From 00e961435e253521f19705e9537084c74b054bc6 Mon Sep 17 00:00:00 2001
From: Eryn Wells <eryn@erynwells.me>
Date: Wed, 2 Mar 2016 02:02:01 -0500
Subject: [PATCH] Add IDT class for managing the interrupt table

---
 src/Descriptors.cc | 84 ++++++++++++++++++++++++++++++++++++++++++++++
 src/Descriptors.hh | 73 +++++++++++++++++++++++++++++++---------
 2 files changed, 142 insertions(+), 15 deletions(-)

diff --git a/src/Descriptors.cc b/src/Descriptors.cc
index 7370326..29cd019 100644
--- a/src/Descriptors.cc
+++ b/src/Descriptors.cc
@@ -140,4 +140,88 @@ GDT::load()
         : "%eax");
 }
 
+/*
+ * Static
+ */
+
+IDT&
+IDT::systemIDT()
+{
+    static IDT sIDT;
+    return sIDT;
+}
+
+/*
+ * Public
+ */
+
+Descriptor
+IDT::DescriptorSpec::descriptor()
+    const
+{
+    Descriptor descriptor = 0;
+
+    if (type != Type::Task) {
+        descriptor = offset & 0xFFFF0000;
+    }
+
+    const auto p   = uint8_t(isPresent);
+    const auto dpl = uint8_t(privilegeLevel);
+    const auto d   = uint8_t((type == Type::Task) ? 0 : is32BitGate);
+    const auto typ = uint8_t(type);
+
+    descriptor |= p << 15;
+    descriptor |= dpl << 13;
+    descriptor |= d << 11;
+    descriptor |= typ << 8;
+
+    // Shift everything up by 32 to make room for the lower 4 bytes
+    descriptor <<= 32;
+
+    descriptor |= segment << 16;
+
+    if (type != Type::Task) {
+        descriptor |= offset & 0x0000FFFF;
+    }
+
+    return descriptor;
+}
+
+
+IDT::IDT()
+    : mTable{0}
+{ }
+
+
+void
+IDT::setDescriptor(size_t index,
+                   const DescriptorSpec& spec)
+{
+    if (index >= Size) {
+        return;
+    }
+    mTable[index] = spec.descriptor();
+}
+
+
+void
+IDT::setNullDescriptor(size_t index)
+{
+    if (index >= Size) {
+        return;
+    }
+    mTable[index] = 0;
+}
+
+
+void
+IDT::load()
+    const
+{
+    PseudoDescriptor idt{Size * sizeof(Descriptor) - 1, uint32_t(&mTable)};
+    asm volatile(
+        "lidt %0\n"
+        : : "m" (idt) :);
+}
+
 } /* namespace kernel */
diff --git a/src/Descriptors.hh b/src/Descriptors.hh
index 3b24fa6..054dc5f 100644
--- a/src/Descriptors.hh
+++ b/src/Descriptors.hh
@@ -9,24 +9,24 @@
 
 namespace kernel {
 
-void initGDT();
+/**
+ * SegmentDescriptors are entries in the GDT and LDT that describe memory
+ * segments. Each descriptor is two double-words (8 bytes, 64 bits) long.
+ */
+typedef uint64_t Descriptor;
+
+
+/** Descriptor privilege level. */
+enum class DPL {
+    Ring0 = 0x0,
+    Ring1 = 0x1,
+    Ring2 = 0x2,
+    Ring3 = 0x3
+};
+
 
 struct GDT
 {
-    /**
-     * SegmentDescriptors are entries in the GDT and LDT that describe memory
-     * segments. Each descriptor is two double-words (8 bytes, 64 bits) long.
-     */
-    typedef uint64_t Descriptor;
-
-    /** Descriptor privilege level. */
-    enum class DPL {
-        Ring0 = 0x0,
-        Ring1 = 0x1,
-        Ring2 = 0x2,
-        Ring3 = 0x3
-    };
-
     /** A four bit value describing the type of the segment. */
     enum class Type {
         // Data segment types
@@ -93,4 +93,47 @@ private:
     Descriptor table[Size];
 };
 
+
+struct IDT
+{
+    enum class Type {
+        Task = 0x5,
+        Interrupt = 0x6,
+        Trap = 0x7,
+    };
+
+    struct DescriptorSpec
+    {
+        uint16_t segment;
+        uint32_t offset;
+        bool isPresent;
+        DPL privilegeLevel;
+        bool is32BitGate;
+        Type type;
+
+        Descriptor descriptor() const;
+    };
+
+    static IDT& systemIDT();
+
+    IDT();
+
+    /** Set the descriptor at the given `index` to the value of `spec`. */
+    void setDescriptor(size_t index, const DescriptorSpec& spec);
+
+    /** Set the descriptor at the given `index` to the NULL descriptor. */
+    void setNullDescriptor(size_t index);
+
+    void load() const;
+
+private:
+    /**
+     * Size of the table. IDTs shouldn't have any more than this many
+     * descriptors.
+     */
+    static const size_t Size = 256;
+
+    Descriptor mTable[Size];
+};
+
 } /* namespace kernel */