Lecture: booting

Preparation

• read the xv6 book: “Appendix A, PC hardware” and “Appendix B, The boot loader”
• start on the booting exercise

Class structure

• goals
  • understand low-level systems
  • learn to work with systems software: reading code/manuals, debugging, etc.
• two OSes
  • xv6 (exercises): a Unix-style OS on x86
  • JOS (labs): an exokernel-style OS on x86
• lectures: basic/more modern concepts
• sections: help you with exercises/labs
• final exam: open book/notes/laptop; no Internet or other communication

Overview

• the PC hardware
  • example: IBM T42
  • abstract model: CPU, Memory, and I/O
    • CPU interprets instructions: IP (Instruction Pointer) to memory
    • memory stores instructions and data
    • I/O to external world: memory-mapped I/O & port I/O
  • in this class we use QEMU to emulate the PC
  • concepts apply to non-x86 architectures
• how does printing “hello” work on this PC?
  • example: a tiny hello “kernel” (tar and source code)
  • run make qemu
    • VGA text buffer at physical memory address 0xb8000
    • one byte for character, one byte for attribute
    • memory-mapped I/O: 0xb8000 + (row * 80 + column) * 2;
      • talk to devices through memory address space
      • will see more examples in later labs
  • perhaps the lowest-level “hello world” you have seen so far
  • what if we have two applications trying to print?
• what’s an OS / why do we need an OS
  • a reusable library
  • a set of programming abstractions
  • a mediator between applications/hardware
• a brief history
  • Kirk McKusick - A Narrative History of BSD
  • what’s the OS on your phone/laptop/…?
  • see more on the readings page

The booting process

• what happens after power on?
  • high-level: firmware → bootloader → OS kernel
  • what’re the “handover protocols” / machine state after each handover
• example: BIOS-bootloader handover
  • BIOS firmware: a program stored in non-volatile memory
  • detect & initialize hardware
  • start in real mode: 16-bit registers, 20-bit (1 MB) addresses
  • load the master boot record (the first sector, 512 bytes) to 0x7c00 & jump there
• example: bootloader-kernel handover via Multiboot
  • the xv6 bootloader: bootasm.S and bootmain.c
  • switch to protected mode: 32-bit registers/addresses
  • load the kernel (in ELF) from hard drive
  • prepare machine state & handover: jump to the entry point in entry.S
• why do we need the bootloader
  • hard to fit the entire OS in 512 bytes
  • more options for users: boot from cdrom, network, usb, etc.
  • can have multiple stages
• BIOS-booting pros & cons
  • simple but restrictive
  • arch-specific: x86 real to protected (to long) mode
  • drivers?
  • Unified Extensible Firmware Interface (UEFI)
• the first xv6 exercise & JOS lab 1
  • start now!
  • tomorrow’s sections
  • review x86 calling convention from CSE 351