Wiki Combinational logic Priority decode

Priority decode

if/else chains vs unique/priority case, and how synthesis picks a winner.

When more than one select can be true, hardware must pick a winner. if / else if is a priority mux. unique case says the labels are mutually exclusive.

flowchart LR
  r3[req3] --> pri[priority_mux]
  r2[req2] --> pri
  r1[req1] --> pri
  r0[req0] --> pri
  pri --> g[grant]

Priority chain

always_comb begin
  grant = 2'd0;
  if (req[3])      grant = 2'd3;
  else if (req[2]) grant = 2'd2;
  else if (req[1]) grant = 2'd1;
  else if (req[0]) grant = 2'd0;
end

Earlier branches win. That is correct for interrupt / arbiter style decode.

Wrong vs right

Wrong — overlapping case labels with no unique/priority and no default (latch + X in sim):

always_comb
  case (1'b1)
    req[1]: grant = 2'd1;
    req[0]: grant = 2'd0;
  endcase

Right — state the intent:

always_comb begin
  grant = 2'd0;
  unique case (1'b1)
    req[1]: grant = 2'd1;
    req[0]: grant = 2'd0;
    default: grant = 2'd0;
  endcase
end

Use priority case when overlaps are legal and you want the first match. Use unique case when two hits are a bug.

Priority chain

Wrong vs right

See also