Add more comments to GDT code

1 files changed, 42 insertions, 10 deletions

diff --git a/src/gdt.h b/src/gdt.h
index 99a6da2..244908b 100644
--- a/src/gdt.h
+++ b/src/gdt.h
@@ -33,15 +33,37 @@ public:
 
   class SegmentDescriptor {
   public:
-    SegmentDescriptor(uint32_t base = 0, uint32_t limit = 0, uint8_t type = 0);
+    enum Ring : unsigned int { Ring0 = 0x0, Ring1 = 0x1, Ring2 = 0x2, Ring3 = 0x3 };
+    struct Access {
+      bool accessed : 1 = false;              // if 0, is set by processor when accessed
+      bool r_w : 1 = false;                   // on code segment: read toggle
+                                              // on data segment: write toggle
+      bool direction_conforming : 1 = false;  // TODO
+      bool exe : 1 = false;                   // true for code segment, false for data segment
+      bool segment : 1 = false;               // segment bit: 1 for code/data segments
+                                              //              0 for gates and tss
+      Ring privilege : 2 = Ring0;             // descriptor privilege level
+      bool present : 1 = true;                // must be 1 for every active segment
+    } __attribute__((packed));
+
+    SegmentDescriptor() { access.present = false; };
+
+    template <uint32_t limit>
+    static SegmentDescriptor make(uint32_t base, Access a) {
+      static_assert(limit <= 0xffffu || (limit & 0xfff) == 0xfff);
+      return SegmentDescriptor(base, limit, a);
+    }
+
     [[nodiscard]] uint32_t base() const;
     [[nodiscard]] uint32_t limit() const;
 
   private:
+    SegmentDescriptor(uint32_t base, uint32_t limit, Access a);
+
     /*
-     * |31|  |  |  |  |  |  |24|23|22|  |20|19|  |  |16|15|  |  |12|11|  |  | 8| 7|  |  |  |  |  |  | 0|
-     * | base (24~31)          | G|DB|  | A| lim(16~19)| P| DPL | S|    Type   |   base (16~23)        |
-     * | base (0~15)                                   | limit (0~15)                                  |
+     * |31|  |  |  |  |  |  |24|23|22|21|20|19|  |  |16|15|  |  |12|11|  |  | 8| 7|  |  |  |  |  |  | 0|
+     * |   base_31_24          | G|DB|  | A| lim_19_16 | P| DPL | S|   Type    |   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)
@@ -50,12 +72,21 @@ public:
      * flags    defines the segment chunks and 16/32 bit
      */
 
-    uint16_t limit_low;
-    uint16_t base_low;
-    uint8_t base_high;
-    uint8_t type;
-    uint8_t flags_limit_high;
-    uint8_t base_vhigh;
+    unsigned int limit_15_0 : 16 = 0;  // low bits of segment limit
+    unsigned int base_15_0 : 16 = 0;   // low bits of segment base address
+    unsigned int base_23_16 : 8 = 0;   // middle bits of segment base address
+
+    /* access byte */
+    Access access;
+
+    unsigned int limit_19_16 : 4 = 0;  // high bits of segment limit
+    /* flags */
+    [[maybe_unused]] unsigned int a : 1 = 0;    // unused, available for software use
+    [[maybe_unused]] unsigned int rsv : 1 = 0;  // reserved
+    unsigned int db : 1 = 0;           // 0: 16-bit segment; 1: 32-bit segment
+    unsigned int granularity : 1 = 0;  // limit scaled by 4k when set
+
+    unsigned int base_31_24 : 8 = 0;   // high bits of segment address
   } __attribute__((packed));
 
 private:
@@ -64,3 +95,4 @@ private:
 
 static_assert(sizeof(GDT::Pointer) == 6);
 static_assert(sizeof(GDT::SegmentDescriptor) == 8);
+static_assert(sizeof(GDT::SegmentDescriptor::Access) == 1);