You can use Macro Assembler for this purpose. It is a free and VERY good assembler which provides the READ instruction. This examplecode is used in the manual.

IF MomPass=1
READ "Buffer size",Buffersize
ENDIF

By the way ... this assembler also supports local labels, macros, case-sensitivity (or not) and all the rest I ever dreamt of. Find the assembler at
http://john.ccac.rwth-aachen.de:8000/as/
and download the latest beta version, because it works much better than the older ones.