Click above to get retro games delivered to your door ever month!

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.