1 files changed, 55 insertions, 0 deletions
diff --git a/i686/include/gdt.h b/i686/include/gdt.h
new file mode 100644
index 0000000..2bdfb22
--- /dev/null
+++ b/i686/include/gdt.h
@@ -0,0 +1,55 @@
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+
+enum Ring { Ring0 = 0x0, Ring1 = 0x1, Ring2 = 0x2, Ring3 = 0x3 };
+
+struct __attribute__((packed)) Access {
+  unsigned accessed : 1;   /* if 0, is set by processor when accessed */
+  unsigned readwrite : 1;  /* code seg: read toggle; data seg: write toggle */
+  unsigned direction : 1;  /* code seg: conforming bit; data seg: direction bit */
+  unsigned executable : 1; /* executable bit */
+  unsigned segment : 1;    /* true for code/data; false for gates/tss */
+  unsigned privilege : 2;  /* descriptor privilege level */
+  unsigned present : 1;    /* true for every active segment */
+};
+/* _Static_assert(sizeof(struct Access) == 1, "access byte size"); */
+
+static const struct Access null_access = {0, 0, 0, 0, 0, Ring0, 0};
+static const struct Access ktext_access = {0, 1, 0, 1, 1, Ring0, 1};
+static const struct Access kdata_access = {0, 1, 0, 0, 1, Ring0, 1};
+
+/* Segment Descriptor
+ * A memory structure (part of a table) that tells the CPU the attributes of a given segment
+ * |31|  |  |  |  |  |  |24|23|22|21|20|19|  |  |16|15|  |  |  |  |  |  | 8| 7|  |  |  |  |  |  | 0|
+ * |   base_31_24          | G|DB|  | A| lim_19_16 |   access              |   base_23_16          |
+ * |   base_15_0                                   |   limit_15_0                                  |
+ * |31|  |  |  |  |  |  |  |  |  |  |  |  |  |  |16|15|  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0|
+ * limit    size of segment - 1, either in bytes or in 4KiB chunks (check flags)
+ * base     address of segment
+ * access
+ * flags    defines the segment chunks and 16/32 bit */
+struct __attribute__((packed)) SegmentDescriptor_t {
+  uint16_t limit_15_0;      /* low bits of segment limit */
+  uint16_t base_15_0;       /* low bits of segment base address */
+  uint8_t base_23_16;       /* middle bits of segment base address */
+  uint8_t access;           /* access byte */
+  unsigned limit_19_16 : 4; /* high bits of segment limit */
+  /* flags */
+  bool a : 1;           /* unused, available for software use */
+  bool rsv : 1;         /* reserved */
+  bool db : 1;          /* false => 16-bit seg; true => 32-bit seg */
+  bool granularity : 1; /* limit scaled by 4k when set */
+  uint8_t base_31_24;   /* high bits of segment address */
+};
+/* _Static_assert(sizeof(struct SegmentDescriptor_t) == 8, "segment descriptor size"); */
+
+void SegmentDescriptor(struct SegmentDescriptor_t *self, unsigned base, unsigned limit, uint8_t access);
+
+void gdt_install();
+
+enum SegmentIndex {
+  ktextDescriptor = 2 * sizeof(struct SegmentDescriptor_t),
+  kdataDescriptor = 3 * sizeof(struct SegmentDescriptor_t)
+};