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Administrative Overview

Administrative Overview. 6 Projects Design Review: Monday before 6:30pm Lab Friend Center 010 (“Fishbowl”). COS 318 Project 1 Bootloader. Problem. We will write an Operating System Manages programs, resources, users, etc. How are programs loaded? The OS takes care of this

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Administrative Overview

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  1. Administrative Overview • 6 Projects • Design Review: Monday before 6:30pm • Lab Friend Center 010 (“Fishbowl”)

  2. COS 318 Project 1Bootloader

  3. Problem • We will write an Operating System • Manages programs, resources, users, etc. • How are programs loaded? • The OS takes care of this • How is the OS loaded? …

  4. Booting a ComputerOn Startup… • The BIOS is loaded • Typically doesn’t know anything about the OS • Minimal functionality • The BIOS loads & runs the first sector of a boot device. • An OS cannot fit in just one sector

  5. Bootup Details • Start at 0xFFFF0 • Self test & initialization • Search for a boot device • Hard disk • Floppy • Flash • …

  6. Bootup Details • 1st sector loaded to 0x7c00 • Jump to 0x7c00 • 512 bytes to load the kernel

  7. Bootloader Kernel Bootloader Memory Disk

  8. Entering the Bootloader • %dl = Boot device number • Load the kernel from this device • %cs = Code segment • NO STACK! (%ss, %sp unset) • %ds unset (set it to 0x07c0 before fetching from memory!) • Other registers unset

  9. The kernel might be big Kernel

  10. Solution • Move the bootloader Kernel

  11. Loading the kernel • Load to address 0x0000:1000 • Set up the stack • %ss = 0x9000, %sp=0xfffe • %ds = 0 • Long Jump to 0x0000:1000

  12. Design Reviewprint_char, print_string Example: _start: jmp past_data welcome_msg: .asciz “OS Project 1\n\rLoading…” past_data: push welcome_msg call print_string jmp somewhere_else

  13. Design Review • Bootloader • How to find the kernel on disk? • Where to copy the kernel? • How to copy the kernel? • Createimage • How do you find the code segments in ELF using header information? • Give a formula for the disk/image address of segment i in the program header. • Where do you use padding?

  14. How long will this take? • Bootloader • About 80 lines assembly • Simple instructions (int, mov, cld) • Createimage • About 200 lines of C • Hardest part: navigating ELF structs Debugging will take the most time.

  15. Addressing • Real Mode • 1 MB • Format: 0x0000:0000 • Physical address = (segment << 4)+offset • Ex: 0x07c0:0000 = 0x0000:7c00 • Protected Mode • 4 GB (32-bit) • Format: 0x0000:00000000 (32-bit) • Virtual Addressing (user mode) • Physical address = a bit more complicated…

  16. Registers General Registers

  17. AT&T Syntax • Registers: %ax, %ah, %eax ,… • Definitions • .equ BOOT_SEGMENT, 0x07c0 • Constants: $0x0100, $4 • Labels • _start: • print_string: • Memory access • movw %ax, (0x40) • movb %dl, (a_label) • movw %es:(%ax), %dx • Comments • /* multiline */ • # to the end of the line • Directives • .equ, .byte, .word, .ascii, .asciz

  18. Assembly Pitfalls .equ BOOT_SEGMENT, 0x07c0 movw BOOT_SEGMENT, %ds

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