Why do you need to sync the serial and the external interrupts? From what you've described, there is only one interrupt source - INT0. When INT0 gets fired, you need to read the state of the input port pin and shift this into a register. Once you've got 8bits into this register, copy the register to external memory.

So, you need to keep a count of the number of interrupts you've received so you can count off 8bits.


The serial uart is another matter - don't get confused.

This sounds like homework, so don't expect us to give you the whole solution. We can assist you though.

If you're using assembler -read up on the shift and rotate instructions.