41 lines
1.4 KiB
C
Executable File
41 lines
1.4 KiB
C
Executable File
#include "../drivers/ports.h"
|
|
|
|
void _start() {
|
|
/* Screen cursor position: ask VGA control register (0x3d4) for bytes
|
|
* 14 = high byte of cursor and 15 = low byte of cursor. */
|
|
port_byte_out(0x3d4, 14); /* Requesting byte 14: high byte of cursor pos */
|
|
/* Data is returned in VGA data register (0x3d5) */
|
|
int position = port_byte_in(0x3d5);
|
|
position = position << 8; /* high byte */
|
|
|
|
port_byte_out(0x3d4, 15); /* requesting low byte */
|
|
|
|
/*
|
|
Explaination:
|
|
Since the VGA output on the kernal has a set number of
|
|
positions per row. That number is 80. So by entering 80,
|
|
it goes to the first position on the second row.
|
|
Thus putting 250 is 80+80+80 (3rd row) + 10 columns over.
|
|
*/
|
|
position += port_byte_in(0x3d5) + 250;
|
|
|
|
/* VGA 'cells' consist of the character and its control data
|
|
* e.g. 'white on black background', 'red text on white bg', etc */
|
|
int offset_from_vga = position * 2;
|
|
|
|
/* Now you can examine both variables using gdb, since we still
|
|
* don't know how to print strings on screen. Run 'make debug' and
|
|
* on the gdb console:
|
|
* breakpoint kernel.c:21
|
|
* continue
|
|
* print position
|
|
* print offset_from_vga
|
|
*/
|
|
|
|
/* Let's write on the current cursor position, we already know how
|
|
* to do that */
|
|
char *vga = (char *) 0xb8000;
|
|
vga[offset_from_vga] = 'X';
|
|
vga[offset_from_vga+1] = 0x0f; /* White text on black background */
|
|
}
|