kernel: implement dev zero and null

16 files changed, 622 insertions, 9 deletions

diff --git a/kapi/kapi/devices/error.hpp b/kapi/kapi/devices/error.hpp
index b4812550..cf33daa9 100644
--- a/kapi/kapi/devices/error.hpp
+++ b/kapi/kapi/devices/error.hpp
@@ -14,6 +14,10 @@ namespace kapi::devices
   {
     //! The driver did not match on the given device.
     no_match = 1,
+    //! The device has no signature.
+    no_device_signature = 2,
+    //! The device signature was invalid.
+    invalid_device_signature = 3,
   };
 
   [[nodiscard]] auto driver_match_category() noexcept -> kstd::error_category const &;
diff --git a/kernel/CMakeLists.txt b/kernel/CMakeLists.txt
index 91075adc..3fb3bc4a 100644
--- a/kernel/CMakeLists.txt
+++ b/kernel/CMakeLists.txt
@@ -40,17 +40,20 @@ target_sources("kernel_lib" PRIVATE
 
     # Device Subsystem
     "kernel/devices/block_device_utils.cpp"
+    "kernel/devices/init.cpp"
+    "kernel/devices/pseudo.cpp"
+    "kernel/devices/storage.cpp"
 
     # Bus Sumbsystem
     "kernel/bus/boot_modules.cpp"
+    "kernel/bus/pseudo.cpp"
     "kernel/bus/root.cpp"
 
     # Driver Subsystem
-    "kernel/drivers/storage/ram_disk.cpp"
     "kernel/drivers/init.cpp"
-
-    # Storage Device Subsystem
-    "kernel/devices/storage.cpp"
+    "kernel/drivers/pseudo/null.cpp"
+    "kernel/drivers/pseudo/zero.cpp"
+    "kernel/drivers/storage/ram_disk.cpp"
 
     # Filesystem Subsystem
     "kernel/filesystem/dentry.cpp"
diff --git a/kernel/kapi/devices/device_registry.cpp b/kernel/kapi/devices/device_registry.cpp
index 80ddde65..080ae39b 100644
--- a/kernel/kapi/devices/device_registry.cpp
+++ b/kernel/kapi/devices/device_registry.cpp
@@ -1,5 +1,6 @@
 #include <kapi/devices/device_registry.hpp>
 
+#include <kapi/devices/bus.hpp>
 #include <kapi/devices/device.hpp>
 #include <kapi/system.hpp>
 
@@ -48,7 +49,14 @@ namespace kapi::devices
       return false;
     }
 
-    kstd::println("[OS:DEV] Registering device {}", device->name());
+    if (device->facet<bus>())
+    {
+      kstd::println("[OS:DEV] Registering bus {}", device->name());
+    }
+    else
+    {
+      kstd::println("[OS:DEV] Registering device {}", device->name());
+    }
 
     auto added = false;
     {
diff --git a/kernel/kapi/devices/error.cpp b/kernel/kapi/devices/error.cpp
index 9d814fbe..40320551 100644
--- a/kernel/kapi/devices/error.cpp
+++ b/kernel/kapi/devices/error.cpp
@@ -3,7 +3,6 @@
 #include <kstd/system_error.hpp>
 
 #include <string_view>
-#include <utility>
 
 namespace kapi::devices
 {
@@ -23,6 +22,10 @@ namespace kapi::devices
         {
           case driver_match_errc::no_match:
             return "driver does not match this device";
+          case driver_match_errc::no_device_signature:
+            return "missing device signature";
+          case driver_match_errc::invalid_device_signature:
+            return "device signature is invalid";
           default:
             return "unknown driver matching error";
         }
@@ -30,11 +33,17 @@ namespace kapi::devices
 
