Hi,

I have written code for interrupt driven data capture in Keil (not serial communications). Since the timing requirements for the serial input data stream are very stringent, I am using #pragma asm i.e. inline assembly in the external interrupt 0 ISR. Rest of the code is in C.

On the most fundamental level, I need to sample a port pin on external interrupt 0 very quickly (~15-20us later).

Now my inline assebly code in ISR modifies & uses only two registers: ACC & R0. But the compiler is saving all the registers on entering the ISR & I am missing a major event between this:

CSEG	AT	00003H
AJMP	ext_int_0
RSEG  ?PR?ext_int_0?PS2_INTERR_C_ASM
	USING	0
ext_int_0:
	PUSH 	ACC
	PUSH 	B
	PUSH 	DPH
	PUSH 	DPL
	PUSH 	PSW
	MOV  	PSW,#00H
	PUSH 	AR0
	PUSH 	AR1    ;dont need this onwards
	PUSH 	AR2
	PUSH 	AR3
	PUSH 	AR4
	PUSH 	AR5
	PUSH 	AR6
	PUSH 	AR7    ;~30us wasted after ISR invocation
	USING	0

 

equal time is wasted POPing the unused registers before RETI.

What should I do in order to control which registers are PUSHed & POPed in ISR? Is there any directive I could use?