BIOS color, scroll

bios_scroll.S

@@ -0,0 +1,72 @@
+/*
+BIOS has a scroll function!
+
+Very convenient, otherwise that would be hard to implement.
+
+How it works:
+
+Before scroll:
+
+    a
+     b
+      c
+       d
+
+We then choose to act on the rectangle with corners
+(1, 1) and (2, 2)} given by cx and dx:
+
+    a
+     XX
+     XX
+       d
+
+and scroll that rectangle down by one line (al).
+
+The final outcome is:
+
+    a
+      c
+     NN
+       d
+
+where `N` are new squares generated by the scroll,
+which gets filled with the background color in bh.
+*/
+
+#include "common.h"
+BEGIN
+
+/* Print staircase. */
+CLEAR
+PUTC(61)
+PUTC(0A)
+PUTC(62)
+PUTC(0A)
+PUTC(63)
+PUTC(0A)
+PUTC(64)
+
+/* Function ID. */
+mov $0x06, %ah
+/* nr. of lines to scroll */
+mov $0x01, %al
+/*
+BIOS color attributes.
+Background is the clear color.
+Foreground is set as the new foreground color.
+*/
+mov $0xA4, %bh
+/*
+CH,CL: row,column upper left corner (00:00)
+TODO what does that mean?
+*/
+mov $0x0101, %cx
+/*
+DH,DL: row,column lower right corner (24:79).
+TODO what does it mean?
+*/
+mov $0x0202, %dx
+int $0x10
+
+hlt
+END