[org 0x7c00]
KERNEL_OFFSET equ 0x1000 ; The same one we used when linking the kernel
mov [BOOT_DRIVE], dl ; Remember that the BIOS sets us the boot drive in 'dl' on boot
mov bp, 0x9000
mov sp, bp
mov bx, MSG_REAL_MODE
call print
call print_nl
call load_kernel ; read the kernel from disk
call switch_to_pm ; disable interrupts, load GDT, etc. Finally jumps to 'BEGIN_PM'
[bits 16]
switch_to_pm:
cli ; 1. disable interrupts
lgdt [gdt_descriptor] ; 2. load the GDT descriptor
mov eax, cr0
or eax, 0x1 ; 3. set 32-bit mode bit in cr0
mov cr0, eax
jmp CODE_SEG:init_pm ; 4. far jump by using a different segment
[bits 32]
init_pm: ; we are now using 32-bit instructions
mov ax, DATA_SEG ; 5. update the segment registers
mov ds, ax
mov ss, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ebp, 0x90000 ; 6. update the stack right at the top of the free space
mov esp, ebp
call BEGIN_PM ; 7. Call a well-known label with useful code
[bits 16]
load_kernel:
mov bx, MSG_LOAD_KERNEL
call print
call print_nl
mov bx, KERNEL_OFFSET ; Read from disk and store in 0x1000
mov dh, 16 ; Our future kernel will be larger, make this big
mov dl, [BOOT_DRIVE]
call disk_load
disk_load:
pusha
; reading from disk requires setting specific values in all registers
; so we will overwrite our input parameters from 'dx'. Let's save it
; to the stack for later use.
push dx
mov ah, 0x02 ; ah <- int 0x13 function. 0x02 = 'read'
mov al, dh ; al <- number of sectors to read (0x01 .. 0x80)
mov cl, 0x02 ; cl <- sector (0x01 .. 0x11)
; 0x01 is our boot sector, 0x02 is the first 'available' sector
mov ch, 0x00 ; ch <- cylinder (0x0 .. 0x3FF, upper 2 bits in 'cl')
; dl <- drive number. Our caller sets it as a parameter and gets it from BIOS
; (0 = floppy, 1 = floppy2, 0x80 = hdd, 0x81 = hdd2)
mov dh, 0x00 ; dh <- head number (0x0 .. 0xF)
; [es:bx] <- pointer to buffer where the data will be stored
; caller sets it up for us, and it is actually the standard location for int 13h
int 0x13 ; BIOS interrupt
jc disk_error ; if error (stored in the carry bit)
pop dx
cmp al, dh ; BIOS also sets 'al' to the # of sectors read. Compare it.
jne sectors_error
popa
ret
disk_error:
mov bx, DISK_ERROR
call print
call print_nl
mov dh, ah ; ah = error code, dl = disk drive that dropped the error
call print_hex ; check out the code at http://stanislavs.org/helppc/int_13-1.html
jmp disk_loop
sectors_error:
mov bx, SECTORS_ERROR
call print
disk_loop:
jmp $
DISK_ERROR: db "Disk read error", 0
SECTORS_ERROR: db "Incorrect number of sectors read", 0
ret
[bits 32]
BEGIN_PM:
mov ebx, MSG_PROT_MODE
call print_string_pm
[bits 32] ; using 32-bit protected mode
; this is how constants are defined
VIDEO_MEMORY equ 0xb8000
WHITE_OB_BLACK equ 0x0f ; the color byte for each character
print_string_pm:
pusha
mov edx, VIDEO_MEMORY
print_string_pm_loop:
mov al, [ebx] ; [ebx] is the address of our character
mov ah, WHITE_OB_BLACK
cmp al, 0 ; check if end of string
je print_string_pm_done
mov [edx], ax ; store character + attribute in video memory
add ebx, 1 ; next char
add edx, 2 ; next video memory position
jmp print_string_pm_loop
print_string_pm_done:
popa
ret
call KERNEL_OFFSET ; Give control to the kernel
void main() {
clear_screen();
kprint_at("X", 1, 6);
kprint_at("This text spans multiple lines", 75, 10);
kprint_at("There is a line\nbreak", 0, 20);
kprint("There is a line\nbreak");
kprint_at("What happens when we run out of space?", 45, 24);
}
#include "screen.h"
#include "ports.h"
/* Declaration of private functions */
int get_cursor_offset();
void set_cursor_offset(int offset);
int print_char(char c, int col, int row, char attr);
int get_offset(int col, int row);
int get_offset_row(int offset);
int get_offset_col(int offset);
/**********************************************************
* Public Kernel API functions *
**********************************************************/
/**
* Print a message on the specified location
* If col, row, are negative, we will use the current offset
*/
void kprint_at(char *message, int col, int row) {
/* Set cursor if col/row are negative */
int offset;
if (col >= 0 && row >= 0)
offset = get_offset(col, row);
else {
