HP Vectra Technical Reference Manual page 330

In a 640 KB system if CMOS_SIZE is 620 KB then the EX-BIOS_SIZE data area starts at 620
KB. In this case the EX-BIOS__SIZE data area occupies all the area between the top of RAM
and the memory allocated to DOS.
10.5.2 HP_VECTOIL-TABLE Initialization
Once the EX-BIOS data area has been allocated, and its base address determined, the
HP_VECTOLTABLE is constructed. An image of the default HP_VECTOLTABLE is stored in
the system ROM. This image is transferred from ROM to the base of the EX-BIOS data area. All
free and reserved vectors are initialized to point at V_DOLITILE, a null routine. Some of these
vectors will be initialized to other drivers later in the SYSGEN process.
10.5.3 EX-BIOS Driver Initialization
The next step in the SYSGEN process is the initialization of the EX-BIOS drivers. Each driver is
called with the SF_INIT subfunction. Some of the EX-BIOS drivers add vectors to the table when
called to initialize. For example, the V_HPHIL driver initializes the vector addresses reserved for
the HP-HIL physical device drivers. The HP_VECTOLTABLE is fully initialized to its default state
when each driver has been called in this manner. Additional drivers may be added or substituted
by application programs or system software utilizing the vector maintenance functions of
V_SYSTEM (refer to Section 9 for a description of these functions).
10.5.4 Option ROM Module Integration
The ROM BIOS architecture allows code modules residing on adapter cards to be integrated into
the system. These ROM modules must be in the system address range of OCOOOOH-ODFFFFH.
(Note that only video adapter cards can have base address in the range of oeOOOOH through
OC7FFFH). In addition to ROM modules located on adapter cards, the processor extension card
contains sockets for additional ROMs. These ROMs are addressed from OEOOOOH-OEFFFFH.
ROM modules located on adapter cards or on the processor extension card are integrated into
the system in the same manner.
System Processes 321