Provide an interface for drawing on the screen in VGA text mode

This interface allows the user to write strings and move the cursor in VGA text mode.

This closes #1.

3 files changed, 105 insertions, 10 deletions

diff --git a/kernel/src/io.rs b/kernel/src/io.rs
new file mode 100644
index 0000000..b6b4cc7
--- /dev/null
+++ b/kernel/src/io.rs
@@ -0,0 +1,7 @@
+pub fn out8(address: u16, value: u8) {
+    unsafe {
+        asm!(
+            "out dx, al", in("dx") address, in("al") value
+        )
+    }
+}
\ No newline at end of file
diff --git a/kernel/src/lib.rs b/kernel/src/lib.rs
index 285ecff..b6bc55f 100644
--- a/kernel/src/lib.rs
+++ b/kernel/src/lib.rs
@@ -1,23 +1,23 @@
+#![feature(asm)]
 #![no_std]
 #![no_main]
 
+use crate::terminal::Terminal;
 use core::panic::PanicInfo;
 
+mod io;
+mod terminal;
+
 #[no_mangle]
 pub extern "C" fn kmain() -> ! {
-    let vga_buffer = 0xb8000 as *mut u8;
-
-    for (i, &byte) in b"LisibleOS Hello world".iter().enumerate() {
-        unsafe {
-            *vga_buffer.offset(i as isize * 2) = byte;
-            *vga_buffer.offset(i as isize * 2 + 1) = 0xb;
-        }
-    }
-
+    let mut terminal = Terminal::new();
+    terminal.clear();
+    terminal.put_string(b"LisibleOS\n");
+    terminal.put_string(b"> ");
     loop {}
 }
 
 #[panic_handler]
 fn panic(_info: &PanicInfo) -> ! {
     loop {}
-}
\ No newline at end of file
+}
diff --git a/kernel/src/terminal.rs b/kernel/src/terminal.rs
new file mode 100644
index 0000000..2a5cf1a
--- /dev/null
+++ b/kernel/src/terminal.rs
@@ -0,0 +1,88 @@
+use crate::io;
+
+const VGA_BUFFER_POINTER: *mut u8 = 0xb8000 as *mut u8;
+const VGA_BUFFER_SIZE: u16 = 4000;
+const ROW_COUNT: u8 = 25;
+const COL_COUNT: u8 = 80;
+const VALUE_SIZE: u8 = 2;
+
+pub(crate) struct Terminal {
+    current_row: u8,
+    current_col: u8,
+}
+
+impl Terminal {
+    pub fn new() -> Self {
+        Self {
+            current_row: 0,
+            current_col: 0,
+        }
+    }
+
+    pub fn put_string(&mut self, string: &[u8]) {
+        for &character in string {
+            self.put_char(character);
+        }
+        self.set_cursor_position(self.current_row, self.current_col);
+    }
+
+    pub fn put_char(&mut self, c: u8) {
+        if c == b'\n' {
+            self.current_row += 1;
+            if self.current_row == ROW_COUNT {
+                self.current_row = 0;
+            }
+
+            self.current_col = 0;
+            return;
+        }
+
+        unsafe {
+            *VGA_BUFFER_POINTER.offset(
+                ((self.current_col as isize + self.current_row as isize * COL_COUNT as isize)
+                    * VALUE_SIZE as isize),
+            ) = c;
+        }
+        self.compute_current_position();
+    }
+
+    fn compute_current_position(&mut self) {
+        if self.current_col == COL_COUNT - 1 {
+            if self.current_row == ROW_COUNT - 1 {
+                self.current_col = 0;
+                self.current_row = 0;
+            } else {
+                self.current_row = (self.current_row + 1) % ROW_COUNT;
+            }
+        }
+        self.current_col = (self.current_col + 1) % COL_COUNT;
+    }
+
+    pub fn put_char_at(&mut self, c: u8, row: u8, col: u8) {
+        self.current_row = row;
+        self.current_col = col;
+        self.put_char(c);
+    }
+
+    pub fn clear(&mut self) {
+        for row in 0..ROW_COUNT {
+            for col in 0..COL_COUNT {
+                self.put_char_at(b' ', row, col);
+            }
+        }
+        self.current_col = 0;
+        self.current_row = 0;
+    }
+
+    pub fn set_cursor_position(&mut self, row: u8, col: u8) {
+        let cursor_position: u16 = (row * COL_COUNT + col) as u16;
+        let address_register = 0x3D4;
+        let data_register = 0x3D5;
+        unsafe {
+            io::out8(address_register, 0x0F);
+            io::out8(data_register, (cursor_position & 0xFF) as u8);
+            io::out8(address_register, 0x0E);
+            io::out8(data_register, ((cursor_position >> 8) & 0xFF) as u8);
+        }
+    }
+}