///////////////////////////////////////////////////// Module acs4.v // Hierarchy: decoder_core.v// Module function: //  Module acs4.v calculates the surviving metrics for all four states. The//  inputs are path metrics from previous time slot. New path metrics are//  calculated for next time slot////////////////////////////////////////////////////////////////////////// ////////////////////////// Definitions //////////////////////////////////////`define initial_metric00   4'b0000//`define initial_metric01   4'b1010 //`define initial_metric10   4'b1010//`define initial_metric11   4'b1010// After two trellis stages all the unwanted initial paths (the ones// which do not start from state 00) die out. We have chosen initial// path metric of 10 for the unwanted initial states (01, 10, 11) so that // the maximum possible path metric of these states after two stages of// trellis (10+2+2) does not end up being cut by modulo operation. The// value of 2 represents the maximum possible value of branch metric for// any transition. ///////////////////////////////////////////////////////////////////////////////module  acs4(y2_s1,y1_s1,sel_initial_s1,decisions_s1,path_metrics_s1,phi1,phi2);///////////////////////////////////////////////////////////////////////////////input           sel_initial_s1,                y2_s1,                y1_s1;input           phi1,                phi2;output  [3:0]   decisions_s1;output  [15:0]  path_metrics_s1;// Path metrics vector for testing purposes  wire 	[1:0]	b00_s1, // Hamming distance of input and  00		b01_s1, // Hamming distance of input and  01 		b10_s1, // Hamming distance of input and  10		b11_s1; // Hamming distance of input and  11 wire		d00_s1, // decision bit from ACS00		d01_s1, // decision bit from ACS01		d10_s1, // decision bit from ACS10		d11_s1; // decision bit from ACS11wire   [3:0]    path_metric_out00_s1,                path_metric_out01_s1,		path_metric_out10_s1,		path_metric_out11_s1;/////////////////////////////////////////////////////////////////////////////////////////////////////// Branch metric generation /////////////////////////////assign b00_s1 = y2_s1 + y1_s1;assign b01_s1 = y2_s1 + !y1_s1;assign b10_s1 = !y2_s1 + y1_s1;assign b11_s1 = !y2_s1 + !y1_s1;////////////////////////////////////////////////////////////////////////////////////////////// ACS module for state 00 ///////////////////////////////////////acs1   acs00(.phi1(phi1), .phi2(phi2),             .path_metric0_s1(path_metric_out00_s1),             .path_metric1_s1(path_metric_out01_s1),             .branch_metric0_s1(b00_s1),	     .branch_metric1_s1(b11_s1),             .path_metric_out_s1(path_metric_out00_s1),	     .decision_s1(d00_s1),             .sel_initial_s1(sel_initial_s1),              .initial_metric_s1(4'b0000));          //   .initial_metric_s1(`initial_metric00));///////////////////////////////////////////////////////////////////////////////////////////// ACS module for state 10 ///////////////////////////////////////acs1   acs10(.phi1(phi1), .phi2(phi2),             .path_metric0_s1(path_metric_out00_s1),             .path_metric1_s1(path_metric_out01_s1),             .branch_metric0_s1(b11_s1),	     .branch_metric1_s1(b00_s1),             .path_metric_out_s1(path_metric_out10_s1),	     .decision_s1(d10_s1),             .sel_initial_s1(sel_initial_s1),             .initial_metric_s1(4'b1010));            //  .initial_metric_s1(`initial_metric10));///////////////////////////////////////////////////////////////////////////////////////////// ACS module for state 01 ///////////////////////////////////////acs1   acs01(.phi1(phi1), .phi2(phi2),             .path_metric0_s1(path_metric_out10_s1),             .path_metric1_s1(path_metric_out11_s1),             .branch_metric0_s1(b01_s1),	     .branch_metric1_s1(b10_s1),             .path_metric_out_s1(path_metric_out01_s1),	     .decision_s1(d01_s1),             .sel_initial_s1(sel_initial_s1),             .initial_metric_s1(4'b1010));              // .initial_metric_s1(`initial_metric01));///////////////////////////////////////////////////////////////////////////////////////////// ACS module for state 11 ///////////////////////////////////////acs1   acs11(.phi1(phi1), .phi2(phi2),             .path_metric0_s1(path_metric_out10_s1),             .path_metric1_s1(path_metric_out11_s1),             .branch_metric0_s1(b10_s1),	     .branch_metric1_s1(b01_s1),             .path_metric_out_s1(path_metric_out11_s1),	     .decision_s1(d11_s1),             .sel_initial_s1(sel_initial_s1),             .initial_metric_s1(4'b1010));             // .initial_metric_s1(`initial_metric11));////////////////////////////////////////////////////////////////////////////// assign decisions_s1 = {d11_s1,d10_s1,d01_s1,d00_s1};assign path_metrics_s1 = {path_metric_out11_s1,                          path_metric_out10_s1,                          path_metric_out01_s1,                          path_metric_out00_s1};////////////////////////////////////////////////////////////////////////////endmodule