Relationship between the signal level get from iwconfig and /proc/net/wireless?

[christyyim]

I now currently working on a project to perform network switching according to the received signal strength. From the manual of iwconfig, the explanation for signal level is

Received signal strength (RSSI - how strong the received signal is). May be arbitrary units or dBm, iwconfig uses driver meta information to interpret the raw value given by /proc/net/wireless and display the proper unit or maximum value (using 8 bit arithmetic). In Ad-Hoc mode, this may be undefined and you should use iwspy.

but how they interpret? For example, when the signal strength value(=a)i get from /proc/net/wireless is 202, the signal level(=b) in iwconfig is -54dbm. When i look into the source code of iwcommon.c, i found out that the logic behind is

b = a - 0x100 (= a - 16^2)

here is the part of code in iwcommon.c:

 /********************** STATISTICS SUBROUTINES **********************/
 
/*------------------------------------------------------------------*/
/*
 * Output the link statistics, taking care of formating
 */
void
print_stats(FILE *    stream,
        iwqual *    qual,
        iwrange *    range,
        int        has_range)
{
  /* Just do it */
  if(has_range && (qual->level != 0))
    {
      /* If the statistics are in dBm */
      if(qual->level > range->max_qual.level)
    {
      /* Statistics are in dBm (absolute power measurement) */
      fprintf(stream,
          "Quality:%d/%d  Signal level:%d dBm  Noise level:%d dBm%s\n",
          qual->qual, range->max_qual.qual,
          qual->level - 0x100, qual->noise - 0x100,
          (qual->updated & 0x7) ? " (updated)" : "");
    }
      else
    {
      /* Statistics are relative values (0 -> max) */
      fprintf(stream,
          "Quality:%d/%d  Signal level:%d/%d  Noise level:%d/%d%s\n",
          qual->qual, range->max_qual.qual,
          qual->level, range->max_qual.level,
          qual->noise, range->max_qual.noise,
          (qual->updated & 0x7) ? " (updated)" : "");
    }
    }
  else
    {
      /* We can't read the range, so we don't know... */
      fprintf(stream, "Quality:%d  Signal level:%d  Noise level:%d%s\n",
          qual->qual, qual->level, qual->noise,
          (qual->updated & 0x7) ? " (updated)" : "");
    }
} 
  

can anyone please explain to me the logic behind these code? I cannot figure out how they come out with this conversion. Besides that, where the raw value given in /proc/net/wireless come from?

Thanks a lot!!!