The dmesg area in RAM is a fixed size. It will wrap, if the area fills up.
At power-on and first boot, it will contain the kernel configuration and device scanning output. After that, it will contain any kernel messages sent to the console. If those messages fill the dmesg area, kernel configuration information will be overlayed. It can still be obtained from /var/run/dmesg.boot.
Kernel messages are also logged in /var/log/messages.
You may notice, if RAM is not wiped by your BIOS during subsequent boots, a series of kernel configuration and scan messages in your dmesg output, rather than a single one. On boot, the OS will check the dmesg area looking for existing messages from previous boots, and if it finds a valid set of messages, will pick up where it left off. Just in case you need the prior information for debugging purposes.
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