6/2/20151 This presentation will probably involve audience discussion, which will create action items. Use PowerPoint to keep track of these action items

04/18/23 1

ENCM515 -- Interrupts“Review” and “C”-interrupts

M. R. Smith, Electrical and Computer Engineering

University of Calgary, Alberta, Canadasmithmr @ ucalgary,ca

04/18/23 ENCM515 -- Interrupts Review -- Part 1

Copyright [email protected] 2 / 24

To be tackled today Subroutines and Interrupts Example “C” code (68K)

subroutine assembly code interrupt service routine assembly

Example “C” code (21K) The “C” wrapper interrupts using IRQ1 button interrupts using 21K timer



Subroutines and Interrupts

Very similar in concept Very different in implementation

Subroutines occur as part of your normal program flow. You tackle a certain task, written as a subroutine, at a certain location in your code.

Time of starting plays no part in the design of a subroutine.



WAITING for an event to happen

Sometimes your code must wait for an external event to occur before the code can proceed

float array_in[200], array_out[200];main( ) {

For Ever {Wait till array_in() is filled;Process(array_in,array_out);DoRestOfCode();

} Stupid way to code -- unless this is the only thing the

processor will do



POLLING to check for events

int event1, event2, event3;float array_in[200], array_out[200];main( ) {

For Ever {if (event1 is TRUE) Process(array_in,array_out);if (event2 is TRUE) Proce????if (event3 is TRUE) Proce????DoRestofCodef();

}

Problem -- what if many events? Problem -- what if DoRestofCode() takes a long time? Problem -- what if event2 must be handled THE MOMENT it

occurs?



Solution -- Interrupts

void InterruptServiceRoutine(void);char isr_count; // Semaphore

float array_in[200], array_out[200];void main() { SetUpInterrupts(pointer to InteruptServiceRoutine() ); ActivateInterrupts(Based on event1); isr_count = 0;

while ( isr_count < 8 ) { // Totally un-exitable normally DoRestofCode();

}}



Sub-task as ISR (68K “C”)

void InterruptServiceRoutine(void);extern char isr_count; // Semaphore

#pragma interrupt() -- Signals to 68K compilerthat InterruptServiceRoutine is not a subroutine

void InterruptServiceRoutine(void);{Acknowledge Interrupt(); This is a normal subroutine call isr_count++;Process(array_in, array_out); This is a normal subroutine call

}

HOWEVER -- InterruptServiceRoutine IS NOT a normal subroutine call!!!



Subroutines are not interrupts!

Subroutines occur when the programmer wants them to occur Specific location in program code where

called Specific location in code program where

will return to Can prepare for when they will occur so

can pass parameters to them Rules are -- save non-volatile registers to

stack IF you are going to use them, otherwise use scratch (volatile) registers.



Interrupts are not subroutines

Interrupts occur when the interrupt wants to occur NO Specific location in program code where called NO Specific location in program code where will

return to Can’t prepare for when interrupts will occur so

can’t pass normal parameters to them -- need to use semaphores and messages instead

Interrupts may not want to stop Interrupts may want to use volatile registers

but subroutines are already using them!



Interrupt response must be fast

68K interrupt -- not too fast to get into Finish current instruction (8 cycles) Save next instruction address (12 cycles) Save status register (4 cycles at least) Look in “Vector Table” to find the starting address

of the ISR routine (8 cycles to fetch) Fetch the first instruction in ISR Save registers

Just as slow to get out of the interrupt service routine! Reverse of above



Interrupts on 21K -- designed to be fast

Finish current instruction (1 cycle) (but what about pipeline issues?)

Save next instruction address (hardware) Save status register???? Starting address of the ISR routine at fixed location

(0 cycles) Alternate set of registers available (sometimes) Some interrupts simply steal cycles on the bus

(DMA) and don’t cause “interrupts” as we know them!

Other issues -- Remember 3 PC’s in sequencer?



Rough Code for 68K style main( )

#define SIZE 200float buffer1[SIZE], buffer2[SIZE], buffer3[SIZE];int event1 =1, event2 = 1, event3 = 1;

main( ) { SetUpInterrupts(pointer to ISR() ); ActivateInterrupts(Based on Timer); forever {

while (event1 = = 1) /* Do nothing */ ;Process(buffer1); /* ISR is filling buffer2 at this time */while (event2 = = 1) /* Do nothing */;Process(buffer2);etc;

}



The ISR routine (68K style “C”)

int count = 0;float *whichbuff = buffer1;int *whichevent = &event1;

#pragma interrupt()void ISR(void) {

*whichbuff = ReadD2A();whichbuff++;count++;if (count = = SIZE) {

count = 0;whichbuff = buffer2;*whichevent = 0;etc;

}}



SECTION codeXDEF _ISR

_ISRMOVEM.L D0/D1/A0/A1,-(A7)

