Booting

Execution

• A CPU loads the next instruction from memory
• This instruction is executed
• This process repeats

Where does the first instruction come from?

Volatility

• Most operating systems are stored in non-volatile memory
• CPUs in most computers execute instructions from volatile memory
• A set of operations is needed to move the operating system to a location that is executable

First-stage boot loader

• Most modern systems include multiple stages of booting
• The first step typically involves loading and executing firmware from a dedicated storage device
• Examples include BIOS and UEFI

Second-stage boot loader

• The initial boot loader is usually tightly coupled to the device
• It is basic and has limited size
• It is used to load a more complete bootloader that loads the full OS
• Examples include GRUB, Syslinux, NTLDR, and BOOTMGR

GRUB

• Popular boot loader used with Linux systems
• Supports multiple operating systems

Kernel Initialization

1. Bootloader transfers control to the kernel
2. Kernel initializes memory and hardware
3. Kernel mounts the root filesystem
4. Kernel starts the first process

Uptime

cat /proc/uptime

Processes

• Running instances of programs
• Identified by a Process identifier

top

top can be used to show an interactive list of running processes

Process Tree

pstree

PID 0

• PID 0 is not a regular process
• Created during kernel initialization
• Manages power when the CPU is not busy

PID 1

• PID 1 is the first user space process
• Commonly init or systemd
• Serves as the parent of all other processes
• Handles system shutdown and service management

Process Status

cat /proc/1/status

Kernel Log

• The kernel ring buffer contains messages from the kernel
• Useful for diagnosing hardware and boot issues
• Accessible via the filesystem at /dev/kmsg (requires root) or dmesg

sudo head /dev/kmsg