1. Introduction

What you need

2. Basic hard drive/BIOS shit

As soon as you flip that switch, your CPU starts executing shit located at F000:FFF0. This area contains the BIOS, Basic Input/Output System. This code is written in assembly and is stored in chips called EPROMs in your computer. This code will perform something known as POST, Power On Self Test. This checks for installed devices and checks if they all work. In particular it checks for the video card and runs the video BIOS usually located at C000h. Next it checks for other ROMs to see if they have installed BIOSes. Usually it then finds and executes the hard drive BIOS located at C8000h. Then it starts something like a "system inventory" where it checks for other installed devices and tests them. It does some more stuff that's all basicly useless for us right now, until it finally transfers control over to the operating system. That's the part that we're interested in. Back in the old days, only one OS was installed on a computer. If you bought a certain computer, you could only run the OS that was made for it. Nothing else. Obviosly that wasn't such a good thing as you would have to buy a new computer if you wanted a different OS, so BIOS makers came up with the Boot Sector. In case you didn't know yet, a Sector is the smallest area your hard drive can access. According to the ATA standards each sector is exactly 512 bytes. However ATA standards only apply to hard drives, things like floopies can use whatever they want. Knowing this we can move on to the boot sector.

3. Making a Boot Sector

After the BIOS has successfully completed the POST it calls interrupt 19h. You can actually see this by dumping the memory located at F000:FFF0. For example, on my box I used debug with the following result:
-d f000:fff0
F000:FFF0 CD 19 E0 00 F0 31 31 2F-32 36 2F 39 39 00 FC 81 .....11/26/99...

As you should know, CD = INT. INT 19h attempts to read in the Boot Sector of the 1st floppy disk. If it fails it does the same thing on the 1st hard drive. If that fails it returns an error message. A valid boot sector must have its last two bytes set to AA55h. Assuming a valid boot sector is found, the code is loaded into memory at location 0000:7C00 and interrupt 19h jumps there to start executing the code. Since a boot sector has to fit into one sector (512 bytes) it can't really do much, usually it does a search for another file on another sector, then executes it. Our boot sector won't do that. For now it is enough that it displays a message and reboots when you press a key. Since DOS is not loaded yet, we have to use BIOS interupts to do all this. First we display a messages using interupt 10h. Next we wait for the user to press a key using interrupt 16h, and finally we make a FAR jump to FFFF:0000 which we restart the computer. So lets code this bitch:

First we use the code

	MOV AX,0x0003
	INT 0x10
to get into video mode. The registers have to be set up like this:
AH	Function number (00h, video)
AL	Video Mode (03, 80x25x16)
Next we print the message using:
	MOV AX,0x1301
	MOV BX,0x0007
	MOV CX,0x23
	MOV BP,MSG
	ADD BP,0x7C00
	INT 0x10
AH	Function number (13h: print string)
AL	Write Mode (01h: string is characters only, attribute in BL, cursor moved)
BH	Video Page number (00h)
BL	Attributes of characters (07h)
CX	Length of string, excluding any attributes (23h = 35 characters)
BP	ES:BP must point to the string, since a boot sector starts at 07C00, we add that to BP after we loaded it. You could also set the entry point of the program to 07C00, or change the data segment register to point to 07C00, but since it's just one instruction, this is fine for now.
Now we wait for the key to be pressed:
	MOV AH,0x00
	INT 0x16

Registers:
AH - 00, Read keyboard buffer, wait till full if not already.
The buffer will be empty since the computer didn't get time to put anything into it yet. Finally we reboot the computer by simply jumping to 0000:FFFF:

DB 0xEA
DW 0x0000
DW 0xFFFF

This looks a bit wierd but it's actualy quite simple. When declaring "variables" in assembly, the assembler simply puts the value into a memory location. Usually you use interrupts or something to point to them in order to use and manipulate them, but we could also put code there. This is what we're doing here. If you get a Hex to Mnemonix chart you will notice that EA is a Far Jump. So we put that into memory, followed by the location to jump to.

Next we fill the the remaining memory with NULL:

TIMES 510-($-$$) DB 0

This could also be done in TASM with something like TIMES 510 DUP (0). Finally we have to add those two bytes to the end so that the BIOS will know that this is a valid boot sector. This is done with the simple statement:

SIGNATURE DW 0xAA55

Here is the full code to everything we just discussed:

START:
	MOV AX,0x0003
	INT 0x10
PRINT_STRING:
	MOV AX,0x1301
	MOV BX,0x0007
	MOV CX,0x23
	MOV BP,MSG
	ADD BP,0x7C00
	INT 0x10
WAIT_FOR_KEY_PRESS:
	MOV AH,0x00
	INT 0x16
REBOOT:
	DB 0xEA
	DW 0x0000
	DW 0xFFFF
MSG DB 'pR3sS 4nY k3y 2 k0n71nu3',13,10,'btw, ph33r',0
	TIMES 510-($-$$) DB 0
SIGNATURE DW 0xAA55

