www.ScienceOfAtomics.com/linux
Live CDs
The easiest way to start or repair a system

•  HOME  •  THEORY  •  PRACTICE  •    •  LOGIN  •  FORUM  •  CONTACT  • 

Basic procedure

Switch the computer on (in other words, power up and boot the system) and allow it to boot as normal. Put the CD (or DVD) into the drive and restart the computer (reboot the system). One of four things can happen:

  1. The system boots the CD. In this case just follow the instructions and the Live System should install itself in RAM and display the initial screen. If this process fails, read the rest of this page to find out why.
  2. The system boots as usual. This means that the computer's BIOS is set to boot straight from the HDD. Read the rest of this page to learn how to change this setting.
  3. The system starts booting the CD, but the screen becomes unreadable at some point. This is unusual, and due to video hardware that the Live CD does not recognize. Reboot the Live CD and explore the options available; there is usually one or more relating to the "framebuffer" (i.e. video memory) or other similar option. These should be tried in turn.
  4. The system starts booting the CD, but halts or repeats at some point. This is very unusual, and can be caused either by a damaged CD or by unknown or faulty hardware. Try using another CD, or the same CD in a different computer in order to isolate the fault.

The BIOS setup screen

The first PCs always contained one or more floppy disk drives (FDD's), and the first of them was the default boot device. Floppies were also the default installation medium for software, sometimes requiring twenty or more disks to install a large package. Today floppy disks are becoming obsolete, but so much legacy equipment is still in use that it will be many years before they disappear completely. The default installation media today are CD's and DVD's. The early BIOS's only knew how to boot from floppy and HDD, but modern BIOS's can boot from a number of different media, including a network.

The BIOS contains a list of boot devices, and will pass control to the first device on which it finds an operating system. If an operating system is installed on the HDD and it is the first device on the list, the system will always boot from it. In order to boot from a CD or other device, it must be higher on the boot list than any other device containing an operating system.

The Boot List is controlled from the BIOS Setup Screen. This does not appear during the normal boot sequence, and must be invoked by pressing a key during the first few seconds of the boot sequence whilst the initial screen is being displayed. The required key varies according to the installed BIOS and computer manufacturer, but two keys are in common use.

Invoking the BIOS Setup Screen

AMD BIOS's use the Delete (DEL) key.

Phoenix BIOS's use the F2 key.

Other BIOS's may use different keys; you will need to investigate or experiment further.

The appearance of the Setup Screen also varies across platforms, but the functions performed are similar. It is worth a few minutes to learn how to operate the Setup Screen and to examine the options available. The Boot List is usually on the second or third screen. The layout and choices vary, but all of them result in a sequential list of devices that the CPU attempts to boot from. The first device is tried; if it cannot be booted, the second device is tried; and so on. The sequence should be set to allow booting of a CD before the default Hard Disk.

Note that good security practice is not to allow booting of other devices during normal operation in case an inexperienced or malicious User damages the system. Most Home Users will not have this concern, and can leave their system set to boot from CD and USB (and floppy if required) ahead of the HDD.

With the BIOS Boot List correctly configured, the Live CD should boot through to the initial screen. A complete Linux distro is now available, but Users should allow that it will run somewhat slower than an installed version due to the need to read and decompress instruction and application code from the CD prior to executing it.

If the boot process fails to complete, several options are available that will often bypass the source of trouble. If you're not sure how to proceed, go to the Help Page and decide how best to proceed. However, it's a good idea to have some understanding of the boot process before going further. The introduction that follows will give you a better idea of where your problem may lie.

Understanding the boot process

The sequence of events occurring when a computer starts operation is called the "boot sequence". The name comes from the early days of computing. At power-up the computer's instruction memory contains garbage, and a valid sequence of instructions must be written into it telling the CPU how to find and load the operating system. In the earliest computers this was done by hand using switches on the computer's front panel. The operators likened this to "picking yourself up by the bootstraps", so the term "bootstrap code" was used to refer to this initial set of instructions. This was later shortened to the verb "to boot".

In modern machines the bootstrap code is contained in a ROM chip on the mainboard. ROM is an acronym for Read Only Memory, although some machines use "flash memory" which can be reprogrammed if required. The code it contains is called the BIOS, or Basic Input/Output System, and there are two dominant vendors of BIOS code, AMD (American Micro Devices) and Phoenix. Both BIOS's perform the same major functions, but additional features have been introduced in different versions over the years, such as the ability to boot from USB memory sticks.

The POST

The first operations performed by the ROM-BIOS at boot are called the Power-On Self Test or POST. This checks the operation of the CPU itself and determines its type, model, and features, then moves outwards to memory, storage devices, keyboard, screen, mouse, printer, and other peripherals. In the event that a screen is not available for displaying any errors detected, the system speaker (a small speaker inside the case, not the external sound system) emits a series of "beeps" conforming to a pre-programmed "beep code" that identifies the fault. Under normal operation, a single "all OK" beep is emitted and the boot sequence proper is entered.

