Commented by: @AlexPeshkoff

Dmitry, please take into an account for the future - doing reinterpret_cast<const USHORT*>(UCHAR*) (or any other cast leading to stronger alignment requirements) is very dangerous. If original data is not aligned properly, it will break all RISC builds (like sparc, parisc).
I understand that doing development on machines where alignment is not 'must be' thing makes us forget about alignment issues, but please remember that firebird is also used on other CPUs.

Commented by: @ibprovider

Alexander, I know about problems with bad-alignment by RISC machines. Thanks :)

And I know, also, than separation of generic "csconvert" on "csconvert_ucs2_to_mbc" and "csconvert_mbc_to_ucs2" can prevent this problems.