Lecture 0: Booting

Preparation

• Read “Appendix A, PC hardware” of the xv6 book.
• Skim the Minimal Intel Architecture Boot Loader.

Don’t be dismayed by unfamiliar technical jargon; you’ll get a better idea once you start the labs.

Class structure

• Goals
  • understand OS concepts and apply to labs
  • learn to work with systems software: reading code/manuals, debugging, etc.
• Lectures: concepts, reading OS code, recent topics, lab review
• Sections: help you with labs
• Labs: JOS, an exokernel-style OS on x86
• Final exam: open book, open notes, open laptop; no Internet or other communication
• Overload form

See the 451 class website for details.

Overview

• What’s an OS?
  • a library
  • a set of programming abstractions
  • a mediator between applications, and between applications and hardware
• 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
  • for labs, we use QEMU to emulate the PC
  • concepts apply to non-x86 architectures as well: JOS/arm demo (Raspberry Pi)

The booting process

• What happens after power on?
  • high-level: firmware → bootloader → OS kernel
  • JOS: BIOS → boot/* → kern/*
  • why three steps
  • what’re the “handover protocols”
• BIOS: a program stored in non-volatile memory
  • real mode: 16-bit registers, 20-bit (1 MB) addresses
  • detect & initialize hardware
    • Lab 1: E820 memory map
    • later: ACPI tables for multiprocessor, etc.
  • load the master boot record (the first sector, 512 bytes) to 0x7c00
  • BIOS-bootloader handover: jump to that address
• Bootloader: why do we need it?
  • hard to fit the entire OS in 512 bytes
  • more options for users: boot from cdrom, network, usb, etc.
  • can have multiple stages
• Example: JOS bootloader
  • boot/boot.s and boot/main.c
  • basic setup
  • get the E820 memory map
  • 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
    • the JOS bootloader-kernel handover protocol is called Multiboot
    • eax: magic number 0x1BADB002
    • ebx: set to a struct multiboot_info (for the E820 map)
• BIOS-booting pros & cons
  • simple but restrictive
  • arch-specific: x86 real to protected (to long) mode
  • drivers?
  • Unified Extensible Firmware Interface (UEFI)
• More examples
  • xv6
  • Linux
  • how about your laptops/phones
• Summary
  • handover protocols: firmware-bootloader & bootloader-kernel
  • understand the machine state after each handover

The JOS kernel

• Lab 1: kern/entry.S and kern/init.c
  • set up paging & jump to C function i386_init()
  • set up console: we can finally print out something!
  • print CPU and memory information
  • drop into the kernel monitor, for now
• How does JOS print to screen?
  • read/write memory: crt_buf at KERNBASE + 0xB8000
  • one byte for character, one byte for attribute
  • memory-mapped I/O
    • talk to devices through memory address space
    • will see more examples in later labs
  • alternative: port I/O (not our focus)
    • through special instructions in and out
    • I/O address space is separate from memory address space
• Lab 1
  • start now!
  • tomorrow’s sections
  • review x86 calling convention from CSE 351