slackware extlinux installation

how to install extlinux as bootloader for BIOS based systems, with GPT partitions. this assumes slackware, the drive is sda and the /boot partition is the first one. everything here can be done from the slackware installer.

partitions

create GPT partitions as you like (gdisk, parted, etc.)
set the legacy boot flag on the partition (classically: the one holding /boot) where extlinux files will be installed.
parted, toggle legacy_boot flag on partition 1: toggle 1 legacy_boot
sgdisk, set legacy_boot flag on partittion 1: sgdisk /dev/sda --attributes=1:set:2
install extlinux files: extlinux --install /boot/extlinux
install the MBR to the disk: dd bs=440 count=1 conv=notrunc if=/usr/share/syslinux/gptmbr.bin of=/dev/sda

extlinux configuration in /boot/extlinux/extlinux.conf:

UI menu.c32
PROMPT 0
MENU TITLE boot menu
TIMEOUT 50
DEFAULT slackware

LABEL huge
    MENU LABEL huge
    LINUX ../vmlinuz
    APPEND root=PARTUUID=79995103-39d0-4bbf-82a5-c274f3b1996e ro

LABEL generic
    MENU LABEL generic
    LINUX ../vmlinuz-generic
    APPEND root=UUID=58c840ab-e28e-4efb-8529-e839188768e3 ro
    INITRD ../initrd.gz

note the usage of PARTUUID when not using an initrd. a plain kernel doesn't understand UUID, only PARTUUID.