lab-03: clean up twos complement modules

Gate level implementation for the following components:
- RC_ADD_SUB_64
- TWOSCOMP64
- TWOSCOMP32

logic.v

@@ -20,7 +20,7 @@ output [63:0] Y;
 //input list
 input [63:0] A;
 
-// TBD
+RC_ADD_SUB_64 twoscomp64_sub(.Y(Y), .A(64'b0), .B(A), .SnA(1'b1));
 
 endmodule
 
@@ -31,7 +31,7 @@ output [31:0] Y;
 //input list
 input [31:0] A;
 
-// TBD
+RC_ADD_SUB_32 twoscomp32_sub(.Y(Y), .A(0), .B(A), .SnA(1'b1));
 
 endmodule
 
@@ -143,4 +143,4 @@ input [1:0] I;
 
 // TBD
 
-endmodule
+endmodule

rc_add_sub_32.v

@@ -29,7 +29,21 @@ input [63:0] A;
 input [63:0] B;
 input SnA;
 
-// TBD
+// carry-in bits for each 1-bit full adder
+wire C[0:64];
+buf (C[0], SnA);
+
+genvar i;
+generate
+    for (i = 0; i < 64; i = i + 1)
+    begin : add64_gen_loop
+        wire B_xor;
+        xor (B_xor, B[i], SnA);
+        FULL_ADDER add64_inst(Y[i], C[i+1], A[i], B_xor, C[i]);
+    end
+endgenerate
+
+buf (CO, C[64]);
 
 endmodule
 
@@ -50,11 +64,12 @@ genvar i;
 generate
     for (i = 0; i < 32; i = i + 1)
     begin : add32_gen_loop
-        FULL_ADDER add_inst(Y[i], C[i+1], A[i], B[i] ^ SnA, C[i]);
+        wire B_xor;
+        xor (B_xor, B[i], SnA);
+        FULL_ADDER add32_inst(Y[i], C[i+1], A[i], B_xor, C[i]);
     end
 endgenerate
 
-//assign CO = C[32];
 buf (CO, C[32]);
 
 endmodule