1 files changed, 118 insertions, 0 deletions
diff --git a/src/kernel/arch/amd64/32/gdt.c b/src/kernel/arch/amd64/32/gdt.c
new file mode 100644
index 0000000..2229330
--- /dev/null
+++ b/src/kernel/arch/amd64/32/gdt.c
@@ -0,0 +1,118 @@
+#include <kernel/arch/amd64/32/gdt.h>
+#include <kernel/arch/generic.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+extern char _isr_mini_stack;
+
+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 reserved0;
+	uint64_t rsp[3];
+	uint64_t ist[8];
+	uint64_t reserved2;
+	uint16_t reserved3;
+	uint16_t iopb;
+} __attribute__((packed));
+
+struct lgdt_arg {
+	uint16_t limit;
+	uint32_t base;
+} __attribute__((packed));
+
+__attribute__((section(".shared")))
+struct gdt_entry GDT[SEG_end];
+__attribute__((section(".shared")))
+struct tss_entry TSS;
+
+struct lgdt_arg lgdt_arg;
+
+
+static void *memset32(void *s, int c, size_t n) {
+	uint8_t *s2 = s;
+	for (size_t i = 0; i < n; i++)
+		s2[i] = c;
+	return s;
+}
+
+
+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,
+		.base_low   = 0,
+		.base_high  = 0,
+
+		.ring       = ring,
+		.code       = code,
+
+		.accessed   = 0,
+		.rw         = 1,
+		.conforming = 0,
+		.codeordata = 1,
+		.present    = 1,
+		.long_mode  = 1,
+		.available  = 1,
+		.x32        = 0,
+	};
+}
+
+void gdt_init(void) {
+	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);
+
+	lgdt_arg.limit = sizeof(GDT) - 1;
+	lgdt_arg.base = (uint64_t) &GDT;
+
+
+	memset32(&TSS, 0, sizeof(TSS));
+	for (int i = 0; i < 3; i++)
+		TSS.rsp[i] = (uint64_t) &_isr_mini_stack;
+	TSS.ist[1] = (uint64_t) &_isr_mini_stack;
+
+	uint64_t tss_addr = (uint64_t) &TSS;
+	GDT[SEG_TSS] = (struct gdt_entry) {
+		.limit_low  = sizeof(TSS),
+		.limit_high = sizeof(TSS) >> 16,
+		.gran       = 0,
+		.base_low   = tss_addr,
+		.base_high  = tss_addr >> 24,
+
+		.accessed   = 1,
+		.rw         = 0,
+		.conforming = 0,
+		.code       = 1,
+		.codeordata = 0,
+		.ring       = 0, // was 3 pre-port
+		.present    = 1,
+		.available  = 1,
+		.long_mode  = 0,
+		.x32        = 0,
+	};
+	*((uint64_t*)&GDT[SEG_TSS2]) = (tss_addr >> 32);
+}