It's pretty clever. One shift register held a bit to indicate the current cursor position. To write a character, the circuit waited until the cursor bit appeare

It's pretty clever. One shift register held a bit to indicate the current cursor position. To write a character, the circuit waited until the cursor bit appeared and then updated the character shift registers. At the same time, the cursor bit was delayed to advance it to the next character. Note that you could only write characters sequentially; you couldn't write to arbitrary locations on the screen.