HP Vectra Technical Reference Manual page 250

Discs using installable device drivers can not be used as boot devices, since they are loaded in
RAM by the operating system. Further, operating systems other than DOS may not recognize the
disc in the system. For more information on installable device drivers consult the Vectra MS-DOS
Programmer's Reference Manual.
Using the option ROM entry mechanism described in the following section, the external hard disc
becomes an integrated part of the system and is treated as if it were an internal drive. The first
physical hard disc drive, 80H, can then be used as the system boot device.
8.1.5 INT 13H Disc Chain
The INT 13H disc driver chain is a linked list of disc driver entry points. This chain accesses the
BIOS based flexible disc driver and the hard disc driver. This linked list is configured during
SYSGEN. The following description specifies how the disc chain is implemented and how it is
created in the system.
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8.1.5.1 INT 13H Disc Chain Linkage
The hard disc driver is linked into the INT 13H driver chain during the option ROM initialization.
The process can be extended by other option ROM based disc adapters. The following is a
description of how the HP hard disc ROM BIOS enters the INT 13H device chain during the
option ROM initialization process. The relationship between the flexible and hard disc driver code
modules is illustrated in figure 8.1 .
• SYSGEN first configures INT 13H to point to the entry point of the flexible disc driver code.
• SYSGEN detects the hard disc driver's option ROM module and call the initialization entry
point of the option ROM.
• The hard disc driver code initializes the disc adapter and the disc drive.
• After the disc adapter hardware is initialized the disc drive is ready to install itself in the INT
13H disc driver chain. The hard disc driver calls the INT 13H function F13_GET_HPARMS
(08H) to determine how many other hard discs have entered the system. The lowest hard
disc device number (80H) is used for the call. If the call is successful, then the DL register
contains the current number, 'n', of hard discs already linked in the INT 13H disc chain. If
there are no other hard discs linked into the system, the call will return the status, bad
command error, and the current number of drives 'n' is set to DOH.
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