Gracias. Ya mas o menos voy entendiendolo.
A ver, ahora tengo este bootloader:
Código C++:
Ver original[BITS 16] ; We need 16-bit intructions for Real mode
[ORG 0x7C00] ; The BIOS loads the boot sector into memory location 0x7C00
reset_drive:
mov ah, 0 ; RESET-command
int 13h ; Call interrupt 13h
or ah, ah ; Check for error code
jnz reset_drive ; Try again if ah != 0
mov ax, 0
mov es, ax
mov bx, 0x1000 ; Destination address = 0000:1000
mov ah, 02h ; READ SECTOR-command
mov al, 02h ; Number of sectors to read = 1
mov ch, 0 ; Cylinder = 0
mov cl, 02h ; Sector = 2
mov dh, 0 ; Head = 0
int 13h ; Call interrupt 13h
or ah, ah ; Check for error code
jnz reset_drive ; Try again if ah != 0
cli ; Disable interrupts, we want to be alone
xor ax, ax
mov ds, ax ; Set DS-register to 0 - used by lgdt
lgdt [gdt_desc] ; Load the GDT descriptor
mov eax, cr0 ; Copy the contents of CR0 into EAX
or eax, 1 ; Set bit 0
mov cr0, eax ; Copy the contents of EAX into CR0
jmp 08h:clear_pipe ; Jump to code segment, offset clear_pipe
[BITS 32] ; We now need 32-bit instructions
clear_pipe:
mov ax, 10h ; Save data segment identifyer
mov ds, ax ; Move a valid data segment into the data segment register
mov ss, ax ; Move a valid data segment into the stack segment register
mov esp, 090000h ; Move the stack pointer to 090000h
jmp 08h:01000h ; Jump to section 08h (code), offset 01000h
gdt: ; Address for the GDT
gdt_null: ; Null Segment
dd 0
dd 0
gdt_code: ; Code segment, read/execute, nonconforming
dw 0FFFFh
dw 0
db 0
db 10011010b
db 11001111b
db 0
gdt_data: ; Data segment, read/write, expand down
dw 0FFFFh
dw 0
db 0
db 10010010b
db 11001111b
db 0
gdt_end: ; Used to calculate the size of the GDT
gdt_desc: ; The GDT descriptor
dw gdt_end - gdt - 1 ; Limit (size)
dd gdt ; Address of the GDT
times 510-($-$$) db 0 ; Fill up the file with zeros
dw 0AA55h ; Boot sector identifyer
y este kernel en C:
lo compilo con este script:
nasm -f bin bootsect.asm -o bootsect.bin
gcc -ffreestanding -c main.c -o main.o
ld -e main -Ttext 0x1000 -o kernel.o main.o
ld -i -e main -Ttext 0x1000 -o kernel.o main.o
objcopy -R .note -R .comment -S -O binary kernel.o kernel.bin
Y creo la imagen con:
rm boot.img
dd if=/dev/zero of=boot.img bs=512 count=15
dd if=bootloader.bin of=boot.img bs=512 count=1
dd if=kernel.bin of=boot.img bs=512 count=14 seek=1
Pero cuando prueba la imagen en la maquina virtual en un diskete, me sale el error que no encuentra un sector booteable.
¿Estoy creando mal la imagen?
Gracias.
C/C++ Hola mundo sin Sistema Operativo 
