1) REXX also treats numbers as numbers, not differenciating between integer and FP.

2) Once your integer grow beyond 53-bits, you will start to loose precision in the conversion to FP.

print $_, ': ', 2**$_ == 2**$_ -1 for 33 .. 64;
33 :
34 :
35 :
36 :
37 :
38 :
39 :
40 :
41 :
42 :
43 :
44 :
45 :
46 :
47 :
48 :
49 :
50 :
51 :
52 :
53 :
54 :  1
55 :  1
56 :  1
57 :  1
58 :  1
59 :  1
60 :  1
61 :  1
62 :  1
63 :  1
64 :  1
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On a 64-bit OS, this would not be the case as there would be no need to premote the number to a floating point. Equally, any OS/CRT/FPP that used 80-bit IEEE floating point instead of 64-bit would get a different set of results again--hence non-portable.

3) Depends...