Method and apparatus for handling an interrupt from a real-time clock to increment a program clock

1. In a computer system having a processor and a program running on the processor, the program having a program clock that is incremented by the processor in response to an interrupt received from a real-time clock, a method of incrementing the program clock, comprising the steps of:

(a) storing a count representing a cumulative interval of time that has elapsed without a corresponding incrementing of the program clock; and

(b) in response to an interrupt from the real-time clock:

(1) incrementing the cumulative interval of time by an actual interval of time that has elapsed since a previous real-time clock interrupt; and

(2) if the cumulative interval of time is greater than an expected interval of time between real-time clock interrupts, incrementing the program clock and decrementing the cumulative interval of time to reflect incrementing of the program clock.

2. The method of claim 1 in which the steps of incrementing the program clock and decrementing the cumulative interval of time are performed iteratively until the cumulative interval of time is less than the expected interval of time between real-time clock interrupts.

3. The method of claim 1 in which the steps of incrementing the program clock and decrementing the cumulative interval of time are performed unconditionally on a first iteration and are performed conditionally on one or more additional iterations until the cumulative interval of time is less than the expected interval of time between real-time clock interrupts.

4. The method of claim 1 in which the processor transfers control to an interrupt handling routine for performing steps (b)(1) and (b)(2) in response to the interrupt from the real-time clock.

5. The method of claim 1 in which step (b)(1) includes the step of determining the interval of time that has elapsed since the previous real-time clock interrupt.

6. The method of claim 5 in which the step of determining the interval of time that has elapsed since the previous real-time clock interrupt comprises the steps of:

obtaining a current timer value from a timer having a resolution greater than that of the real-time clock; and

comparing the current timer value with a timer value obtained from the timer on the previous real-time clock interrupt.

7. The method of claim 1 in which the program clock is further adjusted upon being incremented a predetermined number of times.

8. In a computer system having a processor and a program running on the processor, the program having a program clock that is incremented by the processor in response to an interrupt received from a real-time clock, apparatus for handling an interrupt from the real-time clock to increment the program clock, comprising:

(a) means for determining an actual interval of time that has elapsed since a previous real-time clock interrupt;

(b) means for incrementing by said actual interval of time a cumulative interval of time that has elapsed since a previous real-time clock interrupt without a corresponding incrementing of the program clock; and

(c) means for incrementing the program clock and decrementing the cumulative interval of time to reflect incrementing of the program clock if the cumulative interval of time is greater than an expected interval of time between real-time clock interrupts.

9. The apparatus of claim 8 in which the means for incrementing the program clock and decrementing the cumulative interval of time operates iteratively until the cumulative interval of time is less than the expected interval of time between real-time clock interrupts.

10. The apparatus of claim 8 in which the means for incrementing the program clock and decrementing the cumulative interval of time operates unconditionally on a first iteration and operates conditionally on one or more additional iterations until the cumulative interval of time is less than the expected interval of time between real-time clock interrupts.

11. The apparatus of claim 8 in which means (a)–(c) are part of an interrupt handling routine responsive to the interrupt from the real-time clock.

12. The apparatus of claim 8 in which the means for determining the interval of time that has elapsed since the previous real-time clock interrupt comprises:

means for obtaining a current timer value from a timer having a resolution greater than that of the real-time clock; and

means for comparing the current timer value with a timer value obtained from the timer on the previous real-time clock interrupt.

13. The apparatus of claim 8 in which the program clock is further adjusted upon being incremented a predetermined number of times.

14. A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for incrementing a program clock in a computer system having a processor and a program running on the processor, the program having a program clock that is incremented by the processor in response to an interrupt received from the real-time clock, the method steps comprising:

(a) storing a count representing a cumulative interval of time that has elapsed without a corresponding incrementing of the program clock; and

(b) in response to an interrupt from the real-time clock:

(1) incrementing the cumulative interval of time by an actual interval of time that has elapsed since a previous real-time clock interrupt; and

(2) incrementing the program clock and decrementing the cumulative interval of time to reflect incrementing of the program clock if the cumulative interval of time is greater than an expected interval of time between real-time clock interrupts.

15. The program storage device of claim 14 in which the steps of incrementing the program clock and decrementing the cumulative interval of time are performed iteratively until the cumulative interval of time is less than the expected interval of time between real-time clock interrupts.

16. The program storage device of claim 14 in which the steps of incrementing the program clock and decrementing the cumulative interval of time are performed unconditionally on a first iteration and are performed conditionally on one or more additional iterations until the cumulative interval of time is less than the expected interval of time between real-time clock interrupts.

17. The program storage device of claim 14 in which the processor transfers control to an interrupt handling routine for performing steps (b)(1) and (b)(2) in response to the interrupt from the real-time clock.

18. The program storage device of claim 14 in which step (b)(1) includes the step of determining the interval of time that has elapsed since the previous real-time clock interrupt.

19. The program storage device of claim 14 in which the step of determining the interval of time that has elapsed since the previous real-time clock interrupt comprises the steps of:

obtaining a current timer value from a timer having a resolution greater than that of the real-time clock; and

comparing the current timer value with a timer value obtained from the timer on the previous real-time clock interrupt.

20. The program storage device of claim 14 in which the program clock is further adjusted upon being incremented a predetermined number of times.