Linux Console - Raspberry Pi System Software Reference - page 62

Table 5-2. The Meaning of Flags Displayed in Parentheses

Flag Meaning

E The console is enabled.

C Is the preferred console.

B Primary boot console.

p Used for printk buffer.

b Not a TTY, but is a Braille device.

a Safe to use when CPU is offline.

Serial Console
If you wired up a serial console to your Raspberry Pi, you can use a utility such as PuTTY
( http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html ) on your
laptop or desktop computer to connect to it. The serial console sees the following first few
lines at boot (long lines are edited):

Uncompressing Linux… done, booting the kernel.
[ 0.000000] Initializing cgroup subsys cpu
[ 0.000000] Linux version 3.2.27+ (dc4@dc4-arm-01) \
(gcc version 4.7.2 20120731 (prerelease) \
(crosstool -NG linaro -1.13.1+bzr2458 - Linaro GCC 2012.08 )) \
#250 PREEMPT Thu Oct 18 19:03:02 BST 2012
[ 0.000000] CPU: ARMv6-compatible processor [410fb767] \
revision 7 (ARMv7), cr=00c5387d
[ 0.000000] CPU : PIPT /VIPT nonaliasing data cache, \
VIPT nonaliasing instruction cache
[ 0.000000] Machine: BCM2708

Using the dmesg command, you can see almost the same thing:

$ dmesg | head −5
[ 0.000000] Initializing cgroup subsys cpu
[ 0.000000] Linux version 3.2.27+ (dc4@dc4−arm−01) \
(gcc version 4.7.2 20120731 (prerelease) \
(crosstool −NG linaro −1.13.1+bzr2458 − Linaro GCC 2012.08)) \
#250 PREEMPT Thu Oct 18 19:03:02 BST 2012
[ 0.000000] CPU: ARMv6−compatible processor [410fb767] \
revision 7 (ARMv7), cr=00c5387d
[ 0.000000] CPU:PIPT/VIPT nonaliasing data cache, \
VIPT nonaliasing instruction cache
[ 0.000000] Machine:BCM2708

The difference is that the initial Uncompressing Linux console output is missing.
Additionally, any debug messages that a new kernel might display can be capturable on a
serial console.