Behavioural - `Non-Synthesisable' RTL

Not all RTL is officially synthesisable, as defined by language standards. However, commercial tools tend to support larger subsets than officially standardised.

RTL with event control in the body of a thread defines a state machine. This is compilable by some tools.

This state machine requires a program counter (PC) register at runtime (implied):

   input clk, din;
   output reg [3:0] q; // Four bits of state are define here.

   always begin
         q <= 1; 
         @(posedge clk) q <= 2;
         if (din) @(posedge clk) q <= 3;
         q <= 4;
         end

How much additional state in the form of PC bits are needed ? Is conditional event control synthesisable ? Does the output `q' ever take on the value 4 ?

22: (C) 2008-15, DJ Greaves, University of Cambridge, Computer Laboratory.

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