Booting

Booting Process - Classical way

Bootstrapping

• CPU needs a first instruction
• Memory needs initial code/data
• I/O needs to know how to communicate

Firmware: Power on sequence

Firmware는 hw와 sw의 중간

1. Installed with a computer in non volatile location (ROM)
2. Initialize low level hardware
3. Hands off control to operating system loader

After power-on

Three-steps

• Firmware → Bootloader → OS kernel
  • xv6: BIOS → bootblock → kernel
  • Linux: BIOS/UEFI → GRUB → vmlinuz

BIOS

Real mode environment

• 16-bit real-mode interface → address space is 64KB

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x86

• real mode
  • backward compatibility
  • 16 bit 주소가 너무 짧아 20 bit 주소 공간 제공
    • 최대 1MB의 메모리에 접근 가능
  • multi-processing 불가
  • 보호 기능 x - 모든 주소 공간에 직접 접근 가능
• protectd mode
  - booting 후 os가 동작하면 protectd로 변경
  - 32bit 주소 공간을 제공하여 훨씬 더 큰 메모리에 접근
  - 가상 메모리 가능
  - 보호 기능 o - 안정적

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x86 Registers (32bit)

General purpose register

• CPU executes operation through registers
• EAX, EBX, ECX, EDX

Stack registers

• ESP: Points to the top of the stack
  • Stack grows down, so ESP is the lowest address
• EBP: Points to the bottom of the current stack frame

Other register

• EIP: Points to the currently executing instruction
• EFLAGS: Bit vector of flags (e.g. carry(after addition), equals zero(after comparison)

반 잘라서 쓸 수 있음 → backward compatibility

Assembly Examples

1. eax ← eax + ebx
2. edx ← ESP

• [esp] : measn deference, pointer
• 현재 stack 합의 값을 읽어서 edx에 놓아라

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Memory Addressing

Memory can be addressed as

• Individual bytes - 가장 작은 주소 지정 단위 8bits, 1 byte
• Multi-byte words, double words, quad words, etc.

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Byte ordering

• How to order bytes in memory
• Variable x has 4-byte 0x01234567
  • 0x는 16진수 표기법, 각 숫자와 문자는 4bit를 나타내므로, 32비트로 해석됨
• Address &x is 0x100

Big Endian

• Least significant byte has highest address (comes last)

Little Endian

• Least significant byte has lowest address (comes first)
  

little endian이면 reverse까지 해줌, 현대는 little

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Segmentation

16bit를 확장시키는 방법

• To represent more than 16-bit address, use segment
• If segment is 4-bit, then $64KB$ X $2^4$ address can be accessed
• e.g.) 0xfffffff0 = 0xffff0000 (base=segment) + 0x000fff0 (EIP)
  • 최종 주소 = 기본 주소 + 오프셋

Segmentation in x86

• 16-bit micro-processor $2^{16}$
• Has 20-bit physical addr $2^{20}$
• Use extra 4 bits from a 16-bit segment register
• Address translation: PA = VA + seg×16

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Using BIOS

BIOS ends by loading a boot loader

Default way is to load the first sector(512 bytes)

• From disk into the memory location at 0x7c00

BIOS then starts executing instructions at the address 0x7c00

• xv6에서는 7c00에서 시작하도록 set

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Booting Sequence

1. System startup
2. Power-On Self Test(POST)
3. Handover to bootloader

• Search bootable devices (0x55, 0xAA = magic number)
• Load the first sector (MBR, 512B) into 0x0000:0x7c00
  • 0000 is segment and 7c00 is offset
  • segment: 메모리의 특정 세그먼트의 시작 주소
  • offset: 해당 세그먼트 내에서의 특정 위치

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Booting Process - UEFI

EFI: Extensible Firmware Interface

Can be programmed in C/C++

UEFI: Unified EFI

Runtime calls must run in same mode as operating system

• A 64-bit OS requires 64-bit UEFI firmware
• A 32-bit OS requires 32-bit UEFI firmware
  • 32-bit UEFI cannot run 64-bit OS

	UEFI	BIOS
Drive size	9 zettabytes	2.2terabytes
Secure Boot	O	X
Running Mode	32, 64 bit	16 bit