LS74181.txt

module NS74181(A,B,F,S,CN,CO,M);
input[3:0]  A,B;
input[3:0]  S;
input  M,CN;
output  CO;
output[3:0]  F;
reg[3:0]  F;
reg[3:0]  ta,tb;
reg  CO;
always @(S)
begin
ta=~A;
tb=~B;
case(S)
'b0000:
begin
if(M) F=ta;
else
begin
if(CN){CO,F}=A;
else {CO,F}=A+1;
end
end
'b0001:
begin
if(M) F=~(A|B); 
else
begin
if(CN){CO,F}=A|B;
else {CO,F }=(A|B)+1;

end
end
'b0010:
begin
if(M) F=(ta)&B; 
else
begin
if(CN) {CO,F }=A|(tb);
else {CO,F}=A|(tb)+1;
end
end
'b0011:
begin
if(M) F=0;
else
begin
if(CN){CO,F}=-1;
else {CO,F }=0;
end
end
'b0100:
begin
if(M) F=~(A&B);
else
begin
if(CN){CO,F}=A+(A&(tb));
else {CO,F }=A+(A&(tb))+1;
end
end
'b0101: