x86_64/vga: fix scrolling implementation

2 files changed, 19 insertions, 2 deletions

diff --git a/arch/x86_64/include/x86_64/boot/boot.hpp b/arch/x86_64/include/x86_64/boot/boot.hpp
index 2c44659..0db2396 100644
--- a/arch/x86_64/include/x86_64/boot/boot.hpp
+++ b/arch/x86_64/include/x86_64/boot/boot.hpp
@@ -60,7 +60,7 @@ namespace teachos::boot
     multiboot2::information_view const * mbi;
 
     //! The index of the next character to be written in the VGA text buffer after handoff.
-    std::size_t vga_buffer_index;
+    std::ptrdiff_t vga_buffer_index;
   };
 
 }  // namespace teachos::boot
diff --git a/arch/x86_64/include/x86_64/vga/text.hpp b/arch/x86_64/include/x86_64/vga/text.hpp
index bb593e7..9f80f94 100644
--- a/arch/x86_64/include/x86_64/vga/text.hpp
+++ b/arch/x86_64/include/x86_64/vga/text.hpp
@@ -3,9 +3,11 @@
 
 #include "kapi/cio.hpp"
 
+#include <cstddef>
 #include <cstdint>
 #include <span>
 #include <string_view>
+#include <utility>
 
 namespace teachos::vga::x86_64::text
 {
@@ -123,14 +125,17 @@ namespace teachos::vga::x86_64::text
     {
       write(text, common_attributes::green_on_black);
     }
+
     auto writeln(std::string_view text) -> void override
     {
       writeln(text, common_attributes::green_on_black);
     }
+
     auto write_error(std::string_view text) -> void override
     {
       write(text, common_attributes::red_on_black);
     }
+
     auto writeln_error(std::string_view text) -> void override
     {
       writeln(text, common_attributes::red_on_black);
@@ -139,11 +144,19 @@ namespace teachos::vga::x86_64::text
   private:
     using glyph = std::pair<char, std::byte>;
 
+    [[nodiscard]] auto column() const noexcept -> std::ptrdiff_t;
+    [[nodiscard]] auto line() const noexcept -> std::ptrdiff_t;
+
     /**
      * @brief Move the cursor to a new line, scrolling the buffer if necessary.
      */
     auto newline() -> void;
 
+    //! Scroll the screen by a number of lines.
+    //!
+    //! @param nof_lines The number of lines to scroll up.
+    auto scroll(std::ptrdiff_t nof_lines = 1) -> void;
+
     /**
      * @brief Write a string of code points to the VGA text buffer.
      *
@@ -155,6 +168,10 @@ namespace teachos::vga::x86_64::text
      */
     auto write(std::string_view code_points, attribute attribute) -> void;
 
+    //! Write a single code point to the VGA text buffer.
+    //!
+    //! @param code_point A single (8-bit) code point
+    //! @param attribute The #attribute to apply to the code point.
     auto write(char code_point, attribute attribute) -> void;
 
     /**
@@ -170,7 +187,7 @@ namespace teachos::vga::x86_64::text
 
     std::span<glyph> static const buffer;
 
-    std::size_t m_position{};
+    std::ptrdiff_t m_position{};
   };
 
 }  // namespace teachos::vga::x86_64::text