BIOS load the first sector into 0000:7c00,then change
CS:IP = 0000:7C00. Please copy the old data of first
to a savety place, and write your self program replace
there. You can do what you want.It's very important to
move your code to another address in RAM. Load old boot
data to 0000:7C00 and change CS:IP = 0000:7C00, you can
exit your boot program.