os: rework kernel architecture

Rework the code structure and architecture of the kernel by separating
platform-dependent and platform-independent code more cleanly. As of
this patchset, full feature parity has not been achieved. Nonetheless, a
sufficient subset of functionality has been ported to the new
architecture to demonstrate the feasibility of the new structure.

1 files changed, 57 insertions, 0 deletions

diff --git a/kernel/src/kapi/cio.cpp b/kernel/src/kapi/cio.cpp
new file mode 100644
index 0000000..66493b6
--- /dev/null
+++ b/kernel/src/kapi/cio.cpp
@@ -0,0 +1,57 @@
+#include "kapi/cio.hpp"
+
+#include <optional>
+#include <string_view>
+#include <utility>
+
+namespace teachos::cio
+{
+  namespace
+  {
+    struct null_device final : public output_device
+    {
+      null_device static instance;
+
+      auto write(std::string_view) -> void override {}
+      auto writeln(std::string_view) -> void override {}
+
+      auto write_error(std::string_view) -> void override {}
+      auto writeln_error(std::string_view) -> void override {}
+    };
+
+    constinit null_device null_device::instance;
+  }  // namespace
+
+  // NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
+  constinit auto active_device = static_cast<output_device *>(&null_device::instance);
+
+  auto set_output_device(output_device & device) -> std::optional<output_device *>
+  {
+    if (&device == active_device)
+    {
+      return {};
+    }
+    return std::exchange(active_device, &device);
+  }
+
+  auto print(std::string_view text) -> void
+  {
+    active_device->write(text);
+  }
+
+  auto println(std::string_view text) -> void
+  {
+    active_device->writeln(text);
+  }
+
+  auto print_error(std::string_view text) -> void
+  {
+    active_device->write_error(text);
+  }
+
+  auto println_error(std::string_view text) -> void
+  {
+    active_device->writeln_error(text);
+  }
+
+}  // namespace teachos::cio
\ No newline at end of file