HP Vectra Technical Reference Manual page 273

The memory size driver (INT 12H) returns a word that indicates the number of 1 KB blocks of
system RAM present. The amount of memory reported does not include any extended memory,
and is adjusted to exclude the amount of RAM occupied by the EX-BIOS data area. For example,
in a system equipped with 640 KB of system RAM using a 4 KB EX-BIOS data area, the amount
of memory reported by this driver will be 636 KB. The default amount of memory is read from a
word of CMOS memory.
9.1.2 Extended System Support
The extended system support driver (INT 15H) provides support for several advanced system
features. It provides "hooks" that allow programs to be written to support multi-tasking at a
future date. In addition, it allows data to be transferred to and from extended memory, and
allows placing the 80286 into its protected mode of operation.
9.1.3 EX-BIOS Driver Support
The V_SYSTEM driver is an EX-BIOS driver that provides support tasks for the EX-BIOS drivers. It
contains functions that allocate RAM in the EX-BIOS data area and manipulate
HP_VECTOLTABLE entries.
9.1.3.1 RAM Allocation
The EX-BIOS data area contains three major data structures; the HP_VECTOLTABLE, the
global data area, and the driver's data area. Within each driver's data area is the driver header,
describe record (if applicable), and variable storage area. Each entry in the HP_VECTOLTABLE
is three words long and consists of: Driver's IP, CS, and DS in that order. The HP_ENTRY_CODE
(INT 6FH) loads the appropriate driver's data segment DS and jumps to the address CS:IP.
The global data area is used by system drivers that need to share data. Data structures like the
EX-BIOS stack and memory management pointers are maintained here.
264 System Drivers