BIOS
gram should always save this vector and restore it before ter
minating.
For more information on the exception vectors, see the
memory map in Appendix K, locations 0-1036 ($0-$40C).
The final BIOS system call pertains to the system timer
interrupt. This is a system interrupt routine that is called pe
riodically to update the GEMDOS date and time. The Tick-
cal() routine returns the number of milliseconds between
timer ticks. For the ST, this value is 20 milliseconds, since
the timer interrupt updates the system clock at the rate of
fifty times per second (even though the interrupt is actually
called 200 times per second). The format for Tickcal() is
long ticklen;
ticklen = Tickcal();
where ticklen is the length of time that passes between timer
ticks, in milliseconds. This call is unnecessary because the
number of milliseconds since the last timer interrupt is
passed on the stack when the timer interrupt handler is
called. This value is also stored in the system variable table
at location 1090 ($442). For more information on using the
timer interrupt vector, see the entry for address 1024 ($400)
in Appendix K.
25
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