From 23e0ccf7b5a0ccea545f231d35dbecc00011a9de Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Thu, 24 Apr 2025 00:21:02 +0200
Subject: multiboot: add multiboot example kernel

---
 x86-bare-metal/multiboot/vga.zig | 117 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 117 insertions(+)
 create mode 100644 x86-bare-metal/multiboot/vga.zig

(limited to 'x86-bare-metal/multiboot/vga.zig')

diff --git a/x86-bare-metal/multiboot/vga.zig b/x86-bare-metal/multiboot/vga.zig
new file mode 100644
index 0000000..10d222c
--- /dev/null
+++ b/x86-bare-metal/multiboot/vga.zig
@@ -0,0 +1,117 @@
+/// VGA text mode console [1]
+///
+/// 8 colors + 8 bright variants:
+///   - 0x0  black      (0x8  dark gray)
+///   - 0x1  blue       (0x9  light blue)
+///   - 0x2  green      (0xa  light green)
+///   - 0x3  cyan       (0xb  light cyan)
+///   - 0x4  red        (0xc  light red)
+///   - 0x5  magenta    (0xd  pink)
+///   - 0x6  brown      (0xe  yellow)
+///   - 0x7  light gray (0xf  white)
+///
+/// [1] https://en.wikipedia.org/wiki/VGA_text_mode
+pub fn vga(fg: u4, bg: u3) type {
+    return struct {
+        const Self = @This();
+
+        // Text mode dimensions (mode 3h text mode 80x25).
+        const COLS = 80;
+        const ROWS = 25;
+        // Slice into VGA video memory.
+        const VIDEO: []u16 = @as([*]u16, @ptrFromInt(0xB8000))[0 .. COLS * ROWS];
+
+        /// Current index into last line.
+        col: usize,
+
+        /// Default value.
+        pub const init: Self = .{
+            .col = 0,
+        };
+
+        /// Write string to screen and move cursor.
+        pub fn puts(self: *Self, msg: []const u8) void {
+            for (msg) |char| {
+                self.putcImpl(char);
+            }
+            self.updateCursor();
+        }
+
+        /// Write character to screen and move cursor.
+        pub fn putc(self: *Self, char: u8) void {
+            self.putcImpl(char);
+            self.updateCursor();
+        }
+
+        /// Write character to screen.
+        fn putcImpl(self: *Self, char: u8) void {
+            if (char == '\n') {
+                self.lf();
+                return;
+            }
+
+            const idx = (ROWS - 1) * COLS + self.col;
+            VIDEO[idx] = vgaEntry(char, fg, bg);
+
+            self.col += 1;
+            if (self.col == COLS) {
+                self.lf();
+            }
+        }
+
+        /// Handle linefeed (newline).
+        /// Move up all lines by one, shifting out the upper most line.
+        fn lf(self: *Self) void {
+            // Shift all rows one up, copy rows [1. ROWS[-> [0, ROWS-1[.
+            for (0..(ROWS - 1) * COLS) |idx| {
+                VIDEO[idx] = VIDEO[1 * COLS + idx];
+            }
+            // Clear ROWS-1 (last row).
+            for (0..COLS) |idx| {
+                VIDEO[(ROWS - 1) * COLS + idx] = vgaEntry(' ', fg, bg);
+            }
+            self.col = 0;
+        }
+
+        /// Update cursor to current position.
+        /// https://wiki.osdev.org/Text_Mode_Cursor#Without_the_BIOS
+        fn updateCursor(self: Self) void {
+            const idx = (ROWS - 1) * COLS + self.col;
+
+            outb(0x3d4, 0xf);
+            outb(0x3d5, @truncate(idx & 0xff));
+            outb(0x3d4, 0xe);
+            outb(0x3d5, @truncate(idx >> 8 & 0xff));
+        }
+
+        const Writer = @import("std").io.GenericWriter(*Self, anyerror, struct {
+            fn write(ctx: *Self, bytes: []const u8) anyerror!usize {
+                ctx.puts(bytes);
+                return bytes.len;
+            }
+        }.write);
+
+        pub fn writer(self: *Self) Writer {
+            return .{ .context = self };
+        }
+    };
+}
+
+// Each VGA (text mode) video buffer entry is 16bit wide.
+//      [15] blink
+//   [14:12] bg color (3 bit)
+//   [11: 8] fg color (4 bit)
+//   [ 7: 0] ascii character
+//
+// https://en.wikipedia.org/wiki/VGA_text_mode
+inline fn vgaEntry(char: u8, fg: u4, bg: u3) u16 {
+    return @as(u16, bg) << 12 | @as(u16, fg) << 8 | char;
+}
+
+inline fn outb(port: u16, val: u8) void {
+    _ = asm volatile ("outb %[al], %[dx]"
+        : // no outputs
+        : [al] "{al}" (val),
+          [dx] "{dx}" (port),
+    );
+}
-- 
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