add descriptor_table access_byte

4 files changed, 103 insertions, 2 deletions

diff --git a/arch/x86_64/include/arch/context_switching/descriptor_table/access_byte.hpp b/arch/x86_64/include/arch/context_switching/descriptor_table/access_byte.hpp
new file mode 100644
index 0000000..b3bce11
--- /dev/null
+++ b/arch/x86_64/include/arch/context_switching/descriptor_table/access_byte.hpp
@@ -0,0 +1,87 @@
+
+#ifndef TEACHOS_ARCH_X86_64_CONTEXT_SWITCHING_DESCRIPTOR_TABLE_POINTERS_HPP
+#define TEACHOS_ARCH_X86_64_CONTEXT_SWITCHING_DESCRIPTOR_TABLE_POINTERS_HPP
+
+#include <bitset>
+#include <cstddef>
+#include <cstdint>
+
+namespace teachos::arch::context_switching::descriptor_table
+{
+  enum class privilege_level
+  {
+    KERNEL = 0,
+    ADMIN = 1,
+    PRIVILEGED_USER = 2,
+    USER = 3
+  };
+
+  /// @brief Defines the access byte of a Descriptor Table
+  struct access_byte
+  {
+    /**
+     * @brief Possible set bits in our underlying std::bitset and the meaning when they are set.
+     */
+    enum bitset : uint8_t
+    {
+      PRESENT = 1U << 0U,                    ///< Present bit; Allows an entry to refer to a valid segment.
+                                             ///< Must be set (1) for any valid segment.
+      PRIVILEGE_LEVEL = 1U << 1U | 1 << 2U,  ///< Descriptor privilege level field.
+                                             ///< Contains the CPU Privilege level of the segment.
+      DESCRIPTOR_TYPE = 1U << 3U,            ///< Defines a system segment (if 0) or a code/data segment (if 1).
+      EXECUTABLE = 1U << 4U,  ///< Defines a data segment (if 0) or a code segment which can be executed from (if 1).
+      CONFORMING = 1U << 5U,  ///< For data selectors: Direction bit
+                              ///< Grows up (if 0), grows down (if 1)
+                              ///< For code selectors: Conforming bit
+                              ///< Code can only be executed from the DPL ring (if 0)
+                              ///< Code can be executed from equal or lower DPL ring (if 1)
+      READ_WRITE = 1U << 6U,  ///< For code segments: Readable bit
+                              ///< For data segments: Writeable bit (read is always allowed)
+      ACCESSED = 1U << 7U     ///< Set, when the segment is accessed. If GDT descriptor is stored in read only pages and
+                              ///< this bit is set to 0, the CPU trying to set this bit will trigger a page fault.
+    };
+
+    /**
+     * @brief Constructor.
+     *
+     * @param flags Actual value read from the elf section header, which should be converted into a std::bitset, to
+     * allow reading the state of single bits more easily.
+     */
+    explicit access_byte(uint8_t flags)
+        : flags(flags)
+    {
+      // Nothing to do
+    }
+
+    /**
+     * @brief Checks if the given std::bitset is a subset or equivalent to the underlying std::bitset.
+     *
+     * @note Meaning that all bits that are set in the given std::bitset also have to be set in the underlyng
+     * std::bitset. Any additional bits that are set are not relevant.
+     *
+     * @param other Flags that we want to compare against and check if the underlying std::bitset has the same bits set.
+     * @return Whether the given flags are a subset or equivalent with the underlying std::bitset.
+     */
+    auto contains_flags(std::bitset<8U> other) const -> bool;
+
+    /**
+     * @brief Returns the privilege level of the current access_byte
+     *
+     * @return privilege_level
+     */
+    auto get_privilege_level() const -> privilege_level;
+
+    /**
+     * @brief Allows to compare the underlying std::bitset of two instances.
+     *
+     * @param other Other instance that we want to compare with.
+     * @return Whether the underlying std::bitset of both types is the same.
+     */
+    auto operator==(access_byte const & other) const -> bool = default;
+
+  private:
+    std::bitset<8U> flags;  ///< Underlying bitset used to read the flags from
+  };
+}  // namespace teachos::arch::context_switching::descriptor_table
+
+#endif  // TEACHOS_ARCH_X86_64_CONTEXT_SWITCHING_DESCRIPTOR_TABLE_POINTERS_HPP
\ No newline at end of file
diff --git a/arch/x86_64/include/arch/context_switching/descriptor_table/access_bytes.hpp b/arch/x86_64/include/arch/context_switching/descriptor_table/access_bytes.hpp
deleted file mode 100644
index e69de29..0000000
--- a/arch/x86_64/include/arch/context_switching/descriptor_table/access_bytes.hpp
+++ /dev/null
diff --git a/arch/x86_64/src/boot/boot.s b/arch/x86_64/src/boot/boot.s
index 2197dce..dbea42a 100644
--- a/arch/x86_64/src/boot/boot.s
+++ b/arch/x86_64/src/boot/boot.s
@@ -197,10 +197,10 @@ _start:
     call enable_paging
     call enable_sse
 
-    cli // Clears the interrupt flag during the GDT setup
+    cli /* Clears the interrupt flag during the GDT setup */
     lgdt (global_descriptor_table_pointer)
     jmp $global_descriptor_table_code,$_transition_to_long_mode
-    // The interrupt flag is set in cpp after setting up the GDT
+    /* The interrupt flag is set in cpp after setting up the GDT */
 
     call halt
 
diff --git a/arch/x86_64/src/context_switching/descriptor_table/access_byte.cpp b/arch/x86_64/src/context_switching/descriptor_table/access_byte.cpp
new file mode 100644
index 0000000..15a0947
--- /dev/null
+++ b/arch/x86_64/src/context_switching/descriptor_table/access_byte.cpp
@@ -0,0 +1,14 @@
+#include "arch/context_switching/descriptor_table/access_byte.hpp"
+
+#include <bitset>
+
+namespace teachos::arch::context_switching::descriptor_table
+{
+  auto access_byte::contains_flags(std::bitset<8U> other) const -> bool { return (flags & other) == other; }
+
+  auto access_byte::get_privilege_level() const -> privilege_level
+  {
+    constexpr uint8_t PRIVILEGE_MASK = 0b0110'0000;
+    return static_cast<privilege_level>(flags.to_ulong() & PRIVILEGE_MASK);
+  }
+}  // namespace teachos::arch::context_switching::descriptor_table