Hi Andy,

please can you explain, what the difference between a software interrupt and simple calling a subroutine.

I see no difference, except a call was more flexible, since it can be directed anywhere in the program space, not only to a defined vector.

But you can use any unused hardware interrupt. You must only set the appropriate interrupt pending flag.
E.g.:

setb ri ;using UART interrupt (0023h)
jb ri $ ;wait until interrupt serviced

org 0023h
clr ri
;... handle software interrupt
reti


Peter