This is a set of retargettable assemblers and linkage editors, which was developed by Alan Baldwin. John Hartman created the version for 8051, and I (Sandeep) have made some enhancements and bug fixes for it to work properly with SDCC.
SDCC uses an about 1999 branch which unfortunately is not compatible with the more advanced (f.e. macros, more targets) ASxxxx Cross Assemblers nowadays available from Alan Baldwin http://shop-pdp.kent.edu/. In 2009 Alan made his ASxxxx Cross Assemblers available under the GPL licence (GPLv3 or later), so a reunion could be possible. Thanks Alan!