x86-bare-metal-examples/bios_scroll.S

/*
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

CLEAR
PRINT_STRING $stair

/* 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
stair:
    .asciz "a\nb\nc\nd"