__asm uint32_t test (uint32_t data) problem

Want to declare an assembler function (to be called from standard C) for a Cortex-M3 (LPC1343)
like this:

__asm uint32_t RevBytes (uint32_t data)
{ rev r0, r0
bx lr
}

Using armcc.exe V5.06 update 5 one gets error message "error: expected '(' after 'asm'"

So it seems that keyword "__asm" is not known by Compiler.

What is the correct syntax to use?

Best regards
Juergen