Chapter 30.

MMIX Dependent Features

30.1. Command line Options

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 doubt 

ful.

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 (Sec 

tion 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 (Section 30.4 Differences to

mmixal

) specifies that global registers allo 

cated 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

.

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 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.