Assembling for a MIPS ECOFF target supports some additional sections
besides the usual .text, .data and .bss. The
additional sections are .rdata, used for readonly data,
.sdata, used for small data, and .sbss, used for small
common objects.
When assembling for ECOFF, the assembler uses the $gp
($28) register to form the address of a "small object". Any
object in the .sdata or .sbss sections is considered
"small" in this sense. For external objects, or for objects in the
.bss section, you can use the GCC `-G' option to control the
size of objects addressed via $gp; the default value is 8,
meaning that a reference to any object eight bytes or smaller will use
$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 has been correctly initialized (normally done
automatically by the startup code). MIPS ECOFF assembly code must not
modify the $gp register.