> Can anybody give me a tip, how I can transform hexadecimal to decimal.
> (the inverse is easy. There is the command hex$(x).)

I presume that you mean converting hexadecimal strings
into numbers? The simplest method is to use:

   hex& = eval("&" + x$)

If you want to be able to deal with invalid hexadecimal
numbers and know which digit was at fault then you may
wish to use something like the following:

proc hex&:(arg$)
   local result&
   local tmp$(1), number$(255), digit$(16)

   : rem Local copy of the argument
   number$ = upper$(arg$)

   : rem Store the hex digits in order
   digit$ = "0123456789ABCDEF"

   : rem Start at zero.
   result& = 0

   : rem Work from left to right.
   while (number$ <> "")
      : rem Extract the leftmost hex digit. In C or
      : rem assembler we would index into the string
      : rem directly like an array.
      tmp$    = left$(number$, 1)
      number$ = right$(number$, len(number$) - 1)

      : rem Shift the result left by one hex place.
      : rem In C/assembler this would be a left shift
      : rem by four bits
      result& = result&*16

      : rem Add in the newly extracted digit. Note
      : rem that LOC() returns a position 1..16 for
      : rem the digits 0..F so we subtract 1. Also
      : rem note that invalid digits produce strange
      : rem results since we treat them as -1. We
      : rem could assume ASCII and convert the value
      : rem of tmp$ into 0..15 directly.
      : rem If we were doing this in C/assembler we
      : rem would use bitwise OR instead.
      result& = result& + (loc(digit$, tmp$) - 1)
   endwh

   return result&
endp