mads (cross assembler for 6502, targetting the Atari 8-bit)

MADS is oriented toward users of QA, XASM, and FA. The syntax is
borrowed from QA, some macro commands and syntax come from XASM, and
SpartaDOS X (SDX) syntax is inherited from FA. Additional characters
are allowed in label names. Also added are support for the 65816 CPU,
macros, procedures, splitting across virtual memory banks, and nested
label names.

/usr/doc/mads-$VERSION/examples has a lot of sample code that's well
worth looking at. The DOS/Windows package of MADS ships with various
executables in examples/compression. These are not included in the
package. "Squash", "deflate", and "rle" ship with source, so you
can compile these yourself if you need them. "Exomizer", "lz4", and
"pucrunch" exist on SlackBuilds.org, in the system/ category.

Syntax highlighting for the vim and scite editors is included.
