12-bit Unsigned Adder-subtractor

Operands x and y are 12-bit unsigned integers.

Behavior

• mode=0: s receives the low 12 bits of x+y; ovf_unf receives the carry-out.
• mode=1: s receives x-y modulo 4096; ovf_unf is 1 when x<y.
• The result must be purely combinational.
• Extend operands to 13 bits before the operation to preserve carry or borrow information.