Add more comments to GDT code

1 files changed, 39 insertions, 0 deletions

diff --git a/src/gdt/segmentdescriptor.cc b/src/gdt/segmentdescriptor.cc
new file mode 100644
index 0000000..94205c8
--- /dev/null
+++ b/src/gdt/segmentdescriptor.cc
@@ -0,0 +1,39 @@
+#include "../gdt.h"
+
+GDT::SegmentDescriptor::SegmentDescriptor(uint32_t base, uint32_t limit, GDT::SegmentDescriptor::Access a) : access(a) {
+  // store base
+  base_15_0 = base & 0xffff;
+  base_23_16 = (base >> 16) & 0xff;
+  base_31_24 = (base >> 24) & 0xff;
+
+  // store limit
+  if (limit <= 0xffffu) {  // 16-bit address space
+    limit_15_0 = limit & 0xffff;
+    limit_19_16 = 0;
+  } else {
+    db = 1;
+    // limit is uint32_t, which we need to store in limit_19_16 and limit_15_0, a total of 20 bits
+    limit = limit >> 12;
+    limit_15_0 = limit & 0xffff;
+    limit_19_16 = (limit >> 16) & 0xf;
+  }
+
+  granularity = 1;
+}
+
+uint32_t GDT::SegmentDescriptor::base() const {
+  uint32_t base = base_31_24;
+  base = (base << 8) + base_23_16;
+  base = (base << 16) + base_15_0;
+  return base;
+}
+
+uint32_t GDT::SegmentDescriptor::limit() const {
+  uint32_t limit = 0;
+  if (db == 1) {
+    limit = limit_19_16;
+    limit = limit << 16;
+  }
+  limit += limit_15_0;
+  return 0;
+}