170

Chapter 29. MIPS Dependent Features

uses

$gp

. Passing

 G 0

to

as

prevents it from using the

$gp

register on the basis of object size (but

the assembler uses

$gp

for objects in

.sdata

or

sbss

in any case). The size of an object in the

.bss

section is set by the

.comm

or

.lcomm

directive that defines it. The size of an external object may be

set with the

.extern

directive. For example,

.extern sym,4

declares that the object at

sym

is 4

bytes in length, whie leaving

sym

otherwise undefined.

Using small ecoff objects requires linker support, and assumes that the

$gp

register is correctly ini 

tialized (normally done automatically by the startup code). mips ecoff assembly code must not modify

the

$gp

register.

29.3. Directives for debugging information

mips ecoff

as

supports several directives used for generating debugging information which are not

support by traditional mips assemblers. These are

.def

,

.endef

,

.dim

,

.file

,

.scl

,

.size

,

.tag

,

.type

,

.val

,

.stabd

,

.stabn

, and

.stabs

. The debugging information generated by the three

.stab

directives can only be read by gdb, not by traditional mips debuggers (this enhancement is

required to fully support C++ debugging). These directives are primarily used by compilers, not as 

sembly language programmers!

29.4. Directives to override the ISA level

gnu

as

supports an additional directive to change the mips Instruction Set Architecture level on the

fly:

.set mipsn

.

n

should be a number from 0 to 5, or 32, 32r2, 64 or 64r2. The values other than

0 make the assembler accept instructions for the corresponding isa level, from that point on in the

assembly.

.set mipsn

affects not only which instructions are permitted, but also how certain macros

are expanded.

.set mips0

restores the isa level to its original level: either the level you selected

with command line options, or the default for your configuration. You can use this feature to permit

specific r4000 instructions while assembling in 32 bit mode. Use this directive with care!

The directive

.set mips16

puts the assembler into MIPS 16 mode, in which it will assemble in 

structions for the MIPS 16 processor. Use

.set nomips16

to return to normal 32 bit mode.

Traditional mips assemblers do not support this directive.

29.5. Directives for extending MIPS 16 bit instructions

By default, MIPS 16 instructions are automatically extended to 32 bits when necessary. The directive

.set noautoextend

will turn this off. When

.set noautoextend

is in effect, any 32 bit instruc 

tion must be explicitly extended with the

.e

modifier (e.g.,

li.e $4,1000

). The directive

.set

autoextend

may be used to once again automatically extend instructions when necessary.

This directive is only meaningful when in MIPS 16 mode. Traditional mips assemblers do not support

this directive.

29.6. Directive to mark data as an instruction

The

.insn

directive tells

as

that the following data is actually instructions. This makes a difference

in MIPS 16 mode: when loading the address of a label which precedes instructions,

as

automatically

adds 1 to the value, so that jumping to the loaded address will do the right thing.