GPS140 Datasheet, PDF (31/38 Pages) List of Unclassifed Manufacturers – GPS Positioning Module

Before you install your ISR, temporarily mask out the IRQ you will be using. This
prevents the IRQ from requesting an interrupt while you are installing and initializing
your ISR. To mask the IRQ, read the current IMR at I/O port 21h, and set the bit that
corresponds to the IRQ. The IMR is arranged so that bit 0 is for IRQ0 and bit 7 is for
IRQ7. See the paragraph entitled Interrupt Mask Register (IMR) earlier in this
discussion for help in determining your IRQ's bit. After setting the bit, write the new
value to I/O port 21h.
With the startup IMR saved and the interrupts temporarily disabled, you can assign the
interrupt vector to point to your ISR. Again you can overwrite the appropriate entry in
the vector table with a direct memory write, but this is not recommended. Instead use
the DOS function 25h (Set Interrupt Vector) or, if your compiler provides it, the library
routine for setting up interrupt vectors. Remember that interrupt vector 8 corresponds
to IRQ0, vector 9 for IRQ1 etc.
If you need to program the source of your interrupts, do that next. For example, if you
are using transmitted or received messages as an interrupt source program it to do
that. Finally, clear the mask bit for your IRQ in the IMR. This will enable your IRQ.
Common Interrupt mistakes
Remember hardware interrupts are from 8-15, XT IRQ's are numbered 0-7. Do not
forget to clear the IRQ mask bit in the IMR Forgetting to send the EOI command after
ISR code. Disables further interrupts.
GPS140HR
31
RTD Embedded Technologies, Inc.