The number on the top changed with every link that went into the machine, 
and it seemed to determine, in a limited way, what the machine would do next; 
for example, she had learned that the number happened to be 09, and if the next 
link in the chain was in the vertical position (which the Duke referred to as a 
one), the drums would spin around and change the number to 23. But if the next 
link was, instead, a zero (as the Duke referred to links with horizontal 
toggles), the number drums would change to 03. But that wasn't all: In this 
case, the machine would, for some reason, reverse the
direction in which the chain was moving through the machine, and also flick the 
toggle from zero to one. That is, the machine could write on the chain as well 
as read from it.