Thanks for your time, Russell.

Yes it does take a lot of work to update the outputs if one were to use the memory mapped I/O. Reason why I was looking for simply setting or resetting BITs and then write the BYTE.

In fact if I can do what I intented, then I can very simply SET or RESET the bits in the main line code as desired and the BYTE can be written to the output in an interrupt that is fired at the required interval, say 50ms or 100ms. That way the main line code just alters the output image BYTE that gets copied to the output at regular intervals.

Your idea of SET_BIT and RESET_BIT is good.

Raghu