Pascal/Turbo Pascal - Ayuda! Necesito los codigo echo y una explicacion facil de enteder

{Espero esto te allude}

program algunos_ejercicios;

  uses

     crt;

   const

    disesm : array[1..7] of char = ('L','M','X','J','V','S','D');

    mesesbi : array[1..12] of word = (31, 29, 31, 30, 31, 30, 31, 31, 30,

                                                       31, 30, 31);

    meses : array[1..12] of word = (31, 28, 31, 30, 31, 30, 31, 31, 30,

                                                       31, 30, 31);

   var

     cadena : string;

     diass, mm, yy, x, y : integer;

     letra : char;

     arrayd : array[1..12] of integer;

     i, p, di : integer;

     ayos, messe : word;

   procedure inversa(c : string);

   var

     d : integer;

     begin

        for d := length(c) downto 1 do

        write(c[d]);

        readkey;

     end;

   function mueve(l : char) : boolean;

   begin

      if upcase(l) in['A','B','I','D'] then

      begin

      gotoxy(x,y);write(' ');

      end;

      case l of

   'a','A' : begin

                y := y - 1;

                if y < 1 then

                y := 1;

                mueve := true;

                end;

   'b','B' : begin

                y := y + 1;

                if y > 24 then

                y := 24;

                mueve := true;

                end;

   'i','I' : begin

                x := x - 1;

                if x < 1 then

                x := 1;

                mueve := true;

                end;

   'd','D' : begin

                x := x + 1;

                if x > 79 then

                x := 79;

                mueve := true;

                end;

   'f','F' : mueve := false;

     end;

     if upcase(l) in['A','B','I','D'] then

     begin

     gotoxy(x,y);write(letra);

     end;

   end;

   procedure caracter;

   var

      tec : char;

   begin

      clrscr;

      write('   Entre Letra : ');

      repeat

         letra := upcase(readkey);

      until letra in['A'..'Z'];

      x := 80 div 2;

      y := 25 div 2;

      clrscr;

      gotoxy(12,25);write('Mueven [A=Arriba B=Ab ajo I=Izquierda D=Derecha F=Fin ]');

      gotoxy(x,y);write(letra);

    repeat

        tec := readkey;

    until mueve(tec) = false;

   end;

   procedure espalindromo;

    var

      temp, cad : string[20];

      z, i : integer;

    begin

       write('  Entre Cadena : ');

       readln(cad);

       z := 1;

       for i := length(cad) downto 1 do

       begin

          temp[z] := cad[i];

          temp[0] := chr(z);

          z := z + 1;

       end;

         writeln;

         if temp = cad then

         writeln('  Es Palindromo')

      else

         writeln('  No Es Palindromo');

         writeln;

         writeln('    Pulse Una Tecla');

         readkey;

    end;

   function bisiesto(ano3 : word) : boolean;

  var

     bb, bx, by : real;

  begin

     bx := frac(ano3/4);

     by := frac(ano3/100);

     bb := frac(ano3/400);

     if ((bx = 0) and (by > 0)) or ((bx = 0) and (bb = 0)) then

     bisiesto := true

   else

     bisiesto := false;

  end;

  function diasemana(an, ms, di : word) : char;

  var

     bis, sig, any, mes, dia : real;

     pos : integer;

     le : char;

  begin

     case an of

  1600..1699 : sig := 7;

  1700..1799 : sig := 5;

  1800..1899 : sig := 3;

  1900..1999 : sig := 1;

  2000..2099 : sig := 0;

  2100..2199 : sig := -2;

  2200..2299 : sig := -4;

      end;

   any := (round(an mod 100)) div 4;

   any := any + (round(an mod 100));

   if (bisiesto(an) = true) and (ms in[1,2]) then

   bis := 1

 else

   bis := 0;

   case ms of

 1,10 : mes := 6;

 2,3,11 : mes := 2;

 4,7 : mes := 5;

 5 : mes := 0;

 6 : mes := 3;

 8 : mes := 1;

 9,12 : mes := 4;

   end;

 dia := di;

  pos := round((bis + sig + any + mes + dia));

  pos := pos mod 7;

  case pos of

  1 : le := 'L';

  2 : le := 'M';

  3 : le := 'E';

  4 : le := 'J';

  5 : le := 'V';

  6 : le := 'S';

  0 : le := 'D';

  end;

    diasemana := le;

  end;

  function dia_del_ano(ano, mes, dia : word) : integer;

  var

    yj, dt, da, cot : integer;

    sali : boolean;

  begin

      dia_del_ano := 0;

      if bisiesto(ano) = true then

      begin

          diass := 366;

          for dt := 1 to 12 do

          arrayd[dt] := mesesbi[dt];

       end

     else

         begin

            diass := 365;

            for dt := 1 to 12 do

            arrayd[dt] := meses[dt];

          end;

     sali := false;

     da := 1;

     cot := 0;

     dt := 1;

     yj := 0;

   repeat

        if dt > arrayd[da] then

        begin

            cot := cot + arrayd[da];

            dt := 1;

            inc(da);

        end;

     if (da = mes) and (dt = dia) then

     begin

         sali := true;

         dia_del_ano := cot + dt;

         yj := cot + dt;

     end;

     inc(dt);

  until (da > mes) or (sali = true);

  end;

  procedure presenta_calendario(ayo, mess : word);

  var

    tt, y, x : integer;

    se : char;

  begin

    clrscr;

    if bisiesto(ayo) = true then

    tt := mesesbi[mess]

  else

    tt := meses[mess];

    y := 3;

    x := 10;

    gotoxy(x,y);write(' A',chr(164),'O ',ayo,'   Mes ',mess);

    y := y + 2;

    gotoxy(x,y);write('L   M   M   J   V   S   D');

    y := y + 1;

    for i := 1 to tt do

    begin

    se := diasemana(ayo, mess, i);

    case  se of

  'L' : x := 10;

  'M' : x := 14;

  'E' : x := 18;

  'J' : x := 22;

  'V' : x := 26;

  'S' : x := 30;

  'D' : x := 34;

    end;

    gotoxy(x,y);write(i);

    if se = 'D' then

    y := y + 2;

    end;

  end;

  procedure menu;

  var

    tec : char;

    sal : boolean;

  begin

     sal := false;

   repeat

     clrscr;

     writeln('   Menu General');

     writeln;

     writeln('    1 = Parametro inverso');

     writeln('    2 = Desplazar caracter');

     writeln('    3 = Cadena es palindromo');

     writeln('    4 = Visualice mes');

     writeln('    5 = Salir');

     writeln;

     writeln('   Elija Opcion');

     repeat

        tec := readkey;

     until tec in['1','2','3','4','5'];

     clrscr;

    case tec of

  '1' : begin

           write('  Entre cadena : ');

           readln(cadena);

           inversa(cadena);

        end;

  '2' : caracter;

  '3' : espalindromo;

  '4' : begin

          write('   Entre A¥O : ');

          readln(ayos);

          write('   Entre MES : ');

          readln(messe);

          presenta_calendario(ayos, messe);

          readkey;

        end;

  '5' : sal := true;

    end;

   until sal = true;

  end;

   begin

      menu;

   end.