Here's a boot sector I whipped up that helps me run a dual-boot FreeBSD/Windows system using NTLDR with much elegance. It is derived from a boot sector I wrote for a mystery OS, and includes a debug routine that isn't used but was left in there because I was proud of it. I haven't looked at this in quite a while, but it works. It is in NASM (not MASM) syntax. ;;; =========================================================================== ;;; This is the source to a fake boot sector to help the Windows NT bootloader, ;;; ntldr, load other operating systems. All this does is load the "real" boot ;;; sector from a drive and pass control to it. The drive number is hard-coded ;;; by the DRIVE %define below. You need NASM to assemble this file. ;;; =========================================================================== ;; BIOS drive number to load from %define DRIVE 0x80 ; <---- CHANGE FOR EACH BOOT SECTOR NEEDED ;; I hate "jmp short" %macro jmps 1 jmp short %1 %endmacro ;; Where we want to be org 0x7E00 ;;; =========================================================================== ;;; CODE ;;; =========================================================================== ;; Setup stack, move code somewhere else, and jump there cli xor ax, ax mov ds, ax mov es, ax mov ss, ax mov sp, 0x7C00 mov si, sp mov di, 0x7E00 mov cx, 0x0100 cld rep movsw jmp moved + 0x200 moved: sti ;; Reset drive mov dl, DRIVE int 0x13 jc error ;; Load real boot sector mov ax, 0x0201 mov bx, 0x7C00 mov cx, 0x0001 mov dh, ch mov dl, DRIVE int 0x13 jc error ;; Jump to real boot sector mov dl, DRIVE jmp 0x7C00 ;; In case of error, print msg and lock up error: mov si, errmsg call print jmps lockup ;; debug ;; Print the value of the registers and wait for a keystroke debug: push ax push cx push dx push bp pusha push ds push es push cs push ss mov bp, sp add bp, 22 mov si, regs .reg: call print mov cx, 4 ; print 4 digits mov dx, [bp] ; load word into dx .digit: rol dx, 4 ; rotate so that lowest 4 bits are used mov ax, 0x0E0F ; ah = request, al = mask for nybble and al, dl add al, 0x90 ; convert al to hex digits in ASCII daa adc al, 0x40 daa int 0x10 loop .digit cmp bp, sp je .done sub bp, 2 jmps .reg .done: call print add sp, 24 xor ax, ax int 0x16 pop bp pop dx pop cx pop ax ret ;;; print ;;; prints the zero-terminated string pointed to by SI. print: push ax push bx mov ah, 0x0E xor bx, bx .char: lodsb or al, al jz .done int 0x10 jmps .char .done: pop bx pop ax ret ;; lockup ;; Locks the machine way up. lockup: hlt jmps lockup ;;; =========================================================================== ;;; DATA ;;; =========================================================================== ;; Error message errmsg: db 'Error reading boot sector. System halted.', 0 ;; Register names regs: db 13, 10, "AX=", 0 db 13, 10, "CX=", 0 db 13, 10, "DX=", 0 db 13, 10, "BX=", 0 db 13, 10, "SP=", 0 db 13, 10, "BP=", 0 db 13, 10, "SI=", 0 db 13, 10, "DI=", 0 db 13, 10, "DS=", 0 db 13, 10, "ES=", 0 db 13, 10, "CS=", 0 db 13, 10, "SS=", 0 db 13, 10, 0 ;; Zero-padding to 512 bytes pad: times 510-($-$$) db 0x00 ;; Magic boot sector number magic: dw 0xAA55 -- Jeffrey S. Sharp jss(a)subatomix.com