The Real-Time Clock

A dedicated chip on the mainboard contains a hardware Real-Time Clock (RTC) powered by a small battery, typically a lithium button cell with a lifetime of up to ten years. The RTC also contains a small amount of RAM (Random Access Memory that is rewritable). The BIOS stores its settings in the RTC RAM, so if its battery goes flat, the BIOS settings will be lost along with time-of-day. Should this occur, the battery must be replaced.

The BIOS drivers

As the name suggests, the BIOS code "knows how" to operate the essential system peripherals – keyboard, screen, and, in earlier days, a simple printer. Modern BIOS's also contain basic drivers for a range of popular peripherals on the assumption that they conform to the ISA standard. The Industry Standard Architecture is a set of elementary specifications based on the design of the early IBM PC, XT, and AT machines to which all modern PC's conform. Most modern peripherals switch to ISA operation on power-up or reset, and can be used in this minimal mode by the BIOS. During the operating system initialization sequence, each peripheral is interrogated for its type and model number, the appropriate driver is installed, and the peripheral is reprogrammed to a more capable mode; for example, the initial video screen setting corresponds to a text-only display having twenty-five lines of eighty characters. This soon changes to a graphics screen, perhaps at 640 x 480 pixels with 256 colours, and then to 1024 x 768 pixels with millions of colours, or whatever the User has selected.

If the operating system does not "recognize" a peripheral, it will leave the original BIOS driver installed. This often occurs when a User installs a new piece of hardware, and, depending on what it is, has consequences ranging from sub-standard performance to system failure. The correct driver must be installed in order to remedy the situation.

The boot device

Once the BIOS has installed its drivers, it searches the system for a device containing an operating system. Typically this will be a HDD, but the procedure followed is very similar for all boot devices. Data on HDD's is stored in sectors, and the first sector on the drive always serves as the boot sector. Within the boot sector is an instruction telling the CPU where to find the entrance to the operating system loader. This is a small, specialized utility that "knows how" to load an operating system into memory and pass control to it. The two most popular Linux loaders are GRUB and LILO. The BIOS reads the loader code into memory and passes control to it; the loader in turn reads the kernel initialization code into memory, sets the values of any required parameters, passes control to it, and the operating system then proceeds to initialize itself.

The kernel and shells

The process of operating system initialization varies according to its design. The following discussion applies to Linux, but is sufficiently general as to apply to most modern operating systems. The first stage is to install a kernel or core, so named from the analogy of a nut, which consists of a hard outer shell and an inner kernel. The reason for the analogy is seldom appreciated, but is important for proper use of modern operating systems. During normal operation, the User is permitted to access and alter data in memory and on disk, but must be prevented from accessing system code and data, and data belonging to other Users. These considerations fall under the topic of system security. Because Unix and Linux were designed from the very beginning as multi-user systems, they have excellent security, as well as high flexibility in managing access to system resources. All access in Unix and Linux systems is supervised by the kernel. Users can only access their own files directly; all other access must be done via the kernel using system calls within the shell. The shell is thus visualized as an impenetrable barrier surrounding the kernel, as in a nut.

Once the kernel is in memory, hardware drivers are installed and connected to it. System logs are opened and updated, and one or more user consoles are installed. Each console contains a shell interfacing to the kernel via a command line, thereby giving the user access to the system under supervision of the kernel. The commandline is still the most direct and powerful way to access the operating system, although most Users find it too daunting, simply because they do not understand the basic concepts on which it operates.

The Graphical User Interface

Until 1984, this was the end of the boot sequence, since the commandline was the default User Interface. In 1981 Xerox developed the first Graphical User Interface or GUI known as 8010 Star, but never released it to the public. It was the first computer to use WIMP - Windows, Icons, Menus, and Pointing – the basic elements of a GUI. In 1984 Apple released its version of a GUI based on the Xerox Star system, the first Apple Macintosh. Steve Jobs (Apple's MD) had been able to see this work when he agreed to sell Xerox $1 million dollars worth of stock to Xerox back in 1981. Until then, IBM had almost completely dominated the personal computer market since their 1981 debut. In one swift move, Apple shifted the focus of the market and initiated a preference for the GUI's that are now ubiquitous.

The boot sequence of modern operating systems therefore continues past the commandline stage, and installs a GUI, of which there are many. MS Windows users have no choice in this regard, but Linux Users have literally hundreds of alternatives from which to choose. Most will use the default supplied with their distro. A GUI consists of several components – window manager, file manager, utilities, applications, and services, to name a few. These are often combined into a suite called a Desktop incorporating all of them. The two most popular Linux Desktops are KDE and Gnome. Both can be expanded and customized ad infinitum, and provide very powerful operating environments.