Echelon Neuron - Manual (Page 136)

The value of TOS after the first subtraction is 1 because 3-2 = 1. In this case, the
value of the Carry flag is cleared because the unsigned subtraction did not
require an extra carry bit.
The value of TOS after the second subtraction is 255 (h’FF) because 2-3 = -1. In
this case, the value of the Carry flag is set because NEXT was greater than TOS
and the unsigned subtraction required an extra carry bit.
126 Neuron Assembly Language Instruction Statements