- Pass the parameter of log_buf_len=n to kernel, where n is a power of two number (n = 2^x). The default for slackware is 2^15 or 32768 (32k). You can change n to 65536 (2^16) or 131072 (2^17). In this way, you don't need to recompile your current kernel.
- Compile or recompile your kernel. Before you compile the kernel yourself or recompile it yourself, you have to change the kernel parameter in General Setup > Kernel log buffer size (see figure) to 16 (65536) or 17 (131072). After compilation, don't forget to copy kernel image, config, and System.map into /boot directory and set up your bootloader (lilo/grub) properly.
Choose one of them then you will get full dmesg output on your screen. It will be very long message log.