offset = get_cursor_offset();
row = get_offset_row(offset);
col = get_offset_col(offset);
}
/* Loop through message and print it */
int i = 0;
while (message[i] != 0) {
offset = print_char(message[i++], col, row, WHITE_ON_BLACK);
/* Compute row/col for next iteration */
row = get_offset_row(offset);
col = get_offset_col(offset);
}
}
void kprint(char *message) {
kprint_at(message, -1, -1);
}
/**********************************************************
* Private kernel functions *
**********************************************************/
/**
* Innermost print function for our kernel, directly accesses the video memory
*
* If 'col' and 'row' are negative, we will print at current cursor location
* If 'attr' is zero it will use 'white on black' as default
* Returns the offset of the next character
* Sets the video cursor to the returned offset
*/
int print_char(char c, int col, int row, char attr) {
unsigned char *vidmem = (unsigned char*) VIDEO_ADDRESS;
if (!attr) attr = WHITE_ON_BLACK;
/* Error control: print a red 'E' if the coords aren't right */
if (col >= MAX_COLS || row >= MAX_ROWS) {
vidmem[2*(MAX_COLS)*(MAX_ROWS)-2] = 'E';
vidmem[2*(MAX_COLS)*(MAX_ROWS)-1] = RED_ON_WHITE;
return get_offset(col, row);
}
int offset;
if (col >= 0 && row >= 0) offset = get_offset(col, row);
else offset = get_cursor_offset();
if (c == '\n') {
row = get_offset_row(offset);
offset = get_offset(0, row+1);
} else {
vidmem[offset] = c;
vidmem[offset+1] = attr;
offset += 2;
}
set_cursor_offset(offset);
return offset;
}
int get_cursor_offset() {
/* Use the VGA ports to get the current cursor position
* 1. Ask for high byte of the cursor offset (data 14)
* 2. Ask for low byte (data 15)
*/
port_byte_out(REG_SCREEN_CTRL, 14);
int offset = port_byte_in(REG_SCREEN_DATA) << 8; /* High byte: << 8 */
port_byte_out(REG_SCREEN_CTRL, 15);
offset += port_byte_in(REG_SCREEN_DATA);
return offset * 2; /* Position * size of character cell */
}
void set_cursor_offset(int offset) {
/* Similar to get_cursor_offset, but instead of reading we write data */
offset /= 2;
port_byte_out(REG_SCREEN_CTRL, 14);
port_byte_out(REG_SCREEN_DATA, (unsigned char)(offset >> 8));
port_byte_out(REG_SCREEN_CTRL, 15);
port_byte_out(REG_SCREEN_DATA, (unsigned char)(offset & 0xff));
}
void clear_screen() {
int screen_size = MAX_COLS * MAX_ROWS;
int i;
char *screen = VIDEO_ADDRESS;
for (i = 0; i < screen_size; i++) {
screen[i*2] = ' ';
screen[i*2+1] = WHITE_ON_BLACK;
}
set_cursor_offset(get_offset(0, 0));
}
int get_offset(int col, int row) { return 2 * (row * MAX_COLS + col); }
int get_offset_row(int offset) { return offset / (2 * MAX_COLS); }
int get_offset_col(int offset) { return (offset - (get_offset_row(offset)*2*MAX_COLS))/2; }
unsigned char port_byte_in (unsigned short port) {
unsigned char result;
/* Inline assembler syntax
* !! Notice how the source and destination registers are switched from NASM !!
*
* '"=a" (result)'; set '=' the C variable '(result)' to the value of register e'a'x
* '"d" (port)': map the C variable '(port)' into e'd'x register
*
* Inputs and outputs are separated by colons
*/
__asm__("in %%dx, %%al" : "=a" (result) : "d" (port));
return result;
}
void port_byte_out (unsigned short port, unsigned char data) {
/* Notice how here both registers are mapped to C variables and
* nothing is returned, thus, no equals '=' in the asm syntax
* However we see a comma since there are two variables in the input area
* and none in the 'return' area
*/
__asm__("out %%al, %%dx" : : "a" (data), "d" (port));
}
unsigned short port_word_in (unsigned short port) {
unsigned short result;
__asm__("in %%dx, %%ax" : "=a" (result) : "d" (port));
return result;
}
void port_word_out (unsigned short port, unsigned short data) {
__asm__("out %%ax, %%dx" : : "a" (data), "d" (port));
}
jmp $ ; Stay here when the kernel returns control to us (if ever)
BOOT_DRIVE db 0 ; It is a good idea to store it in memory because 'dl' may get overwritten
MSG_REAL_MODE db "Started in 16-bit Real Mode", 0
MSG_PROT_MODE db "Landed in 32-bit Protected Mode", 0
MSG_LOAD_KERNEL db "Loading kernel into memory", 0
; padding
times 510 - ($-$$) db 0
dw 0xaa55
