The MMIX version of as has some machine-dependent options.
When -fixed-special-register-names is specified, only the register names specified in Section 30.3.3 Register names are recognized in the instructions PUT and GET.
You can use the -globalize-symbols to make all symbols global. This option is useful when splitting up a mmixal program into several files.
The -gnu-syntax turns off most syntax compatibility with mmixal. Its usability is currently doubtful.
The -relax option is not fully supported, but will eventually make the object file prepared for linker relaxation.
If you want to avoid inadvertently calling a predefined symbol and would rather get an error, for example when using as with a compiler or other machine-generated code, specify -no-predefined-syms. This turns off built-in predefined definitions of all such symbols, including rounding-mode symbols, segment symbols, BIT symbols, and TRAP symbols used in mmix "system calls". It also turns off predefined special-register names, except when used in PUT and GET instructions.
By default, some instructions are expanded to fit the size of the operand or an external symbol (refer to Section 30.2 Instruction expansion). By passing -no-expand, no such expansion will be done, instead causing errors at link time if the operand does not fit.
The mmixal documentation specifies that global registers allocated with the GREG directive and initialized to the same non-zero value, will refer to the same global register. This isn't strictly enforceable in as since the final addresses aren't known until link-time, but it will do an effort unless the -no-merge-gregs option is specified. (Register merging isn't yet implemented in ld.)
as will warn every time it expands an instruction to fit an operand unless the option -x is specified. It is believed that this behaviour is more useful than just mimicking mmixal's behaviour, in which instructions are only expanded if the -x option is specified, and assembly fails otherwise, when an instruction needs to be expanded. It needs to be kept in mind that mmixal is both an assembler and linker, while as will expand instructions that at link stage can be contracted. (Though linker relaxation isn't yet implemented in ld.) The option -x also imples -linker-allocated-gregs.
Usually a two-operand-expression without a matching GREG directive is treated as an error by as. When the option -linker-allocated-gregs is in effect, they are instead passed through to the linker, which will allocate as many global registers as is needed.