Two's Complement addition
1100 in four bits is -4

extending this to 6 bits, do sign extension to the left, adding 1's
because the left bit above is 1, and get 111100

this is still -4 in two's complement

0101 is 5 in decimal

extending it to the left, adding 0's, because the left bit is 0, 
and get 000101

adding these two together:

 111100
 000101
 ======
1000001


the carry bit (first one on the left) is discarded.

The result then is just 1, which matches adding in decimal, -4 + 5 = 1.