I have some query about the priority of if else in verilog. For example.

  If (a)
  else if c
  else if e

At here, those a,b,c...g are all searched ? How do those have priorities in if~else in verilog?

  • 1
    \$\begingroup\$ What do you mean by 'searched'? There's no precedence, or at least it depends on what b,d,f,g are. \$\endgroup\$ May 19, 2015 at 8:25
  • \$\begingroup\$ @Vladmir Cravero How does it synthesis? \$\endgroup\$
    – gmotree
    May 19, 2015 at 8:38
  • \$\begingroup\$ They're all synthesised, provided a/c/e aren't compile-time constants. \$\endgroup\$
    – pjc50
    May 19, 2015 at 8:46
  • \$\begingroup\$ @ pjc50 I meant as gate. \$\endgroup\$
    – gmotree
    May 19, 2015 at 9:03
  • \$\begingroup\$ Well. I beg to differ. AFAIK (and pls correct me if I'm wrong), if else construct infers a 'Priority routing Network', with the outermost if carrying highest priority. All of it is synthesized using a ladder of multiplexers (2:1 Mux I believe). \$\endgroup\$ May 19, 2015 at 9:46

1 Answer 1


An if-else construct infers a priority routing network. Following example is taken directly from a verilog book :

r = a + b + c ;
else if (m > n )
r = a - b ;
r = c + 1 ;

The circuit is as shown :

enter image description here

As you can see, the outermost if is given a priority in the sense that it is routed first if a match is found, bypassing others.

Ref : Fpga prototyping by verilog

  • 1
    \$\begingroup\$ Thanks . There are priorities in there as cuicuit. First m and n equality check then m and n size check. \$\endgroup\$
    – gmotree
    May 19, 2015 at 10:17
  • \$\begingroup\$ Yes. And finally a default case when none of the above exists. \$\endgroup\$ May 19, 2015 at 10:21
  • \$\begingroup\$ Mind giving a reference to the book? \$\endgroup\$
    – Dzarda
    May 19, 2015 at 10:25
  • \$\begingroup\$ Added the link to the book. \$\endgroup\$ May 19, 2015 at 10:29
  • \$\begingroup\$ Should the synthesis tool recognize the three cases as mutually exclusive and be able to do one parallell mux? \$\endgroup\$
    – Moberg
    Dec 3, 2019 at 22:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.