Since version 2.5, LWASM is able to generate OS9 modules. The syntax is basically the same as for other assemblers. A module starts with the MOD directive and ends with the EMOD directive. The OS9 directive is provided as a shortcut for writing system calls.
LWASM does NOT provide an OS9Defs file. You must provide your own. Also note that the common practice of using "ifp1" around the inclusion of the OS9Defs file is discouraged as it is pointless and can lead to unintentional problems and phasing errors. Because LWASM reads each file exactly once, there is no benefit to restricting the inclusion to the first assembly pass.
As of version 4.5, LWASM also implements the standard data/code address streams for OS9 modules. That means that between MOD and EMOD, any RMB, RMD, RMQ, or equivalent directives will move the data address ahead and leave the code address unmodified. Outside of an actual module, both the code and data addresses are moved ahead equally. That last bit is critical to understand because it means any directives that follow an EMOD directive may have different results than other assemblers.
Additionally, within a module body, the ORG directive sets only the data address, not the code address. However, outside a module body, ORG sets both addresses.
Both code and data addresses are reset to 0 by the MOD directive.
As of version 4.5, LWLINK also supports creation of OS9 modules.