51. Opcode Decoder

Syntax:

if (expression)
   statement;
else if (expression)
   statement;
else
   statement;

Key Rules

• expression must evaluate to a 1-bit result (0, 1, x, z).
• Execution is first true branch wins (checked top to bottom).
• If none match and no else, no action occurs.

Common Uses

• Combinational decision making inside always @(*).
• Priority logic (highest branch gets priority).

Guidelines

• Always use full coverage (else or default) to avoid unintended latch inference.
• Use begin…end for multiple statements.
• Use case for parallel decisions; if-else is better for priority logic.

Example

always @(*) begin
  if (sel == 2'b00)     y = a;
  else if (sel == 2'b01) y = b;
  else if (sel == 2'b10) y = c;
  else                   y = d; // avoids latch
end

General Syntax:

case (expression)
   value1: statement;
   value2: statement;
   ...
   default: statement;
endcase

Case

• Exact match (bit-by-bit, including x/z).
• Good for one-hot encoding or multi-way decode.

Casez

• Treats Z and ? as don’t-cares.
• Useful for partial matches and encoders.

casez (opcode)
   4'b1???: alu_op = ADD;
   4'b01??: alu_op = SUB;
   default: alu_op = NOP;
endcase

Casex

• Treats X, Z, and ? as don’t-cares.
• Risky → can mask real unknowns in simulation.
• Synthesis tools may treat it differently; use sparingly.

Guidelines

• Always include default to prevent latches.
• casez is preferred over casex in synthesizable RTL.
• Case statements are parallel (not priority) unless overlapping values are used.

Syntax:

for (init; condition; step) statement;

Rules

• All loop control variables must be declared (integer i;).
• Loop unrolls during elaboration if bounds are static.
• Synthesizable only with constant bounds.

Example

integer i;
always @(*) begin
  sum = 0;
  for (i=0; i<8; i=i+1) begin
    sum = sum + data[i];
  end
end

Guidelines

• Use for loops to describe repeated hardware structures (not like software loops).
• Avoid unbounded or runtime-dependent loops in RTL → not synthesizable.
• For simulation-only, forever and while are also allowed.