The 1802 CPU now supports Interrupt handling.
Added support for reading the serial RX, it also sets the interrupt flag when it's FIFO has data. You can use INP N1 to get bytes from the FIFO and check if it has data with BN2.
I also have the write button set the interrupt flag, and using BN4 you can check the state of it.
Here's a picture of the output from a terminal, of a program I made to test the functionality.
When you press the Write button it send you the current byte toggled into the switches in hex. "Button: 00"
And when you type on the terminal it stores the byte in a string buffer and when you hit enter it sends that buffer back to you. "Got: testing"