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 used an about 1998 branch of asxxxx version 2.0 which unfortunately is not compatible with the more advanced (f.e. macros, more targets) ASxxxx Cross Assemblers nowadays available from Alan Baldwin https://shop-pdp.net/. In 2009 Alan made his ASxxxx Cross Assemblers version 5.0 available under the GPL license (GPLv3 or later), so a reunion is now a work in progress. Thanks Alan!