Assemble with "nasm filename.asm". This will get you a file called "filename", no extension. It is a raw binary image of the code. Get out a floppy and type "debug filename". Enter this at the prompt: w 100 0 0 1. You should know what this does from my assembly tutorial, if not it simply means write whatever is in memory to location 100 on disk 0 (A:), starting from sector 0 to sector 1. Now try booting from this disk. You should get the message:

pR3sS 4nY k3y 2 k0n71nu3
btw, ph33r

And when you press a key, the keyboard buffer gets filled so interupt 16h is finished and we move on to the restart procedure. Obviously this was just a simply example, instead of printing a string, waiting for a key press and restarting, you could've put anything in there, just as long as you don't use DOS interupts. One nice thing might be to get into Protected Mode, or you could even do some graphics shit which might run faster than in DOS or Windows since nothing is in memory except what you want to be there.

4. Making a program to write a boot sector

If you tryed to access the disk with your boot sector on it, you'll notice that you can't. At least not using DOS. That's because DOS uses a few bytes of memory for data that it needs to know in order to determine what kind of disk it is, our program however uses those bytes for the code. Now, you could look up those memory areas and declare them at the start of your program, but instead we will just create a program that will write any kind of file directly to the boot sector of a disk, regardless of what's on that disk. This sounds harder than it really is. In fact, the resulting program is a mere 73 bytes. First of all we have to open the file we want to write to the boot sector using the code:

READFILE:

MOV AX,3D00h
MOV DX,OFFSET FILENAME
INT 21h

AH = 3Dh, Open file
AL = 00, open file as read only
DX = Points to file name. This has to be a ASCIIZ string, meaning it's terminated with a NULL character (0).

This will return the file handle in AX. If an error has occured, the carry flag will be set and the error code stored in AH. In that case, branch:

JC ERROR

Otherwise proceed to reading in the file:

MOV BX,AX
MOV AH,3Fh
MOV CX,0200h
MOV DX,OFFSET SHIT
INT 21h

First we move the file handle from AX into BX, then set up the other registers as follows:
AH = 3Fh, Read file
CX = 200h, Amount of data to read. Since a boot sector will always be 512 bytes long we read in 200h bytes (512d).
DX = Points to memory area to hold contents of file
Again, the carry flag will be set if an error occured, so branch:

JC ERROR

Now we're getting to the actual writing part. First we reset the floppy disk controller with the code:
WRITE_SECTOR:

MOV AH,0h
MOV DL,0
INT 13h

Next we write the data:

MOV AX,0301h
MOV CX,1
MOV DX,0
MOV BX,OFFSET SHIT
INT 13h

This is one of the more complicated interupts, and you have to know some shit about how hard drives are made up.
AH = 03h, Write Sector
AL = 1, Number of sectors to write on same track and head
CH = 0, Track number to write
CL = 1, Sector number to start writing from
DH = 0, Head number to write
DL = 0, Drive number to write (0 = A, 1 = B, etc)
BX = Buffer to write sector(s) from

Again the carry flag is set if an error occurs, but I like to keep things interesting and used a different method to check for an error. The error code is stored in AH, if AH is 0 there was no error. So to check for an error I can simply XOR AH, AH and Jump if Not Zero.

XOR AH,AH
JNZ ERROR

Otherwise, we're done and can terminate the program:

INT 20h

So the finished program looks like this:

MAIN SEGMENT

;the usual setup I use for .com files

ASSUME CS:MAIN,DS:MAIN,ES:MAIN,SS:MAIN
ORG 100h

START:

READFILE:

;reads file as explained above

MOV AX,3D00h
MOV DX,OFFSET FILENAME
INT 21h

JC ERROR

MOV BX,AX
MOV AH,3Fh
MOV CX,0200h
MOV DX,OFFSET SHIT
INT 21h

JC ERROR

WRITE_SECTOR:

;writes sectors as explained above

MOV AH,0h
MOV DL,0
INT 13h

MOV AH,03h
MOV AL,1
MOV CX,1
MOV DX,0
MOV BX,OFFSET SHIT
INT 13h

XOR AH,AH
JNZ ERROR

INT 20h

ERROR:

MOV AH,09h

;displays error message

MOV DX, OFFSET SHIT1
INT 21h
INT 20h

SHIT	DB ?	;uninitialized array to hold contents of file
SHIT1	DB 'Error$'	;Bad ass error message
FILENAME	DB 'ph33r',0	;filename to write
MAIN ENDS
END START

Now this thing is very very basic. There are many areas you could improve on. For example:

1. Make the filename a user inputed value. To do so, make FILENAME an array of 12 unitialized bytes (DOS filenames can't be longer than that). Than load that array into SI and call interupt 16h, function 0h. Loop it until enter is pressed, store the value in SI, incrementing SI each time.
2. Add more error messages, maybe even something that checks the error code and response with an appropriate message
3. This program wont wait for the motor to start up, so make a loop that loops about 3 times, checking if the disk drive is ready. If all tries fail, return an error saying that the disk is not in the drive or something. The error code is returned in AH, so you can make a simple check and respond with the corrosponding error message.
4. Display a (C) Microsoft message

5. Other