Return from Interupt is different on all ECU's. When the ECU gets an interupt, it drops everything on the stack, and then goes off and performs your interupt routine (I can think of a few in the EEC - Smartlock, Auto Trans Control, Ignition Timing, Fueling etc)

RETI is different from Ret in that it automatically picks up the registers etc from the stack, and continues on processing from where the main code was "at" (as long as youve been good with your stack processing).

RET just returns from your routine, without correcting the registers etc.

Now... thats just normal ECU theory. I have no practical skills on EEC at the moment.

LOL - old 6510 hacker