Making A Bootable Floppy Disk (or, Making A Floppy Disk Bootable):

Writing your own bootable floppy disk code will make you appreciate how handy
operating systems are to have. You won't be able to use any interrupt
functions, so you'll have to make your own. (DOS is, after all, little more
than a collection of interrupts to make programming easier.) Actually, you
can use interrupts which are supplied by the BIOS (as several are), but you
can't use any of the standard INTs which are supplied by DOS.

If there is a floppy disk in the drive and the BIOS is set to boot from the
floppy drive first, it will boot from the floppy drive if there is anything
on the first sector of the floppy. The "Invalid system disk or disk error"
message is NOT produced by the BIOS; The BIOS cannot tell if a disk is a
system disk or not. In fact, when this message appears, the BIOS really has
already booted from the floppy disk, and the reason the message appears is
that the disk itself contains code on the first sector that will make the
computer display this message. (DOS automatically puts this code on the first
sector of every floppy disk it formats.)

To make your own bootable floppy, all you have to do is insert some code on
the first sector of the disk. This will be normal machine code similar to a
compiled assembly language program. In fact, you can write your boot code in
assembler and then use the compiled .COM code on the boot sector. Just
remember not to use INTs supplied by DOS. Also, one interesting fact to keep
in mind when writing boot code: A disk's boot code is loaded into memory
location 0000:7C00 before it is run. This means that normal assembler code,
which is usually assembled assuming a code offset of 100h (the normal offset
for a .COM file) will get messed up unless it is assembled with a code offset
of 7C00. To illustrate a simple assembler program to use on a boot sector,
here's a short booter program to display an error message and then halt the
system:

;(Program begins here)----------------------------------
PUSH CS
POP DS ;These two lines make DS the current code segment

MOV CX,4Ah ;Number of characters to print is 74, or 4Ah
MOV SI,7C15h ;Offset of message

looper:
LODSB ;Get DS:SI into AL
MOV AH,0Eh ;Print character and advance cursor
INT 10h
DEC CX
CMP CX,0
JNE looper

JMP $ ;Enter infinite loop which halts system

;The message contains a 0D and 0A (CR/LF) to make a new line on the screen.

message db 'Welcome to this disk.', 0Dh, 0Ah, 'This is not meant to be a boot disk. System halted.'
;(Program ends here)------------------------------------

This program, once compiled, is *almost* like a normal .COM program under
DOS, except for the use of 7C15h for the offset of the message data. Since
the message is 15h bytes from the start of the program, normally SI would be
set to 115h. (This is what would be compiled into the .COM file if you used
the command "MOV SI,OFFSET message" instead of "MOV SI,7C15h".) Since this
code is loaded into memory offset 7C00, we must manually specify the location
of 7C15. Otherwise, this is a typical assembler program; You can put the
resulting .COM file onto the boot sector of a floppy and boot off that
floppy; This will display the error message and halt the system.
Congratulations! You've made your own bootable floppy disk.