setting up the GDT

1 files changed, 127 insertions, 0 deletions

diff --git a/kernel/gdt.c b/kernel/gdt.c
new file mode 100644
index 0000000..1ee2042
--- /dev/null
+++ b/kernel/gdt.c
@@ -0,0 +1,127 @@
+#include <kernel/tty.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));
+
+enum {
+	SEG_null,
+	SEG_r0data,
+	SEG_r0code,
+	SEG_r3data,
+	SEG_r3code,
+	SEG_TSS,
+
+	SEG_end    
+};
+static struct gdt_entry GDT[6];
+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() {
+	// null segment
+	GDT[0].present = 0;
+
+	// ring0 data
+	GDT[1].limit_low  = 0xFFFF;
+	GDT[1].limit_high = 0xF;
+	GDT[1].gran       = 1; // 4KB * 0xFFFFF = (almost) 4GB
+
+	GDT[1].base_low   = 0;
+	GDT[1].base_high  = 0;
+	GDT[1].accessed   = 0;
+	GDT[1].rw         = 1;
+	GDT[1].conforming = 0;
+	GDT[1].code       = 0;
+	GDT[1].codeordata = 1;
+	GDT[1].ring       = 0;
+	GDT[1].present    = 1;
+	GDT[1].long_mode  = 0; // ???
+	GDT[1].available  = 1; // ???
+	GDT[1].x32        = 1;
+
+	// copy to r0 code
+	GDT[2] = GDT[1];
+	GDT[2].code = 1;
+
+	// r3 data & code
+	GDT[3] = GDT[1];
+	GDT[3].ring = 3;
+	GDT[4] = GDT[2];
+	GDT[3].ring = 3;
+
+	{ // tss
+		// TODO memset(&TSS, 0, sizeof(TSS));
+		TSS.ss0 = 1 << 3; // kernel data segment
+		TSS.esp0 = (uint32_t) &stack_top;
+
+		GDT[5].limit_low  = sizeof(TSS);
+		GDT[5].base_low   = (uint32_t) &TSS;
+		GDT[5].accessed   = 1; // 0 for TSS
+		GDT[5].rw         = 0; // 1 busy / 0 not busy
+		GDT[5].conforming = 0; // 0 for TSS
+		GDT[5].code       = 1; // 32bit
+		GDT[5].codeordata = 0; // is a system entry
+		GDT[5].ring       = 3;
+		GDT[5].present    = 1;
+		GDT[5].limit_high = (sizeof(TSS) >> 16) & 0xf;
+		GDT[5].available  = 0; // 0 for TSS
+		GDT[5].long_mode  = 0;
+		GDT[5].x32        = 0; // idk
+		GDT[5].gran       = 0;
+		GDT[5].base_high  = (((uint32_t) &TSS) >> 24) & 0xff;
+	}
+}
+
+static void gdt_load() {
+	lgdt_arg.limit = sizeof(GDT) - 1;
+	lgdt_arg.base = (uint32_t) &GDT;
+	asm("lgdt (%0)" : : "b" (&lgdt_arg));
+}
+
+void gdt_init() {
+	gdt_prepare();
+	gdt_load();
+	// check if the GDT was set up correctly
+	tty_write("checking gdt...", 15);
+	asm("mov $8, %%eax;"
+	    "mov %%eax, %%ds;"
+	    : : : "%eax");
+	tty_write("ok", 2);
+}