       [[nodiscard]] auto default_error_condition(int value) const noexcept -> kstd::error_condition override
       {
-        if (value == std::to_underlying(driver_match_errc::no_match))
+        switch (static_cast<driver_match_errc>(value))
         {
-          return make_error_condition(kstd::errc::not_supported);
+          case driver_match_errc::no_match:
+          case driver_match_errc::no_device_signature:
+            return make_error_condition(kstd::errc::not_supported);
+            break;
+          case driver_match_errc::invalid_device_signature:
+            return make_error_condition(kstd::errc::invalid_argument);
+          default:
+            return kstd::error_category::default_error_condition(value);
         }
-        return kstd::error_category::default_error_condition(value);
       }
 
     } constexpr driver_match_category_instance{};
diff --git a/kernel/kernel/bus/pseudo.cpp b/kernel/kernel/bus/pseudo.cpp
new file mode 100644
index 00000000..0b002273
--- /dev/null
+++ b/kernel/kernel/bus/pseudo.cpp
@@ -0,0 +1,51 @@
+#include <kernel/bus/pseudo.hpp>
+
+#include <kapi/devices.hpp>
+
+#include <kstd/result.hpp>
+
+#include <algorithm>
+#include <cstdint>
+
+namespace kernel::bus
+{
+
+  namespace
+  {
+
+    struct protocol final : kapi::devices::bus_protocol
+    {
+      auto enumerate(kapi::devices::bus &) -> void override
+      {
+        // Nothing to enumerate.
+      }
+
+      [[nodiscard]] auto match(kapi::devices::device const & device, kapi::devices::driver const & driver) const
+          -> kstd::result<std::uint32_t> override
+      {
+        auto signature = device.facet<pseudo_signature>();
+        auto claim = driver.facet<pseudo_claim>();
+
+        if (!signature || !claim)
+        {
+          return kstd::failure(kapi::devices::driver_match_errc::no_match);
+        }
+
+        auto names = claim->supported_names();
+        auto found = std::ranges::find(names, signature->name());
+
+        if (found == claim->supported_names().end())
+        {
+          return kstd::failure(kapi::devices::driver_match_errc::no_match);
+        }
+
+        return 0u;
+      }
+    } constinit protocol_instance;
+  }  // namespace
+
+  pseudo::pseudo()
+      : kapi::devices::bus{"pseudo", protocol_instance}
+  {}
+
+}  // namespace kernel::bus
\ No newline at end of file
diff --git a/kernel/kernel/bus/pseudo.hpp b/kernel/kernel/bus/pseudo.hpp
new file mode 100644
index 00000000..dace517b
--- /dev/null
+++ b/kernel/kernel/bus/pseudo.hpp
@@ -0,0 +1,38 @@
+#ifndef TEACHOS_KERNEL_BUS_PSEUDO_HPP
+#define TEACHOS_KERNEL_BUS_PSEUDO_HPP
+
+#include <kapi/capabilities/facet_id.hpp>
+#include <kapi/devices.hpp>
+
+#include <span>
+#include <string_view>
+namespace kernel::bus
+{
+
+  struct pseudo_signature
+  {
+    constexpr auto static id = kapi::capabilities::facet_id{"pseudo_signature"};
+
+    virtual ~pseudo_signature() = default;
+
+    [[nodiscard]] virtual auto name() const noexcept -> std::string_view = 0;
+  };
+
+  struct pseudo_claim
+  {
+    constexpr auto static id = kapi::capabilities::facet_id{"pseudo_claim"};
+
+    virtual ~pseudo_claim() = default;
+
+    [[nodiscard]] virtual auto supported_names() const noexcept -> std::span<std::string_view const> = 0;
+  };
+
+  //! The bus for all pseudo devices.
+  struct pseudo final : kapi::devices::bus
+  {
+    pseudo();
+  };
+
+}  // namespace kernel::bus
+
+#endif
\ No newline at end of file
diff --git a/kernel/kernel/devices/init.cpp b/kernel/kernel/devices/init.cpp
new file mode 100644
index 00000000..89f5a796
--- /dev/null
+++ b/kernel/kernel/devices/init.cpp
@@ -0,0 +1,24 @@
