Manually making U disk boot and basic principles for Linux

Basic minimalist directory file after successful production

First, the basic boot process of the system:

First, the system should boot the isolinux.bin executable program. This program is used for booting on the removable media. After the isolinux.bin runs successfully, it will load its configuration file syslinux.cfg, which defines the calling mode of vmlinuz and initrd.img. Vmlinuz is a compressed version of vmlinux, both of which are linux kernels. Isolinux.bin is passed to vmlinuz, which is the Linux kernel and loaded with the kernel according to the initrd.img parameter defined in its configuration mode. Then the Linux kernel will call initrd.img according to the obtained initrd.img call parameter. The basic function of initrd.img is the root path mounter. The meaning of its existence is to save the recompilation of the vmlinuz kernel under different root path hardware. Initrd.img depends on the calling parameters to find the system startup program in the parameter path (here is the installation image of iso), the system will start up.

Second, the basic production process of the boot disk:

The first step: extract the files needed to make the boot image, these files are in the isolinux directory of the Linux installation disk, this directory is a miniature Linux system.

Step 2: Edit the original file isolinux.cfg of the execution configuration file syslinux.cfg of isolinux.bin.

default vesamenu.c32timeout 600 The default defined here is vesamenu.c32c, the role of this vesamenu.32c is to start the menu appearance drawing extension tool. The timeout defined below refers to the user's choice of time limit. 
label linux menu label ^Install CentOS 7 kernel vmlinuz append initrd=initrd.img inst.stage2=hd:LABEL=CENTOS quietlabel check menu label Test this ^media & install CentOS 7 menu default kernel vmlinuz append initrd=initrd. Img inst.stage2=hd:LABEL=CENTOS rd.live.check quietmenu separator # insert an empty line# utilities submenumenu begin ^Troubleshooting menu title Troubleshootinglabel vesa menu indent count 5 menu label Install CentOS 7 in ^basic graphics mode text help Try this Option out if you're having trouble installing CentOS 7. endtext kernel vmlinuz append initrd=initrd.img inst.stage2=hd:LABEL=CENTOS xdriver=vesa nomodeset quietlabel rescue menu indent count 5 menu label ^Rescue a CentOS system text help If The system will not boot, this lets you access files and edit config files to try to get it booting again. endtext kernel vmlinuz append initrd=initrd.img inst.stage2=hd:LABEL=CENTOS rescue quietlabel memtest menu label Run a ^memory Test tex t help If your system is having issues, a problem with your system's memory may be the cause. Use this utility to see if the memory is working correctly. endtext kernel memtestmenu separator # insert an empty linelabel local menu label Boot from ^local drive localboot 0xffffmenu separator # insert an empty linemenu separator # insert an empty linelabel returntomain menu label Return to ^main menu menu exitmenu end These are the specific definitions of each menu, which defines the way each menu corresponds to calling vmlinuz and initrd.img. 
label linux menu label ^Install CentOS 7 kernel vmlinuz append initrd=initrd.img inst.stage2=hd:LABEL=CENTOS quietlabel is a label, similar to grub's title. The kernel entry represents the kernel to be loaded, and append is the suffix parameter, which is equivalent to the role of good-looking line breaks. Initrd=initrd.img is the initrd.img for defining the kernel, the inst.stage2=hd:LABEL=CENTOS quiet is the root path for loading, and the inst.stage2 is the file for loading iso, hd:LABEL=CENTOS The path of the root is in the storage device labeled CENTOS, that is, the U disk, so the name of the boot image and the U disk volume label must be CENTOS. 
 If you don't want to use the menu mode to display the installation file directly, you can save vesamenu.32c and change the default vesamenu.32c at the beginning to default linux, which is the default load label linux definition. 
 Step 3: Make the boot image file CENTOS.iso
 The name of the modified image file is the same as the root path label in syslinux.cfg. Set isolinux.bin as the boot file, check the generate startup information table, this startup information table is the ldlinux.sys startup information file in the last directory. 
 Step 4: Save the image and write it to the USB flash drive by booting -> writing to the hard disk image. At this time, UItral So will automatically generate the syslinux.cfg file box ldlinux.sys file on the U disk, and other files will be copied to the U disk. At this time, the isolinux.cfg in the U disk can be deleted, and finally Copy the image of the installation system to the root directory. 
 Note: The files not mentioned in the U disk are some of the linux detection tools called in the background or other installation options.