From 1a3c20cc9ea191a862eb7e8ac55b3a69ac74ad5e Mon Sep 17 00:00:00 2001
From: Felix Morgner <felix.morgner@ost.ch>
Date: Mon, 24 Nov 2025 16:59:24 +0100
Subject: x86_64/vga: rely less on magic state

---
 arch/x86_64/src/vga/text.cpp | 50 +++++++++++++++++++++++++-------------------
 1 file changed, 29 insertions(+), 21 deletions(-)

(limited to 'arch/x86_64/src/vga')

diff --git a/arch/x86_64/src/vga/text.cpp b/arch/x86_64/src/vga/text.cpp
index 6ecffa3..8b7f01b 100644
--- a/arch/x86_64/src/vga/text.cpp
+++ b/arch/x86_64/src/vga/text.cpp
@@ -1,38 +1,41 @@
 #include "x86_64/vga/text.hpp"
 
+#include "kapi/boot.hpp"
+
 #include "x86_64/boot/boot.hpp"
+#include "x86_64/boot/ld.hpp"
 #include "x86_64/vga/crtc.hpp"
 
 #include <algorithm>
 #include <bit>
 #include <cstddef>
+#include <cstdint>
 #include <string_view>
 #include <utility>
 
 namespace teachos::vga::x86_64::text
 {
-  using boot::x86_64::vga_buffer_pointer;
-
   namespace
   {
-    // NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
-    auto constinit buffer_offset = std::ptrdiff_t{};
-
+    constexpr auto BUFFER_BASE_ADDRESS = std::uintptr_t{0xb8000};
     constexpr auto DEFAULT_TEXT_BUFFER_WIDTH = 80U;
     constexpr auto DEFAULT_TEXT_BUFFER_HEIGHT = 25U;
     constexpr auto CURSOR_ENABLED_BIT = 5U;
-
-    auto write_char(char code_point, attribute attribute) -> void
-    {
-      vga_buffer_pointer[buffer_offset++] = std::pair{code_point, std::bit_cast<std::byte>(attribute)};
-    };
   }  // namespace
 
+  std::span<device::glyph> const device::buffer =
+      std::span{std::bit_cast<device::glyph *>(BUFFER_BASE_ADDRESS +
+                                               std::bit_cast<std::uintptr_t>(&teachos::boot::x86_64::TEACHOS_VMA)),
+                DEFAULT_TEXT_BUFFER_WIDTH * DEFAULT_TEXT_BUFFER_HEIGHT};
+
+  device::device()
+      : m_position{boot::bootstrap_information.vga_buffer_index}
+  {}
+
   auto device::clear(attribute attribute) -> void
   {
-    buffer_offset = 0;
-    std::ranges::fill_n(vga_buffer_pointer.get(), DEFAULT_TEXT_BUFFER_WIDTH * DEFAULT_TEXT_BUFFER_HEIGHT,
-                        std::pair{' ', std::bit_cast<std::byte>(attribute)});
+    m_position = 0;
+    std::ranges::fill(buffer, std::pair{' ', std::bit_cast<std::byte>(attribute)});
   }
 
   auto device::cursor(bool enabled) -> void
@@ -45,30 +48,35 @@ namespace teachos::vga::x86_64::text
 
   auto device::newline() -> void
   {
-    auto current_line = buffer_offset / DEFAULT_TEXT_BUFFER_WIDTH;
+    auto current_line = m_position / DEFAULT_TEXT_BUFFER_WIDTH;
     auto next_line = current_line + 1;
 
     if (std::cmp_greater_equal(next_line, DEFAULT_TEXT_BUFFER_HEIGHT))
     {
-      auto begin = vga_buffer_pointer + DEFAULT_TEXT_BUFFER_WIDTH;
-      auto end = vga_buffer_pointer + DEFAULT_TEXT_BUFFER_WIDTH * DEFAULT_TEXT_BUFFER_HEIGHT;
-      std::ranges::move(begin, end, vga_buffer_pointer.get());
-      buffer_offset = current_line * DEFAULT_TEXT_BUFFER_WIDTH;
+      auto begin = buffer.begin() + DEFAULT_TEXT_BUFFER_WIDTH;
+      auto end = buffer.begin() + DEFAULT_TEXT_BUFFER_WIDTH * DEFAULT_TEXT_BUFFER_HEIGHT;
+      std::ranges::move(begin, end, buffer.begin());
+      m_position = current_line * DEFAULT_TEXT_BUFFER_WIDTH;
     }
     else
     {
-      buffer_offset = next_line * DEFAULT_TEXT_BUFFER_WIDTH;
+      m_position = next_line * DEFAULT_TEXT_BUFFER_WIDTH;
     }
   }
 
   auto device::write(std::string_view code_points, attribute attribute) -> void
   {
-    std::ranges::for_each(code_points, [&](auto code_point) -> void { write_char(code_point, attribute); });
+    std::ranges::for_each(code_points, [&](auto code_point) -> void { write(code_point, attribute); });
   }
 
+  auto device::write(char code_point, attribute attribute) -> void
+  {
+    buffer[m_position++] = std::pair{code_point, std::bit_cast<std::byte>(attribute)};
+  };
+
   auto device::writeln(std::string_view code_points, attribute attribute) -> void
   {
-    std::ranges::for_each(code_points, [&](auto code_point) -> void { write_char(code_point, attribute); });
+    std::ranges::for_each(code_points, [&](auto code_point) -> void { write(code_point, attribute); });
     newline();
   }
 
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