+#include <kernel/devices/init.hpp>
+
+#include <kernel/bus/pseudo.hpp>
+#include <kernel/devices/pseudo.hpp>
+
+#include <kapi/devices.hpp>
+
+#include <kstd/memory.hpp>
+
+namespace kernel::devices
+{
+
+  auto init() -> void
+  {
+    auto root = kapi::devices::get_root_bus();
+
+    auto pseudo_bus = kstd::make_shared<kernel::bus::pseudo>();
+    root->add_child(pseudo_bus);
+
+    pseudo_bus->add_child(kstd::make_shared<devices::pseudo>("null"));
+    pseudo_bus->add_child(kstd::make_shared<devices::pseudo>("zero"));
+  }
+
+}  // namespace kernel::devices
\ No newline at end of file
diff --git a/kernel/kernel/devices/init.hpp b/kernel/kernel/devices/init.hpp
new file mode 100644
index 00000000..563e9c36
--- /dev/null
+++ b/kernel/kernel/devices/init.hpp
@@ -0,0 +1,13 @@
+#ifndef TEACHOS_KERNEL_DEVICES_INIT_HPP
+#define TEACHOS_KERNEL_DEVICES_INIT_HPP
+
+#include <kapi/devices.hpp>
+
+namespace kernel::devices
+{
+
+  auto init() -> void;
+
+}  // namespace kernel::devices
+
+#endif
\ No newline at end of file
diff --git a/kernel/kernel/devices/pseudo.cpp b/kernel/kernel/devices/pseudo.cpp
new file mode 100644
index 00000000..066350e8
--- /dev/null
+++ b/kernel/kernel/devices/pseudo.cpp
@@ -0,0 +1,63 @@
+#include <kernel/devices/pseudo.hpp>
+
+#include <kernel/bus/pseudo.hpp>
+
+#include <kapi/capabilities/facet_id.hpp>
+#include <kapi/devices.hpp>
+
+#include <kstd/string.hpp>
+
+#include <string_view>
+#include <utility>
+
+namespace kernel::devices
+{
+
+  namespace
+  {
+    struct simple_pseudo_signature final : kernel::bus::pseudo_signature
+    {
+      explicit constexpr simple_pseudo_signature(std::string_view name)
+          : m_name{name}
+      {}
+
+      [[nodiscard]] auto name() const noexcept -> std::string_view override
+      {
+        return m_name;
+      }
+
+    private:
+      std::string_view m_name;
+    };
+
+    constinit auto null_signature = simple_pseudo_signature{"null"};
+    constinit auto zero_signature = simple_pseudo_signature{"zero"};
+    constinit auto unknown_signature = simple_pseudo_signature{"<unknown>"};
+  };  // namespace
+
+  pseudo::pseudo(kstd::string name)
+      : kapi::devices::device{std::move(name)}
+  {}
+
+  auto pseudo::query_facet(kapi::capabilities::facet_id facet) -> void *
+  {
+    if (facet == kernel::bus::pseudo_signature::id)
+    {
+      if (name() == "null")
+      {
+        return &null_signature;
+      }
+      else if (name() == "zero")
+      {
+        return &zero_signature;
+      }
+      else
+      {
+        return &unknown_signature;
+      }
+    }
+
+    return device::query_facet(facet);
+  };
+
+}  // namespace kernel::devices
\ No newline at end of file
diff --git a/kernel/kernel/devices/pseudo.hpp b/kernel/kernel/devices/pseudo.hpp
new file mode 100644
index 00000000..fb4ef2c8
--- /dev/null
+++ b/kernel/kernel/devices/pseudo.hpp
@@ -0,0 +1,24 @@
+#ifndef TEACHOS_KERNEL_DEVICES_PSEUDO_HPP
+#define TEACHOS_KERNEL_DEVICES_PSEUDO_HPP
+
+#include <kernel/bus/pseudo.hpp>
+
+#include <kapi/capabilities/facet_id.hpp>
+#include <kapi/devices.hpp>
+
+#include <kstd/string.hpp>
+
+namespace kernel::devices
+{
+
+  struct pseudo final : kapi::devices::device
+  {
+    explicit pseudo(kstd::string name);
+
+  protected:
+    auto query_facet(kapi::capabilities::facet_id facet) -> void * override;
+  };
+
+}  // namespace kernel::devices
+
+#endif
\ No newline at end of file
diff --git a/kernel/kernel/drivers/init.cpp b/kernel/kernel/drivers/init.cpp
index 766011ef..cd196fa8 100644
--- a/kernel/kernel/drivers/init.cpp
+++ b/kernel/kernel/drivers/init.cpp
@@ -1,5 +1,7 @@
 #include <kernel/drivers/init.hpp>
 
