How to add two 16bit numbers using a 32bit adder (Its a FA)?
32bit adder takes 1 cycle to add two numbers.
how to add "p+q" and "r+s" (16bit numbers) in 1 cycle?
Note: 32bit adder is black box, cannot modify anything inside. But you can add combo logic outside.
Electronic – 32b Adder to add two 16bit adders
digital-logic
Best Answer
I am adding one more answer since I have been thinking a bit more about it. It is by far more optimal than my previous solution.
We have 32-bit operands A and B, on which we will perform 32-bit addition (as per the OP restrictions go). The lower 16 bits are Alow and Blow. The higher ones are Ahigh and Bhigh.
Before driving A and B operands to the 32-bit adder, set MSBs (i.e., 16th bits) of Alow and Blow to
0
. This way we prevent overflowing to the 17th bit, which is the LSB of the high operands. After doing that, the high addition is performed correctly. The low part needs a very little correction described below.The MSB of the low result is defined as XOR between MSBs of the low adder result, Alow, and Blow. That performs a single-bit addition with no carry-out signal driving. Other result bits are directly connected to the adder output.
See the example below applied on an 8-bit adder, where two independent 4-bit adds are performed. Your 32-bit addition is analogical.
simulate this circuit – Schematic created using CircuitLab