Siemens MC55 User Manual download pdf (Page 28)

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arranged so that bit 0 is for IRQ0 and bit 7 is for IRQ7. See the paragraph entitled Interrupt Mask

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.

Example on Interrupt vector table setup in C-code:

void far _interrupt new_IRQ1_handler(void ); /* ISR function */

#define IRQ1_VECTOR 3 /* Name for IRQ */

void (interrupt far *old_IRQ1_dispatcher)

(es,ds,di,si,bp,sp,bx,dx,cx,ax,ip,cs,flags); /* Variable to store old IRQ_Vector */

void far _interrupt new_IRQ1_handler(void );

/*----------------------------------------------------------------------

| Function: init_irq_handlers

| Purpose: Set the pointers in the interrupt table to point to

| our functions i.e. setup for ISR's.

|----------------------------------------------------------------------*/

void init_irq_handlers(void)

{

_disable();

old_IRQ1_handler = _dos_getvect(IRQ1_VECTOR + 8);

_dos_setvect(IRQ1_VECTOR + 8, new_IRQ1_handler);

Gi_old_mask = inp(0x21);

outp(0x21,Gi_old_mask & ~(1 << IRQ1_VECTOR));

_enable();

}

|/*----------------------------------------------------------------------

| Function: restore, do this before exiting program

| Purpose: Restore the interrupt vector table.

|----------------------------------------------------------------------*/

void restore(void)

{

/* Restore the old vectors */

_disable();

_dos_setvect(IRQ1_VECTOR + 8, old_IRQ1_handler);

outp(0x21,Gi_old_mask);

_enable();

}

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Siemens MC55 User Manual Page 28

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