Description:	Compute the 2s-complement negative of a 4-bit value.
Input:	A 4-bit value from the front panel.
Output:	The negated value is displayed on the output lamps.
Written by:	bhilpert (2006 Sep)
Assembled:	Ref Tape Assembly --- -------- -------------------------------------------------- 000: 12.10.00 SR1=* / wait for value 001: 03.00.01 SR2=^ / save high bits as well 002: 08.00.02 CR1=SR1 / negate low bits 003: 1B.01.00 NEG 004: 0D.00.0C OR1=CR5 / and display 005: 08.00.03 CR1=SR2 / negate high bits 006: 1B.09.00 NGC 007: 0E.00.0C OR2=CR5 008: 18.00.00 CR1=0 / display 5th bit of result 009: 19.00.00 CR2=0 / 010: 1B.08.00 ADC / add 0+0+carry 011: 0F.00.0C OR3=CR5 /