HP Vectra Technical Reference Manual page 28

2.4.2 EX-BIOS Data Structures
Data structures for the EX-BIOS drivers are located in a block of memory at the top of system
RAM. The address of this block varies depending on the amount of RAM contained in the system
and the hardware configuration.
There are three types of data structures in the EX-BIOS data area. These structures are:
the HPbrVECTOfL..TABLE and its associated HP_ENTRY_CODE, the driver data areas, and the
EX-BIOS global data area.
Each of the 80286 interrupt vectors contains the Code Segment (CS) and Instruction Pointer {lP}
of its associated service routine. The HP_ENTRY interrupt vector (06FH) contains the CS:IP of the
HP_ENTRY_CODE. This routine uses the value contained in the BP register (an offset into the
HP_VECTOfL..TABLE, vector address) to branch to the appropriate EX-BIOS driver. The
HP_VECTOfL..TABLE resides at the base of the EX-BIOS data area. The HP_VECTOfL..TABLE
consists of an array of 3-word (six bytes) entries, one for each EX-BIOS driver. Each entry consists
of the IP, CS, and Data Segment (DS) of a driver.
Figure 2.2 illustrates the relationship between the 80286 interrupt vectors, the
HP_VECTOfL..TABLE, HP_ENTRY_CODE, and the EX-BIOS drivers.
The CS:IP in the HP_ENTRY interrupt vector points to a piece of code which branches to the
desired EX-BIOS driver. The vector address passed in BP must be a multiple of six. The code is as
follows:
HP_ENTRY_CODE:
MOV
JMP
OS, CS:[BP
+
4J
FAR PTR CS:[BPJ
This code resides directly after the last entry in the HP_VECTOfL..TABLE. Therefore, the CS:IP
entry in the HP_ENTRY interrupt vector provides two further pieces of information.
(s:o
is the
"",. starting address of the HP_VECTOfL..TABLE and IP is the length of the HP_VECTOfL..TABLE.
ROM BIOS Overview 19