1 files changed, 79 insertions, 0 deletions
diff --git a/arch/i386/boot/gdt.c b/arch/i386/boot/gdt.c
new file mode 100644
index 0000000..d9bfc1e
--- /dev/null
+++ b/arch/i386/boot/gdt.c
@@ -0,0 +1,79 @@
+#include <kernel/io.h>
+#include <stdint.h>
+
+#define SEG_DESCTYPE(x)  ((x) << 0x04) // Descriptor type (0 for system, 1 for code/data)
+#define SEG_PRES(x)      ((x) << 0x07) // Present
+#define SEG_SAVL(x)      ((x) << 0x0C) // Available for system use
+#define SEG_LONG(x)      ((x) << 0x0D) // Long mode
+#define SEG_SIZE(x)      ((x) << 0x0E) // Size (0 for 16-bit, 1 for 32)
+#define SEG_GRAN(x)      ((x) << 0x0F) // Granularity (0 for 1B - 1MB, 1 for 4KB - 4GB)
+#define SEG_PRIV(x)     (((x) &  0x03) << 0x05)   // Set privilege level (0 - 3)
+ 
+#define SEG_DATA_RD        0x00 // Read-Only
+#define SEG_DATA_RDA       0x01 // Read-Only, accessed
+#define SEG_DATA_RDWR      0x02 // Read/Write
+#define SEG_DATA_RDWRA     0x03 // Read/Write, accessed
+#define SEG_DATA_RDEXPD    0x04 // Read-Only, expand-down
+#define SEG_DATA_RDEXPDA   0x05 // Read-Only, expand-down, accessed
+#define SEG_DATA_RDWREXPD  0x06 // Read/Write, expand-down
+#define SEG_DATA_RDWREXPDA 0x07 // Read/Write, expand-down, accessed
+#define SEG_CODE_EX        0x08 // Execute-Only
+#define SEG_CODE_EXA       0x09 // Execute-Only, accessed
+#define SEG_CODE_EXRD      0x0A // Execute/Read
+#define SEG_CODE_EXRDA     0x0B // Execute/Read, accessed
+#define SEG_CODE_EXC       0x0C // Execute-Only, conforming
+#define SEG_CODE_EXCA      0x0D // Execute-Only, conforming, accessed
+#define SEG_CODE_EXRDC     0x0E // Execute/Read, conforming
+#define SEG_CODE_EXRDCA    0x0F // Execute/Read, conforming, accessed
+ 
+#define GDT_CODE_PL0    SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
+                        SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
+                        SEG_PRIV(0)     | SEG_CODE_EXRD
+ 
+#define GDT_DATA_PL0    SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
+                        SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
+                        SEG_PRIV(0)     | SEG_DATA_RDWR
+ 
+#define GDT_CODE_PL3    SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
+                        SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
+                        SEG_PRIV(3)     | SEG_CODE_EXRD
+ 
+#define GDT_DATA_PL3    SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
+                        SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
+                        SEG_PRIV(3)     | SEG_DATA_RDWR
+
+struct gpt_ptr {
+        uint16_t limit;
+        uint32_t base;
+}__attribute__((packed));
+
+uint64_t desc[3];
+struct gpt_ptr gp;
+
+extern void flush_gdt(struct gpt_ptr *ptr);
+
+uint64_t create_gdt_entry(uint32_t base, uint32_t limit, uint16_t flag) {
+        uint64_t ret;
+
+        ret =   limit           & 0x000F0000;
+        ret |=  (flag << 8)     & 0x00F0FF00;
+        ret |=  (base >> 16)    & 0x000000FF;
+        ret |=  base            & 0xFF000000;
+
+        ret <<= 32;
+
+        ret |= base     << 16;
+        ret |= limit    & 0x0000FFFF;
+
+        return ret;
+}
+
+void gdt_install(void) {
+        gp.limit = 64 * 3 - 1;
+        gp.base = (uint32_t)&desc;
+
+        desc[0] = create_gdt_entry(0, 0, 0);
+        desc[1] = create_gdt_entry(0, 0x000FFFFF, (GDT_CODE_PL0));
+        desc[2] = create_gdt_entry(0, 0x000FFFFF, (GDT_DATA_PL0));
+        flush_gdt(&gp);
+}