_81 BSR _ReadD2A ; *whichbuff = ReadD2A();

MOVE.L _whichbuff,-(A7)BSR __sltos

_82 ADDQ.L #4,_whichbuff ; whichbuff++;_83 ADDQ.L #1,_count ; count++;_84 CMP.L #0xC8,_count ; if (count == SIZE) {

BNE _10_85 CLR.L _count ; count = 0;_86 MOVE.L #_buffer2,_whichbuf; whichbuff = buffer2;_87 MOVE.L _whichevent,A0 ; *whichevent = 0;

CLR.L (A0)_10 MOVEM.L (A7)+,D0/D1/A0/A1

RTE

68K ISR code (defined with #pragma)



The ISR built as subroutine Unintentional Error


// #pragma interrupt() --- Now becomes a subroutinevoid ISR(void) {

*whichbuff = ReadD2A();whichbuff++;count++;if (count = = SIZE) {


}}



SECTION codeXDEF _ISR

_ISR No volatile register saves made since compiler thinks is subroutine_81 BSR _ReadD2A ; *whichbuff = ReadD2A();

MOVE.L _whichbuff,-(A7)BSR __sltos

_82 ADDQ.L #4,_whichbuff ; whichbuff++;_83 ADDQ.L #1,_count ; count++;_84 CMP.L #0xC8,_count ; if (count == SIZE) {

BNE _10_85 CLR.L _count ; count = 0;_86 MOVE.L #_buffer2,_whichbuf; whichbuff = buffer2;_87 MOVE.L _whichevent,A0 ; *whichevent = 0;

CLR.L (A0)No volatile register recovery made since compiler thinks is subroutineRTS --- HOWEVER -- WRONG THINGS ON THE STACK

68K ISR code AS A SUBROUTINE (no #pragma was used) -- BIG PROBLEMS



“C” interrupts running on 21K

We KNOW that exactly the same thing must be happening on 21K as 68K Need to set up interrupt vector equivalent Need to enable interrupts Need to do RTE equivalent

HOWEVER, EVERYTHING IS HIDDEN BY A “C” WRAPPER

Details on 21K interrupts are only needed if we want to go for “REAL SPEED”



The ISR subroutine (21K style)Only works because of hidden “C” wrapper


void ISR(void) {*whichbuff = ReadD2A();whichbuff++;count++;if (count = = SIZE) {


}}



Remember “SOMEWHERE” In the 21K “C” model, there is some TRUE ISR code

that is executed when the interrupt occurs. In that ISR Registers are saved Then your “ISR” is called as a SUBROUTINE Registers are recovered Interrupt is cleared Equivalent to RTE occurs

AND YOU KNOW NOTHING ABOUT IT (and you don’t normally care -- unless speed is important!)

And I think there is a bug in “earlier “C” wrapper -- funny code experiences and also phone messages from Alberta Users



Setting up an interrupt to occur on the 21K IRQ1 flag -- one of the buttons on the board

main( ) { asm(“include <def21060.h>”); asm(“bit set mode2 IRQ1E;”); interrupt(SIG_IRQ1, irq1_handler);

forever {while (event1 = = 1) /* Do nothing -- ISR is filling Buffer 1 */ ;Process(buffer1); /* ISR is filling buffer2 at this time */while (event2 = = 1) /* Do nothing */;Process(buffer2);etc;

}}

void irq1_handler(int sig_num) { /* This is still a subroutine */ISR( );

}



Setting up an interrupt to occur on the 21K internal Timer

main( ) { int data; interruptf(SIG_TMZ, timer_int); /* There’s intterupt, interruptf and interrupts */ data = 20000; /* Every 20000 ticks */ timer_set(data, data); /* That’s the first and reload values */ timer_on();forever {

while (event1 = = 1) /* Do nothing */ ;Process(buffer1); /* ISR is filling buffer2 at this time */while (event2 = = 1) /* Do nothing */;Process(buffer2);etc;

}}void timer_int(int sig_num) { /* This is still a subroutine */

ISR( ); /* Being used to fill a buffer -- perhaps 1 point at a time)}



Setting up the SPORTs interrupt is nasty

Have to set up the CODEC to the right values, sample rate etc

Set up the CODEC to cause interrupts on the SPORT lines

Theninterruptf(SIG_SPTOI, spt0_asserted); /*

transmit */interruptf(SIG_SPROI, spr0_asserted); /*

receive */ And everything works like magic -- Not going to

worry about the details in this lab.



Tackled today Subroutines and Interrupts Example “C” code (68K)

subroutine assembly code interrupt service routine assembly

Example “C” code (21K) The “C” wrapper interrupts using IRQ1 button interrupts using 21K timer

Documents

6/2/20151 This presentation will probably involve audience discussion, which will create action items. Use PowerPoint to keep track of these action items