INT 19h (25)             Bootstrap Loader Service

Attempts to load the sector at head 0, cylinder 0, sector 1, of a
diskette or fixed disk into memory at 0:7C00h, and transfer control
there. This sector usually has an operating system bootstrap loader.

                                                                          

This interrupt attempts to load the sector at head 0, track 0, sector
1, on the first diskette into memory at 0:7C00h. If unable, it then
attempts to load the sector at head 0, track 0, sector 1 of the first
hard disk. If it is unable to load from either the diskette or the
fixed disk, it calls INT 18h, which calls the ROM BASIC.

If INT 19h is successful, control is transferred to the first byte of
the sector, which has been read in at memory location 0:7C00h. That
is, CS is set to 0 and IP is set to 7C00h.

Notes:         This interrupt is not a substitute for a Ctrl-Alt-
Del warm boot. This interrupt is the last action
performed by the Power-On Self Test (POST), which is
activated by Ctrl-Alt-Del. If a user program
attempts to execute this interrupt directly, the
machine may lock up.

To correctly reboot within an application program,
jump to FFFF:0h in memory. That is, set CS to FFFF
and IP to 0. This is the same process that occurs
when the machine is first powered on. Following
these steps will perform a "cold" boot. To perform a
"warm" boot using this method, first set the word at
location 0:472h to 1234h. This will simulate the
pressing of the Ctrl-Alt-Del.

A cold boot initializes all hardware, tests all
hardware, tests RAM, then calls INT 19h to load the
bootstrap loader. This process is performed when the
computer is turned on or a hardware reset button is
pressed.

A warm boot initializes and tests all hardware but
does not test RAM. It then calls INT 19h to load the
bootstrap loader. This process is performed when
Ctrl-Alt-Del is typed.

The sector read from the diskette will in turn read
in the rest of the operating system. A fixed disk
will read in a Partition Table, which will then read
in the correct operating system.

See also: INT 18h
See also: 0:472h

INT 19h (25) Bootstrap Loader Service