Wait a minute.

Can you concurrently keep any combination of keys pressed, or exactly how long should the effect of keypress '1' be active?

It is trivial for a super-loop to contain code:

<pre>
for (;;) {
scan_keys();
if (key_1_pressed) {
port2 = port1;
}
if (key_2_pressed) {
port4 = port3;
}
if (key_3_pressed) {
if (!report_x_sent) {
report_x_sent = TRUE;
copy_report_x_to_uart_send_buffer();
}
} else {
report_x_sent = FALSE;
}
if (key_4_pressed) {
...
}
...
if (uart_rx_bytes) {
process_uart_rx_data();
}
...
}