[Bio-bwa-help] bwa index -a is: Details on database 2GB limit?

Hi.  At http://bio-bwa.sourceforge.net/bwa.shtml one can read for 'bwa
index -a is' that:

"IS linear-time algorithm for constructing suffix array. It requires
5.37N memory where N is the size of the database. IS is moderately
fast, but does not work with database larger than 2GB. IS is the
default algorithm due to its simplicity. The current codes for IS
algorithm are reimplemented by Yuta Mori."

I have few questions I hope someone could help me with:

Q. If one still run it "with [a] database larger than 2GB", what
happens?  What is the error message and when/how soon does it appear?

Q. I'd like to be able to test for this before launching 'bwa index -a
is' (trying to write a "bullet-proof" wrapper used by others).
Exactly what does "database larger than 2GB" refer to?  Is "database"
referring to the FASTA file, the generated BWA index, or the genome
sequence?  Also, does "GB" refer to gigabases (=giganucleotides) or
gigabytes (of a file)?

Thank you,

Henrik
(bwa 0.7.5a-r405)