atasm (a mostly mac/65 compatible cross-assembler)

ATasm is a 6502 command-line cross-assembler that is compatible with the
original Mac/65 macroassembler released by OSS software. Code development
can now be performed using modern editors and compiles with lightning
speed.

This build has been updated for version 1.07d (the last beta release),
but it's still possible to build 1.06 (last stable release) by downloading
the 1.06 source and setting VERSION=1.06 in the environment.
