Initial VGA console output implementation.

- Add VgaWriter struct to handle text output to the VGA text buffer.
- Add raw_write_vga function and support functions for writing directly to the VGA text buffer.
- Add VGA_WRITER static variable to logger module for handling outputting logs to the vGA text buffer.

mythril_core/src/logger.rs

@@ -5,6 +5,68 @@ use core::fmt::Write;
 use spin::Mutex;
 
 static LOG_LOCK: Mutex<()> = Mutex::new(());
+static mut VGA_WRITER: VgaWriter = VgaWriter::new();
+
+const VGA_BASE_ADDR: usize = 0xB8000;
+const VGA_WIDTH: usize = 80;
+const VGA_HEIGHT: usize = 25;
+const VGA_ATTRIB: u16 = 0x0F00; // black background, white text
+
+fn scroll_vga(vga_mem: &mut [[u16; VGA_WIDTH]; VGA_HEIGHT]) {
+    for row in 1..VGA_HEIGHT {
+        for col in 0..VGA_WIDTH {
+            vga_mem[row - 1][col] = vga_mem[row][col];
+        }
+    }
+    clear_line_vga(VGA_HEIGHT - 1, vga_mem);
+}
+
+fn clear_line_vga(row: usize, vga_mem: &mut [[u16; VGA_WIDTH]; VGA_HEIGHT]) {
+    for col in 0..VGA_WIDTH {
+        (*vga_mem)[row][col] = VGA_ATTRIB | 0x20;
+    }
+}
+
+pub fn clear_vga(vga_mem: &mut [[u16; VGA_WIDTH]; VGA_HEIGHT]) {
+    for row in 0..VGA_HEIGHT {
+        clear_line_vga(row, vga_mem);
+    }
+}
+
+pub fn raw_write_vga(
+    s: impl AsRef<str>,
+    mut col: usize,
+    mut row: usize,
+    vga_mem: &mut [[u16; VGA_WIDTH]; VGA_HEIGHT],
+) -> (usize, usize) {
+    for byte in s.as_ref().bytes() {
+        // move cursor on newlines (0x0A) and carriage-returns (0x0D)
+        if byte == 0x0A {
+            row += 1;
+            col = 0;
+            continue;
+        } else if byte == 0x0D {
+            col = 0;
+            continue;
+        }
+
+        if row >= VGA_HEIGHT {
+            scroll_vga(vga_mem);
+            row = VGA_HEIGHT - 1;
+        }
+
+        vga_mem[row][col] = VGA_ATTRIB | (byte as u16);
+
+        col += 1;
+
+        if col >= VGA_WIDTH {
+            row += 1;
+            col = 0;
+        }
+    }
+
+    (col, row)
+}
 
 pub fn write_console(s: impl AsRef<str>) {
     let lock = LOG_LOCK.lock();
@@ -14,6 +76,9 @@ pub fn write_console(s: impl AsRef<str>) {
 
 // NOTE: the caller should hold `LOG_LOCK`
 pub unsafe fn raw_write_console(s: impl AsRef<str>) {
+    // mirror console output to VGA
+    VGA_WRITER.write(s.as_ref());
+
     //FIXME: what about addresses above 4GB?
     let len = s.as_ref().len();
     let ptr = s.as_ref().as_ptr();
@@ -25,6 +90,38 @@ pub unsafe fn raw_write_console(s: impl AsRef<str>) {
          : "volatile");
 }
 
+pub struct VgaWriter {
+    cur_col: usize,
+    cur_row: usize,
+}
+
+impl VgaWriter {
+    pub const fn new() -> Self {
+        VgaWriter { cur_col: 0, cur_row: 0 }
+    }
+
+    pub fn write(&mut self, s: impl AsRef<str>) {
+        let mut vga_mem = unsafe { &mut *(VGA_BASE_ADDR as *mut _) };
+        if self.cur_col == 0 && self.cur_row == 0 {
+            clear_vga(&mut vga_mem);
+        }
+        let (col, row) = raw_write_vga(s, self.cur_col, self.cur_row, &mut vga_mem);
+        self.cur_col = col;
+        self.cur_row = row;
+    }
+}
+
+impl fmt::Write for VgaWriter {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        self.write(s);
+        Ok(())
+    }
+
+    fn write_fmt(&mut self, args: fmt::Arguments) -> Result<(), fmt::Error> {
+        fmt::write(self, args)
+    }
+}
+
 pub struct DirectLogger;
 impl DirectLogger {
     pub const fn new() -> Self {