xa is a high-speed, two-pass portable cross-assembler. It understands mnemonics
and generates code for NMOS 6502s (such as 6502A, 6504, 6507, 6510, 7501, 8500,
8501, 8502 ...), CMOS 6502s (65C02 and Rockwell R65C02) and the 65816.

Key features:
* C-like preprocessor (and understands cpp for additional feature support)
* rich expression syntax and pseudo-op vocabulary
* multiple character sets
* binary linking
* supports o65 relocatable objects with a full linker and relocation suite, as
  well as "bare" plain binary object files
* block structure for label scoping