Fix double buffer allocation

Author: vinc

src/sys/vga/screen.rs

@@ -11,7 +11,8 @@ enum ModeName {
     G640x480x16,
 }
 
-const FRAMEBUFFER: usize = 0xA0000;
+const FRAME_BUFFER_ADDR: usize = 0xA0000;
+const DOUBLE_BUFFER: [u8; 640 * 480] = [0; 640 * 480];
 
 static MODE: Mutex<Option<ModeName>> = Mutex::new(None);
 
@@ -174,14 +175,13 @@ fn set_640x480_mode() {
 
 fn clear_screen() {
     // Clear screen
-    let screen = [0; 640 * 480];
     let size = match *MODE.lock() {
         Some(ModeName::G320x200x256) => 320 * 200,
         Some(ModeName::G640x480x16) => 640 * 480,
         _ => return,
     };
-    let src = screen.as_ptr();
-    let dst = FRAMEBUFFER as *mut u8;
+    let src = DOUBLE_BUFFER.as_ptr();
+    let dst = FRAME_BUFFER_ADDR as *mut u8;
     unsafe {
         core::ptr::copy_nonoverlapping(src, dst, size);
     }