+#include <kernel/drivers/pseudo/null.hpp>
+#include <kernel/drivers/pseudo/zero.hpp>
 #include <kernel/drivers/storage/ram_disk.hpp>
 
 #include <kapi/devices/driver_registry.hpp>
@@ -15,6 +17,8 @@ namespace kernel::drivers
 
     auto & registry = kapi::devices::driver_registry::get();
 
+    registry.add(kstd::make_shared<kernel::drivers::pseudo::null>());
+    registry.add(kstd::make_shared<kernel::drivers::pseudo::zero>());
     registry.add(kstd::make_shared<kernel::drivers::storage::ram_disk>());
   }
 
diff --git a/kernel/kernel/drivers/pseudo/null.cpp b/kernel/kernel/drivers/pseudo/null.cpp
new file mode 100644
index 00000000..d0df9000
--- /dev/null
+++ b/kernel/kernel/drivers/pseudo/null.cpp
@@ -0,0 +1,112 @@
+#include <kernel/drivers/pseudo/null.hpp>
+
+#include <kernel/bus/pseudo.hpp>
+#include <kernel/filesystem/reserved_numbers.hpp>
+
+#include <kapi/devices.hpp>
+#include <kapi/filesystem.hpp>
+
+#include <kstd/memory.hpp>
+#include <kstd/result.hpp>
+#include <kstd/units.hpp>
+
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <optional>
+#include <span>
+#include <string_view>
+#include <utility>
+
+using namespace kstd::units_literals;
+using namespace std::string_view_literals;
+
+namespace kernel::drivers::pseudo
+{
+
+  namespace
+  {
+    struct null_node final : kapi::filesystem::character_special_file
+    {
+    public:
+      [[nodiscard]] auto read(std::span<std::byte>) const -> kstd::result<kstd::units::bytes> override
+      {
+        return 0_B;
+      }
+
+      auto write(std::span<std::byte const> buffer) -> kstd::result<kstd::units::bytes> override
+      {
+        return kstd::units::bytes{buffer.size()};
+      }
+    };
+
+    constexpr auto names = std::array{"null"sv};
+  }  // namespace
+
+  [[nodiscard]] auto null::probe(kapi::devices::device & device) -> kstd::result<void>
+  {
+    auto signature = device.facet<kernel::bus::pseudo_signature>();
+    if (!signature)
+    {
+      return kstd::failure(kapi::devices::driver_match_errc::no_device_signature);
+    }
+
+    if (signature->name() != "null")
+    {
+      return kstd::failure(kapi::devices::driver_match_errc::invalid_device_signature);
+    }
+
+    auto implementation = kstd::make_shared<null_node>();
+    auto published = kapi::devices::publish_facet<kapi::filesystem::character_special_file>(
+        device.shared_from_this(), "null", implementation.get());
+
+    if (!published)
+    {
+      return published;
+    }
+
+    device.set_driver_data(std::move(implementation));
+
+    return kstd::success();
+  }
+
+  [[nodiscard]] auto null::name() const noexcept -> std::string_view
+  {
+    return "Null Source/Sink Device";
+  }
+
+  auto null::unbind(kapi::devices::device & device) -> void
+  {
+    device.set_driver_data(nullptr);
+  }
+
+  auto null::claimed_major() const -> std::optional<std::uint8_t>
+  {
+    return kernel::filesystem::character_major_numbers::memory;
+  }
+
+  auto null::claimed_minor(kapi::devices::device & device) const -> std::optional<std::uint8_t>
+  {
+    if (device.name() == "null")
+    {
+      return kernel::filesystem::character_minor_numbers::null;
+    }
+    return std::nullopt;
+  }
+
+  [[nodiscard]] auto null::supported_names() const noexcept -> std::span<std::string_view const>
+  {
+    return names;
+  }
+
