The MMIX version of
as has some machine-dependent options.
When ‘--fixed-special-register-names’ is specified, only the register names specified in MMIX-Regs are recognized in the instructions
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
By default, some instructions are expanded to fit the size of the operand or an external symbol (see MMIX-Expand). By passing ‘--no-expand’, no such expansion will be done, instead causing errors at link time if the operand does not fit.
mmixal documentation (see mmixsite) specifies that global registers allocated with the ‘GREG’ directive (see MMIX-greg) 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
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 implies ‘--linker-allocated-gregs’.
If instruction expansion is enabled,
as can expand a ‘PUSHJ’ instruction into a series of instructions. The shortest expansion is to not expand it, but just mark the call as redirectable to a stub, which
ld creates at link-time, but only if the original ‘PUSHJ’ instruction is found not to reach the target. The stub consists of the necessary instructions to form a jump to the target. This happens if
as can assert that the ‘PUSHJ’ instruction can reach such a stub. The option ‘--no-pushj-stubs’ disables this shorter expansion, and the longer series of instructions is then created at assembly-time. The option ‘--no-stubs’ is a synonym, intended for compatibility with future releases, where generation of stubs for other instructions may be implemented.
Usually a two-operand-expression (see GREG-base) 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.