+	GDT[SEG_r3code].ring = 3;
+
+	// tss
+	memset(&TSS, 0, sizeof(TSS));
+	TSS.ss0 = SEG_r0data << 3; // kernel data segment
+	TSS.esp0 = (uint32_t) &stack_top;
+
+	GDT[SEG_TSS].limit_low  = sizeof(TSS);
+	GDT[SEG_TSS].base_low   = (uint32_t) &TSS;
+	GDT[SEG_TSS].accessed   = 1; // 1 for TSS
+	GDT[SEG_TSS].rw         = 0; // 1 busy / 0 not busy
+	GDT[SEG_TSS].conforming = 0; // 0 for TSS
+	GDT[SEG_TSS].code       = 1; // 32bit
+	GDT[SEG_TSS].codeordata = 0; // is a system entry
+	GDT[SEG_TSS].ring       = 3;
+	GDT[SEG_TSS].present    = 1;
+	GDT[SEG_TSS].limit_high = (sizeof(TSS) >> 16) & 0xf;
+	GDT[SEG_TSS].available  = 0; // 0 for TSS
+	GDT[SEG_TSS].long_mode  = 0;
+	GDT[SEG_TSS].x32        = 0; // idk
+	GDT[SEG_TSS].gran       = 0;
+	GDT[SEG_TSS].base_high  = (((uint32_t) &TSS) >> 24) & 0xff;
+}
+
+void change_cs(int seg); // temporary
+
+static void gdt_load() {
+	lgdt_arg.limit = sizeof(GDT) - 1;
+	lgdt_arg.base = (uint32_t) &GDT;
+	asm("lgdt (%0)" 
+	    : : "r" (&lgdt_arg) : "memory");
+	asm("ltr %%ax"
+	    : : "a" (SEG_TSS << 3 | 3) : "memory");
+
+	// update all segment registers
+	change_cs(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();
+}