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GHDL c interfaces and extensions: example of VHDL std_logic_vector to c code string.

1. This page shows a simple example, which uses c code to enhance a VHDL test bench. This page shows some other VHDL to c code interface.


2. First is the VHDL bench passes a vector to the c code:
   veco := g_vec( g_v2s(vec) );
   
   Some functions to convert the std_logic_vector to string are required:
   
   1. package body pkg_std_logic_vec is
   2.     function g_v2s(v: std_logic_vector) return string is
   3.       variable result : string (1 to v'length);
   4.       variable r : integer;
   5.     begin
   6.       r := 1;
   7.       for i in v'range loop
   8.         if( v(i) = '1') then result(r) := '1';
   9.         else                 result(r) := '0';
   10.         end if;
   11.         r := r + 1;
   12.       end loop;
   13.       return result;
   14.     end g_v2s;
   17.   function g_vec (s: string) return integer is
   18.   begin
   19.     return getenvS2(s & NUL);
   20.   end g_vec;
   21.   function getenvS2(s : string)
   22.     return integer is
   23.   begin
   24.     assert false report "VHPI" severity failure;
   25.   end getenvS2;
   26. end pkg_std_logic_vec;
   
   



3. Part of the c code is listed below.
   
   
   1. struct int_bounds {
   2.   int left;
   3.   int right;
   4.   char dir;
   5.   unsigned int len;
   6. };
   8. struct ghdl_string {
   9.   char *base;
   10.   struct int_bounds *bounds;
   11. };
   14. long int ghdl_envS2(struct ghdl_string *s) {
   15.   long res;
   16.   int ix;
   17.   int pwr;
   18.   char *r = s->base;
   20.   printf("from c start\n");
   21.   printf("str %s\n", s->base);
   22.   printf("left %d\n", (s->bounds)->left);
   23.   printf("right %d\n", (s->bounds)->right);
   24.   printf("dir %d\n", (s->bounds)->dir);
   25.   printf("len %d\n", (s->bounds)->len);
   27.   res=0;
   28.   pwr=1;
   29.   for(ix=(s->bounds)->len - 2; ix >= 0; ix--) {
   30.     if(r[ix] == '1') //no check for U, X, W ...
   31.     res += pwr * ((unsigned char)r[ix] - 48);
   32.   //printf("%c %d %d\n", r[ix], pwr, res);
   33.     pwr *= 2;
   34.   }
   35.   printf("from c end\n");
   37.   return res;
   38. }
   
   


4. The example is tested using the free VHDL simulator GHDL.
   Compilation, elaboration and simulation is done using a makefile.
   It supports, cleanall, all, run.
   




5. A similar example for a memory array also exists. In this case g++ is used and the cpp code is used. CPP makes it easier to write code for memories using vectors.
   
   extern "C" long int mem_arr_44_wr_c(int addr, struct ghdl_string *s) {
     long res;
   
   



6. For more details please send an e-mail and put in the title: Cpp GHDL std_logic_vec.