wAx is a machine language monitor for the Commodore VIC-20. wAx runs as a BASIC extension. This means that wAx's commands, including assembly, can be executed from BASIC's direct mode, or seamlessly within BASIC programs. wAx has most of the usual monitor tools like assembly, disassembly, hex and text data entry, breakpoint management, search, copy and fill, memory load and save, that kind of thing. But it also has features that are unique or uncommon in native assemblers. wAx is a "somewhat symbolic assembler," bringing labels and forward references, multi-pass assembly capability, and code relocation. It offers optional built-in support of an extended instruction set in the form of 6502 "illegal" opcodes. It provides for code unit testing with an assertion tester and staging environment selection for multiple BASIC programs in memory. It provides a user tool vector and a documented API for development and integration of custom monitor tools.