as: MIPS Small Data
9.26.4 Controlling the use of small data accesses
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It often takes several instructions to load the address of a symbol.
For example, when 'addr' is a 32-bit symbol, the non-PIC expansion of
'dla $4,addr' is usually:
lui $4,%hi(addr)
daddiu $4,$4,%lo(addr)
The sequence is much longer when 'addr' is a 64-bit symbol. 
Directives to override the size of symbols MIPS Symbol Sizes.
In order to cut down on this overhead, most embedded MIPS systems set
aside a 64-kilobyte "small data" area and guarantee that all data of
size N and smaller will be placed in that area. The limit N is passed
to both the assembler and the linker using the command-line option '-G
N', Assembler options MIPS Options. Note that the same value of
N must be used when linking and when assembling all input files to the
link; any inconsistency could cause a relocation overflow error.
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, while leaving
'sym' otherwise undefined.
When no '-G' option is given, the default limit is 8 bytes. The
option '-G 0' prevents any data from being automatically classified as
small.
It is also possible to mark specific objects as small by putting them
in the special sections '.sdata' and '.sbss', which are "small"
counterparts of '.data' and '.bss' respectively. The toolchain will
treat such data as small regardless of the '-G' setting.
On startup, systems that support a small data area are expected to
initialize register '$28', also known as '$gp', in such a way that small
data can be accessed using a 16-bit offset from that register. For
example, when 'addr' is small data, the 'dla $4,addr' instruction above
is equivalent to:
daddiu $4,$28,%gp_rel(addr)
Small data is not supported for SVR4-style PIC.
Info Catalog
as: MIPS Symbol Sizes
as: MIPS-Dependent
as: MIPS ISA