+  auto null::query_facet(kapi::capabilities::facet_id facet) -> void *
+  {
+    if (facet == kernel::bus::pseudo_claim::id)
+    {
+      return static_cast<kernel::bus::pseudo_claim *>(this);
+    }
+
+    return kapi::devices::driver::query_facet(facet);
+  }
+
+}  // namespace kernel::drivers::pseudo
\ No newline at end of file
diff --git a/kernel/kernel/drivers/pseudo/null.hpp b/kernel/kernel/drivers/pseudo/null.hpp
new file mode 100644
index 00000000..f34d3a0a
--- /dev/null
+++ b/kernel/kernel/drivers/pseudo/null.hpp
@@ -0,0 +1,39 @@
+#ifndef TEACHOS_KERNEL_DRIVERS_PSEUDO_NULL_HPP
+#define TEACHOS_KERNEL_DRIVERS_PSEUDO_NULL_HPP
+
+#include <kernel/bus/pseudo.hpp>
+
+#include <kapi/devices.hpp>
+#include <kapi/filesystem.hpp>
+
+#include <kstd/result.hpp>
+
+#include <cstdint>
+#include <optional>
+#include <span>
+#include <string_view>
+
+namespace kernel::drivers::pseudo
+{
+
+  struct null final : kapi::devices::driver, kernel::bus::pseudo_claim
+  {
+    [[nodiscard]] auto probe(kapi::devices::device & device) -> kstd::result<void> override;
+
+    auto unbind(kapi::devices::device & device) -> void override;
+
+    [[nodiscard]] auto name() const noexcept -> std::string_view override;
+
+    [[nodiscard]] auto claimed_major() const -> std::optional<std::uint8_t> override;
+
+    [[nodiscard]] auto claimed_minor(kapi::devices::device &) const -> std::optional<std::uint8_t> override;
+
+    [[nodiscard]] auto supported_names() const noexcept -> std::span<std::string_view const> override;
+
+  protected:
+    auto query_facet(kapi::capabilities::facet_id facet) -> void * override;
+  };
+
+}  // namespace kernel::drivers::pseudo
+
+#endif
\ No newline at end of file
diff --git a/kernel/kernel/drivers/pseudo/zero.cpp b/kernel/kernel/drivers/pseudo/zero.cpp
new file mode 100644
index 00000000..550a97ff
--- /dev/null
+++ b/kernel/kernel/drivers/pseudo/zero.cpp
@@ -0,0 +1,114 @@
+#include <kernel/drivers/pseudo/zero.hpp>
+
+#include <kernel/bus/pseudo.hpp>
+#include <kernel/filesystem/reserved_numbers.hpp>
+
+#include <kapi/devices.hpp>
+#include <kapi/filesystem.hpp>
+
+#include <kstd/memory.hpp>
+#include <kstd/result.hpp>
+#include <kstd/units.hpp>
+
+#include <algorithm>
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <optional>
+#include <span>
+#include <string_view>
+#include <utility>
+
+using namespace kstd::units_literals;
+using namespace std::string_view_literals;
+
+namespace kernel::drivers::pseudo
+{
+
+  namespace
+  {
+    struct zero_node final : kapi::filesystem::character_special_file
+    {
+    public:
+      [[nodiscard]] auto read(std::span<std::byte> buffer) const -> kstd::result<kstd::units::bytes> override
+      {
+        std::ranges::fill(buffer, std::byte{0});
+        return kstd::units::bytes{buffer.size()};
+      }
+
+      auto write(std::span<std::byte const> buffer) -> kstd::result<kstd::units::bytes> override
+      {
+        return kstd::units::bytes{buffer.size()};
+      }
+    };
+
+    constexpr auto names = std::array{"zero"sv};
+  }  // namespace
+
+  [[nodiscard]] auto zero::probe(kapi::devices::device & device) -> kstd::result<void>
+  {
+    auto signature = device.facet<kernel::bus::pseudo_signature>();
+    if (!signature)
+    {
+      return kstd::failure(kapi::devices::driver_match_errc::no_device_signature);
+    }
+
+    if (signature->name() != "zero")
+    {
