"If not specified, the value of Stream_Size for an elementary type should be the number of bits that corresponds to the minimum number of stream elements required by the first subtype of the type, rounded up to the nearest factor or multiple of the word size that is also a multiple of the stream element size."
Followed, except that the number of stream elements is 1, 2, 3, 4 or 8. The Stream_Size may be used to override the default choice.
The default implementation is based on direct binary representations and is
therefore target- and endianness-dependent. To address this issue, GNAT also
supplies an alternate implementation of the stream attributes Read and
Write, which uses the target-independent XDR standard representation for
scalar types. This XDR alternative can be enabled via the binder switch -xdr.