Circuit Cellar Issue 190 Altenburg Listing 1

/* Timer A3 controls the scheduler */
void far vTimerA3ISR(void);                 
#pragma INTERRUPT vTimerA3ISR
void far vTimerA3ISR(void){
#pragma ASM          	
		   ; /* check if we had interrupted the monitor */
	MOV.B 	17[SP], R0H       	; /* get PC(H) */
	AND.B 	R0H, 0FH         	; /* mask flags out of PC(H) */
	MOV.B 	15[SP], R0L       	; /* get PC(M) */
	CMP.W  	#0fc0H, R0        	; /* monitor interrupted ? */
	JLTU   	?+
	POPM   	R0,R1,R2,R3,A0,A1,FB
	REIT                    	; /* ==>> do NOT switch task) */
	NOP
?:
	STC   	SP, _uexc_current_sp  ; /* save stackpointer */
	JSR.a  	_UEXC_CheckTask      	; /* Scheduler */
	POPM  	R0,R1,R2,R3,A0,A1,FB
	REIT   		; /* ==>> exit */
	NOP
#pragma ENDASM
}

Listing 1—Debugging the µexec under the control of the ROM monitor can be difficult. To avoid scheduling into the ROM monitor, the scheduler checks the memory area before changing task. If the ROM monitor is active, scheduling stops.