Legacy IO

CRT and KBD (Monitor and Keyboard) are the simplest way to input and display numbers.
OUT R1, =CRT This will send the value of R1 to CRT
IN R1, =KBD This will take input and store it in R1

KBD is a read only device. CRT is a write only device. Writing to KBD or reading from CRT will result in error.

Picture shows example use. The program has output value 4 to the CRT, and after that it has requested input from KBD. When reading, the machine has to stop until the user presses the send button.