HP Vectra Technical Reference Manual page 106

V_SINPUT chains to the appropriate physical device driver based on the vector index (vector
address divided by six) stored in the Physical ISR Event Record (DL register). It provides an entry
point into the Input System for non-HP-HIL devices. V_SINPUT also provides driver mapping
functions that will be discussed later in this section.
Two physical drivers will be discussed later in this section. The first is the physical GID driver
(PGID) which handles both absolute and relative data. Because PGID can handle both types of
GID data, it can chain to any logical GID driver; this forms the basis for Input System device driver
mapping. The second physical driver is the null device driver (V_PNULL), which serves as a
handler for unsupported devices. The keyboard driver is discussed in Section 5.
4.3.1.1 Device Driver Mapping
Each driver in the Input System has a vector in the HP_VECTOLTABLE, and a driver header.
Each driver header has two fields which determine the mapping of the driver. One field contains
the vector of the driver's parent driver and the other contains the vector of the driver's child
driver. Refer to Section 2 and Appendix G for a detailed description of driver headers.
Calls are made to the vector address contained in the parent field to pass the interrupt on to the
next driver in the device driver chain, moving the data from the hardware toward the application
via the desired logical GID driver. Hardware commands from the application are passed down
the device driver chain to the device via the vector address contained in the child vector field. By
changing the value of the parent or child vector field, the sequence of drivers called to handle an
interrupt or function request is changed. In general an application may re-map a driver by
changing the driver header directly. Functions are provided by the V_SINPUT service to map the
physical GID drivers to the logical GID drivers.
4.3.1.2 Device Emulation
Device emulation occurs when one or more physical devices are mapped to a logical device that
does not represent the original source of the data. For example, mapping a physi·cal mouse driver
to a logical touch screen driver allows the mouse to look like a touch screen to the application.
The key requirement for a logical device driver to emulate other devices is that it accept both
absolute and relative data. Referencing the above example, the logical touch screen driver which
reports absolute data must accept both absolute (touch) data and relative (mouse) data.
Input System and HP-HIL 97