Multi-port register file in HDL

[Goran Dakov]

q1: Do place and route tools synthesize multi-port register files with "write-first" read-during-write behaviour or are they restricted to only "undefined" read-during-write?
q2: In a super-scalar cpu would the multi-ported register files be synthesised ok by place-and-route tools or a specialised compiler is usually used?

 

[pprabhu]

That entirely depends on the HDL itself.

Here's a regfile that implies read during write = "old value"; please note that I'm rusty on 2D arrays/memories so this may not be syntactically perfect.

module regfile(  input clk,
                       input reset,

                       input [3:0] read_addr,
                       output [7:0] read_data,

                       input [3:0] write_addr,
                       input write_valid,
                       input [7:0] write_data );

reg [7:0] memory[0:15];

always @(posedge clk)
begin
  if( reset )
  begin
    read_data <= 8'hxx; // don't care
  end
  else
  begin
    if( read_valid )
    begin
      read_data <= memory[read_addr];
    end

    if( write_valid )
    begin
      memory[write_addr] <= write_data;
    end
  end
end

endmodule

And here's read during write behavior being "new value"; the only change is an additional conditional in the if( read_valid ) block:

---

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always @(posedge clk)
begin
  if( reset )
  begin
    read_data <= 8'hxx; // don't care
  end
  else
  begin
    if( read_valid )
    begin
      if( write_valid )
      begin
        read_data <= write_data; // intercept new data for RDW behavior "new value"
      end
      else
        read_data <= memory[read_addr];
      end
    end
 
    if( write_valid )
    begin
      memory[write_addr] <= write_data;
    end
  end
end
 
endmodule

---

Pretty sure that the number of ports that the regfile has makes a difference with respect to RDW behavior.

q2: I've synthesized multiported regfiles all the time in ISE/VIVADO & QUARTUS so I don't think a "specialized" compiler is needed.