HP Vectra Technical Reference Manual page 280

The second clock is a software clock. It uses Channel 1 of the 8254 counter/timer chip (refer to
the Vectra Technical Reference Manual, Volume I for additional details). Channel 1 of the 8254
generates a hardware interrupt (IRQ 0) approximately 18.2 times per second. The ROM BIOS
keeps time by incrementing a software clock each time the interrupt occurs. The software clock
is used by the operating system for such tasks as time and date stamping of files.
The two clocks operate independently except at boot time. During the boot process the current
time and date maintained by the CMOS clock is read and used to initialize the software clock.
Changing the value of CMOS clock will not affect the software clock until the system is
rebooted.
The STD-BIOS clock driver (INT 1AH) provides a convenient way to read or set the time and date
from either of the system clocks. These functions are detailed later in this chapter.
In addition to keeping time, both clocks issue interrupts that call user or application program
routines. The software clock interrupt service routine performs an INT 1CH each clock tick. If this
vector is modified to point to a user routine, the routine will be called on each clock tick.
The CMOS clock has an "alarm clock" feature. It can be programmed to issue an interrupt at a
specified time. The real-time clock hardware issues an INT 4AH each time the alarm timer is
done. The interrupt 4AH vector can be modified to point to a user-supplied routine.
9.2 Data Structures
The system drivers use several data structures. The data structures for the STD-BIOS system
drivers are contained in the STD-BIOS data area, while those used by the EX-BIOS drivers are in
the EX-BIOS data area.
The STD-BIOS system drivers use four data structures. The memory size and equipment
determination drivers each use a word, the ROM software clock uses five bytes. These data
structures are located at 040: 13H, 040: 1 OH, and 040:6CH respectively. The extended system
support driver uses 9 bytes starting at location 040:98H. The EX-BIOS system drivers use the
global data area. These data structures are described in detail in Appendix B.
System Drivers 271