+      return kstd::failure(kapi::devices::driver_match_errc::invalid_device_signature);
+    }
+
+    auto implementation = kstd::make_shared<zero_node>();
+    auto published = kapi::devices::publish_facet<kapi::filesystem::character_special_file>(
+        device.shared_from_this(), "zero", implementation.get());
+
+    if (!published)
+    {
+      return published;
+    }
+
+    device.set_driver_data(std::move(implementation));
+
+    return kstd::success();
+  }
+
+  [[nodiscard]] auto zero::name() const noexcept -> std::string_view
+  {
+    return "Null-byte Source Device";
+  }
+
+  auto zero::unbind(kapi::devices::device & device) -> void
+  {
+    device.set_driver_data(nullptr);
+  }
+
+  auto zero::claimed_major() const -> std::optional<std::uint8_t>
+  {
+    return kernel::filesystem::character_major_numbers::memory;
+  }
+
+  auto zero::claimed_minor(kapi::devices::device & device) const -> std::optional<std::uint8_t>
+  {
+    if (device.name() == "zero")
+    {
+      return kernel::filesystem::character_minor_numbers::zero;
+    }
+    return std::nullopt;
+  }
+
+  [[nodiscard]] auto zero::supported_names() const noexcept -> std::span<std::string_view const>
+  {
+    return names;
+  }
+
+  auto zero::query_facet(kapi::capabilities::facet_id facet) -> void *
+  {
+    if (facet == kernel::bus::pseudo_claim::id)
+    {
+      return static_cast<kernel::bus::pseudo_claim *>(this);
+    }
+
+    return kapi::devices::driver::query_facet(facet);
+  }
+
+}  // namespace kernel::drivers::pseudo
\ No newline at end of file
diff --git a/kernel/kernel/drivers/pseudo/zero.hpp b/kernel/kernel/drivers/pseudo/zero.hpp
new file mode 100644
index 00000000..7f26cc41
--- /dev/null
+++ b/kernel/kernel/drivers/pseudo/zero.hpp
@@ -0,0 +1,39 @@
+#ifndef TEACHOS_KERNEL_DRIVERS_PSEUDO_ZERO_HPP
+#define TEACHOS_KERNEL_DRIVERS_PSEUDO_ZERO_HPP
+
+#include <kernel/bus/pseudo.hpp>
+
+#include <kapi/devices.hpp>
+#include <kapi/filesystem.hpp>
+
+#include <kstd/result.hpp>
+
+#include <cstdint>
+#include <optional>
+#include <span>
+#include <string_view>
+
+namespace kernel::drivers::pseudo
+{
+
+  struct zero final : kapi::devices::driver, kernel::bus::pseudo_claim
+  {
+    [[nodiscard]] auto probe(kapi::devices::device & device) -> kstd::result<void> override;
+
+    auto unbind(kapi::devices::device & device) -> void override;
+
+    [[nodiscard]] auto name() const noexcept -> std::string_view override;
+
+    [[nodiscard]] auto claimed_major() const -> std::optional<std::uint8_t> override;
+
+    [[nodiscard]] auto claimed_minor(kapi::devices::device &) const -> std::optional<std::uint8_t> override;
+
+    [[nodiscard]] auto supported_names() const noexcept -> std::span<std::string_view const> override;
+
+  protected:
+    auto query_facet(kapi::capabilities::facet_id facet) -> void * override;
+  };
+
+}  // namespace kernel::drivers::pseudo
+
+#endif
\ No newline at end of file
diff --git a/kernel/kernel/main.cpp b/kernel/kernel/main.cpp
index dc57fb55..13697eb6 100644
--- a/kernel/kernel/main.cpp
+++ b/kernel/kernel/main.cpp
@@ -1,4 +1,5 @@
 #include "kernel/filesystem/type_registry.hpp"
+#include <kernel/devices/init.hpp>
 #include <kernel/devices/storage.hpp>
 #include <kernel/drivers/init.hpp>
 #include <kernel/filesystem/open_file_table.hpp>
@@ -141,6 +142,70 @@ auto run_demo() -> void
 
