HP Vectra Technical Reference Manual page 21

2.3 ROM BIOS, Drivers and Functions
The ROM BIOS is comprised of many drivers. For example, there is a driver to perform video
functions, one to perform disc functions, etc. The ROM BIOS drivers are organized into two
components. One component contains the 5TD-B105 drivers that support the 5TD-B105
functions. The second component contains EX-BIOS drivers that support unique HP features.
Each driver supports one or more functions. A function can be viewed as a specific task. For
example, the Video Driver supports 22 separate functions that perform tasks such as setting the
display mode, moving the cursor, and displaying characters.
2.3.1
STD-BIOS Drivers
Drivers in the 5TD-B105 are accessed through an interrupt. 5TD-B105 drivers are accessed through
interrupts OSH and 10H through 1CH. Drivers are accessed by performing a software INT n
instruction, where n is the interrupt number assigned to the driver (refer to table 2.1.)
The function code and any required data are passed in the 80286 registers. Data passing
conventions for 5TD-B105 drivers vary, however, there are aspects which are common.
• Most of the 5TD-B105 drivers support more than one function. Therefore, multi-function
drivers must have the desired function code passed as part of the data. The AH register is
used on all multi-function drivers to pass the function code.
• Byte and word data is passed in the internal registers of the 80286. Registers AL, BX, CX,
and DX are usually used for this purpose. The register assignments and number of registers
used depend on the driver and driver function.
• If the amount of data cannot fit in the internal registers of the 80286, a data buffer in
system memory is used. This buffer is usually pointed to by E5:BX, E5:BP or E5:51.
• Drivers may modify one or more registers. The registers which are maintained and the
registers which are modified vary from driver to driver. The registers which are modified are
listed in each function description.
12 ROM BIOS Overview