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The Syslinux Project covers lightweight bootloaders for

MS-DOS FAT filesystems (SYSLINUX),
network booting (PXELINUX),
bootable "El Torito" CD-ROMs (ISOLINUX), and
Linux ext2/ext3 filesystems (EXTLINUX).

The project also includes MEMDISK, a tool to boot legacy operating systems (such as DOS) from nontraditional media; it is usually used in conjunction with PXELINUX and ISOLINUX.

[출처 : http://syslinux.zytor.com/wiki/index.php/The_Syslinux_Project]
[공식 : http://syslinux.zytor.com/]


F:\syslinux\syslinux.cfg

default vesamenu.c32 timeout 100 menu background splash.jpg menu title Welcome to F10-i686-Live! menu color border 0 #ffffffff #00000000 menu color sel 7 #ffffffff #ff000000 menu color title 0 #ffffffff #00000000 menu color tabmsg 0 #ffffffff #00000000 menu color unsel 0 #ffffffff #00000000 menu color hotsel 0 #ff000000 #ffffffff menu color hotkey 7 #ffffffff #ff000000 menu color timeout_msg 0 #ffffffff #00000000 menu color timeout 0 #ffffffff #00000000 menu color cmdline 0 #ffffffff #00000000 menu hidden menu hiddenrow 5 label linux0   menu label Boot   kernel vmlinuz0   append initrd=initrd0.img root=UUID=00CB-3317 rootfstype=vfat rw liveimg overlay=UUID=00CB-3317 quiet  rhgb menu default label check0   menu label Verify and Boot   kernel vmlinuz0   append initrd=initrd0.img root=UUID=00CB-3317 rootfstype=vfat rw liveimg overlay=UUID=00CB-3317 quiet  rhgb check label memtest   menu label Memory Test   kernel memtest label local   menu label Boot from local drive   localboot 0xffff

APPEND options...

Add one or more options to the kernel command line. These are added both for automatic and manual boots. The options are added at the very beginning of the kernel command line, usually permitting explicitly entered kernel options to override them. This is the equivalent of the LILO "append" option.

APPEND -

Append nothing. APPEND with a single hyphen as argument in a LABEL section can be used to override a global APPEND.

이 외계어들은 머냐면, 문득 liveCD가 어떻게 작동되는지 궁금해서 뒤져보다가 발견한 것들이다.
일단 syslinux 라는 것을 기반으로 USB liveCD가 작동이 되는데, 그 syslinux의 환경설정파일은
syslinux.cfg 파일의 내용 중, persistent overlay를 설정한것과 안한 것의 차이는 저 append의 내용 차이뿐이었다.

initrd= [BOOT] Specify the location of the initial ramdisk root= [KNL] Root filesystem rootfstype= [KNL] Set root filesystem type rw [KNL] Mount root device read-write on boot [출처 : http://lxr.linux.no/linux/Documentation/kernel-parameters.txt]

rhgb = redhat graphical boot - This is a GUI mode booting screen with most of the information hidden while the user sees a rotating activity icon spining and brief information as to what the computer is doing. quiet = hides the majority of boot messages before rhgb starts. These are supposed to make the common user more comfortable. They get alarmed about seeing the kernel and initializing messages, so they hide them for their comfort. [출처 : http://www.redhat.com/archives/fedora-list/2004-May/msg07775.html]

찾아봐도 이정도 밖에 나오지 않는데, syslinux에서 제공하는건지 overlay에 대한 단서는 막막하다.
아무튼 root 와 overlay가 동일한 이름으로 되어 있는 것으로 봐서는, 어떠한 연관이 있어 보인다.