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Bootstrapping on x86

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  1. Bootstrapping on x86 Steve Muckle Wednesday, March 12th 2003 15-412 Spring 2003

  2. Motivation • What happens when you turn on your PC? • How do we get to main() in kernel.c? Carnegie Mellon University

  3. Overview • Requirements of Booting • Ground Zero • The BIOS • The Bootloader • Our projects: Multiboot, OSKit Carnegie Mellon University

  4. Requirements of Booting • Initialize machine to a known state • Make sure basic hardware works • Load a real operating system • Run the real operating system Carnegie Mellon University

  5. Ground Zero • You turn on the machine • Execution begins in real mode at a specific memory address • Real mode: only 1mb of memory is addressable • Start address is in an area mapped to BIOS read-only memory • What’s the BIOS? Carnegie Mellon University

  6. Basic Input/Output System (BIOS) • Code stored in Electrically Erasable Programmable Read Only Memory (EEPROM) on most modern systems • Useful for testing hardware and loading data from storage into memory • Can also be used to configure hardware details like RAM refresh rate or bus speed Carnegie Mellon University

  7. Basic Input/Output System (BIOS) • BIOS performs a Power On Self Test (POST) • BIOS loads the first sector from a boot device- could be a floppy, hard disk, CDROM- without a BIOS, we’d be in a bit of a jam • If the last two bytes are AA55, we’re in business • Otherwise we look somewhere else Carnegie Mellon University

  8. Basic Input/Output System (BIOS) • Sector is copied to 0x7C00 • Execution is transferred to 0x7C00 • If it’s a hard disk or CDROM, there’s an extra step or two (end result is the same) • Now we’re executing the bootloader – the first “software” to execute on the PC Carnegie Mellon University

  9. Bootloader • We’re now executing a bootloader • Some bootloaders exist to load one OS • Others give you a choice of what to load • We use grubhttp://www.gnu.org/software/grub/ Carnegie Mellon University

  10. Bootloader • GRUB is larger than one sector • The sector loaded in by the BIOS just loads…the rest of the bootloader • GRUB then presents you with a boot menu • To load a kernel, it must switch back and forth between real and protected mode • It then jumps to the kernel’s entrypoint- How do we know the kernel’s entrypoint? Carnegie Mellon University

  11. Multiboot Specification • Many OSes require their own bootloader • Multiboot offers a standard way for kernels to communicate entrypoint and other info • The multiboot headermust be located in the8192 bytes • This is the mysteriousmultiboot.o… 0x1badb002 flags checksum header_addr load_addr load_end_addr bss_end_addr entry_addr Carnegie Mellon University

  12. OSkit • The kernel entrypoint is an assembly function in multiboot.o • This calls the first C function, multiboot_main Carnegie Mellon University

  13. OSkit • multiboot_main calls:- base_cpu_setup: init GDT, IDT, and TSS- base_multiboot_init_mem: init LMM- base_multiboot_init_cmdline: parse cmdline passed to kernel by bootloader- main (yes, your main in kernel.c!)- exit, if main ever returns (press a key to reboot…) Carnegie Mellon University

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