Hardware Reference
In-Depth Information
Because of the BCM2835's origins as a multimedia processor targeted at set-top boxes, this
L2 cache is designed to be used by the GPU portion of the chip alone. Unlike a traditional
processor, the CPU doesn't have any L2 cache of its own.
Using
config.txt
, you can tell the BCM2835 to allow its CPU portion access to the L2
cache memory. In some cases, this can improve performance. In other cases, this can harm
performance, due to the physical location of the cache being a relatively long distance away
from the CPU section of the chip and closer to the GPU.
Use of the L2 cache memory also requires a Linux distribution that has been compiled with
the cache memory in mind. Enabling the L2 cache on a distribution built for a standard Pi
with cache disabled may result in strange and unwanted behaviour.
To switch the L2 cache on for CPU access, simply add the following line to the
config.txt
ile:
disable_l2cache=0
As with all
config.txt
settings, the system must be rebooted before the change will take
place. To disable the CPU's access to the cache memory, replace the
0
with a
1
.
Enabling Test Mode
his inal option in
config.txt
is one the overwhelming majority of Pi users won't need to
touch, but is included here for completeness: test mode. Used during production of the
Raspberry Pi at the factory, the test mode—combined with special hardware used to electri-
cally check the board—allows the factory staf to make sure the Pi is operating as it should.
Enabling test mode won't do any permanent damage, but the Pi won't boot into its operating
system until the mode is disabled again and the power to the Pi is switched off and back on.
WARNING
If you're curious to see what the Pi looks like to factory staf, you can enable test mode by
entering the following option into the
config.txt
ile:
test_mode=1
As with other
config.txt
settings, test mode won't be enabled until the Pi is restarted.
Test mode can be disabled again by removing the line in
config.txt
, deleting the ile alto-
gether, or replacing the
1
with a
0
.
