The size of the ROM required to build an 8-bit adder/subtractor with mode…
2014
The size of the ROM required to build an 8-bit adder/subtractor with mode control, carry input, carry output and two’s complement overflow output is given as
- A.
2^16 × 8
- B.
2^18 × 10
- C.
2^16 × 10
- D.
2^18 × 8
Attempted by 195 students.
Show answer & explanation
Correct answer: B
Key insight: the ROM must provide an output for every combination of input signals (address lines) and store all result and status bits (data lines).
Inputs (address lines): two 8-bit operands = 16 bits, plus mode control = 1 bit, plus carry-in = 1 bit; total = 18 address bits.
Number of addresses: 2^18 possible input combinations → 2^18 rows in the ROM.
Outputs (data bits): 8 sum/difference bits + carry-out (1) + two's-complement overflow (1) = 10 data bits.
ROM size: 2^18 × 10 bits.
Therefore the required ROM size is 2^18 × 10 bits.