How would you implement this digital logic in Veri

问题:

I posted an answer to another stackoverflow question which requires some digital logic to be implemented in Verilog or VHDL so that it can be programmed into an FPGA.

How would you implement the following logic diagram in Verilog, VHDL, or any other hardware description language?

The numbered boxes represent bits in a field. Each field has K bits, and the bits for current and mask will be provided by a computer system (using a latched register or equivalent). The bits in next will be read back into that same computer system.

alt text http://img145.imageshack.us/img145/5125/bitshifterlogicdiagramkn7.jpg

See also: this stackoverflow question

回答1:

Something like this?

module scheduler
 #( parameter K = 10 )
  (
   input wire [K:1] current,
   input wire [K:1] mask,
   output reg [K:1] next
   );

   reg [K:1] a;
   reg [K:1] b;

   //'[i+1]' busses that wrap.
   // eg, for a 4-bit bus...
   // a[i]:        a[4],a[3],a[2],a[1] (obviously...)
   // a_wrap[i]:   a[1],a[4],a[3],a[2] 
   wire [K:1] mask_wrap    = { mask[1],mask[K:2] };
   wire [K:1] a_wrap       = { a[1], a[K:2] };
   wire [K:1] current_wrap = { current[1], current[K:2] };

   integer i;
   always @( * ) begin
      for( i=1; i<=K; i=i+1 ) begin
         a[i] = ~current_wrap[i] && b[i];
         b[i] = a_wrap[i] || mask_wrap[i];
         next[i] = ~a[i] && mask_wrap[i];
      end
   end


endmodule

(Disclaimer: linted but not simulated)