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as -o mbr.elf mbr.s

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	.code16	/* CPU is in 16 bits real mode */

start:	
	/* MBR code is loaded at 0x7C00 by the BIOS */
	mov $0x07C0, %ax
	mov %ax, %ds
	/* Stack grows downward from 0x80000 */
	mov $0x8000, %ax
	mov %ax, %ss
	mov $0x00, %sp
	
	movb %dl, (drive) /* Save drive number */

	/* Copy second sector to 0x500 */
	push %dl	/* Drive number */
	mov $0x01, %al	
	push %al	/* Copy 1 sector */
	mov $0x0500, %ax
	push %ax	/* Copy to 0x500 */
	mov $0x02, %al
	push %al	/* Sector 2 */
	mov $0x00, %al
	push %al	/* Head 0 */
	push %al	/* Cylinder 0 */
	call copy

end:
	jmp end
/* ----------------------- Copy function --------------------------

	Copies some sectors to RAM. Push arguments on stack
	in this order:
	+ Drive number (byte)
	+ Number of sectors to copy (byte)
	+ Address of copy (word)
	+ First sector CHS address as :
		- Sector (byte)
		- Head (byte)
		- Cylinder (byte)
	
------------------------------------------------------------------- */
copy:
	pop %al		/* Cylinder */
	mov %al, %ch
	and $0xFF, %ch

	pop %dh		/* Head */

	shr 2, %al
	and $0xC0, %al
	pop %cl		/* Sector */
	or %al, %cl

	pop %ax		/* Address */
	mov %ax, %bx
	shr 4, %ax
	sub %ax, %bx
	mov %ax, %es

	pop %al		/* Number of sectors */
	pop %dl		/* Drive number */

	mov $0x02, %al
	int $0x13
	
	ret

drive:
	.byte 0x00
	
	/* Code space padding */
	.skip 446 - (. - start), 0x90

	/* Partition table */
	.skip 0x40, 0x00

	/* Signature */
	.byte 0x55, 0xAA

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copy:
    push %bp
    mov %sp, bp

    mov 4(%bp), %ax
    mov 6(%bp), %dx

    /* do something with the values */

    pop %bp
    ret