This vector is taken after the POST in order to attempt to load and
execute any bootstrap code on diskette or hard disk.
The data in sector 1 of track 0 of head 0 of the first responding disk
(diskette or hard disk) is loaded at absolute address 0000:7c00 and
control is given to that address. On the oldest IBM-logoed systems,
control goes to INT 18H (to start ROM BASIC) if all disks fail.
On a hard disk, the bootstrap code determines which partition is active,
and then loads and executes the boot sector for that partition.
Note: You can use this service to reboot DOS after changing some values
(be sure 0040:0072 is set to 1234H).
After a reboot, DOS will notice any changes you made.