   // 14) dmp the module after create new directory and file
   // -exec monitor memsave 0xffffffff8025b000 0xA00000 dump_after.bin
+
+  // 15) read from /dev/null
+  auto fd_5 = kapi::filesystem::open("/dev/null");
+  if (fd_5)
+  {
+    kstd::println("--> successfully opened /dev/null as a file with file descriptor {}", fd_5.value());
+  }
+  else
+  {
+    kapi::system::panic("demo failed");
+  }
+
+  auto buffer_4 = kstd::vector<std::byte>{10, std::byte{0xff}};
+  if (auto read = kapi::filesystem::read(fd_5.value(), buffer_4); !read)
+  {
+    kapi::system::panic("demo failed: {}", read.error());
+  }
+  else
+  {
+    bytes_read = read->value;
+  }
+  kstd::println("--> read {} bytes from /dev/null", bytes_read);
+
+  if (auto written = kapi::filesystem::write(fd_5.value(), buffer_4); !written)
+  {
+    kapi::system::panic("demo failed: {}", written.error());
+  }
+  else
+  {
+    bytes_written = written->value;
+  }
+  kstd::println("--> written {} bytes to /dev/null", bytes_written);
+
+  // 16) read from /dev/null
+  auto fd_6 = kapi::filesystem::open("/dev/zero");
+  if (fd_6)
+  {
+    kstd::println("--> successfully opened /dev/zero as a file with file descriptor {}", fd_6.value());
+  }
+  else
+  {
+    kapi::system::panic("demo failed");
+  }
+
+  auto buffer_5 = kstd::vector<std::byte>{10, std::byte{0xff}};
+  if (auto read = kapi::filesystem::read(fd_6.value(), buffer_5); !read)
+  {
+    kapi::system::panic("demo failed: {}", read.error());
+  }
+  else
+  {
+    bytes_read = read->value;
+  }
+  kstd::println("--> read {} bytes from /dev/zero: {::#04x}", bytes_read, buffer_5);
+
+  if (auto written = kapi::filesystem::write(fd_6.value(), buffer_5); !written)
+  {
+    kapi::system::panic("demo failed: {}", written.error());
+  }
+  else
+  {
+    bytes_written = written->value;
+  }
+  kstd::println("--> written {} bytes to /dev/zero", bytes_written);
 }
 
 auto main() -> int
@@ -162,6 +227,9 @@ auto main() -> int
   kernel::drivers::init();
   kstd::println("[OS] Generic drivers initialized.");
 
+  kernel::devices::init();
+  kstd::println("[OS] Generic devices initialized.");
+
   kapi::devices::init_platform_drivers();
   kstd::println("[OS] Platform drivers initialized.");