this ripple carry adder being used, however, when sub = 1 = 4 b = 3, program returning no overflow , sum 1100 instead of 0110. have messed carry behavior?
module fulladder(a, b, cin, sum, cout); input a, b, cin; output sum, cout; assign sum = a^b^cin; assign cout = (cin&a) | (cin&b) |(a&b); endmodule module rca4bit(a, b, c0, sum, c1, overflow); input [3:0] a; input [3:0] b; output [3:0] sum; input c0; output c1; output overflow; wire [2:0] carry; fulladder rca1(a[0], b[0], c0, sum[0], carry[0]); fulladder rca2(a[1], b[1], carry[0], sum[1], carry[1]); fulladder rca3(a[2], b[2], carry[1], sum[2], carry[2]); fulladder rca4(a[3], b[3], carry[2], sum[3], c1); assign overflow = c1 ^ carry[2]; endmodule module rca4bit2cmp(a, b, sub, sum, c1, overflow); input [3:0] a; input [3:0] b; output [3:0] sum; input sub; output c1; output overflow; wire [3:0]invb; assign invb = sub?~b:b; rcl4bit rc4(a, invb, sub, sum, c1, overflow); endmodule
your overflow term doesn't make sense me, think overflow carry out, don't know why you've xor'd carry[2].
other that don't see wrong, don't see how giving 1100 output, , don't see why expect answer 0110 (6).
if 0100, , b 0011, invb 1100, plus cin means b you've got:
0100 (a) 1100 (invb) + 0001 (cin) _________ 10001 overflow 1, , result 0001 (4-3 = 1).
have tried inspecting via waveform see results diverge this?
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