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基于 i386 体系结构的 Linux 启动代码分析

基于 i386 体系结构的 Linux 启动代码分析. xlanchen@2006.6.1. 启动代码文件. linux/arch/i386/boot/bootsect.S linux/arch/i386/boot/setup.S linux/arch/i386/boot/compressed/head.S linux/arch/i386/kernel/head.S linux/arch/i386/kernel/init_task.c linux/arch/init/main.c. 实模式. 实模式是为了兼容早期的 CPU 而设置的 系统总是始于实模式 实模式下

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基于 i386 体系结构的 Linux 启动代码分析

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  1. 基于i386体系结构的Linux启动代码分析 xlanchen@2006.6.1

  2. 启动代码文件 • linux/arch/i386/boot/bootsect.S • linux/arch/i386/boot/setup.S • linux/arch/i386/boot/compressed/head.S • linux/arch/i386/kernel/head.S • linux/arch/i386/kernel/init_task.c • linux/arch/init/main.c Embedded Operating Systems

  3. 实模式 • 实模式是为了兼容早期的CPU而设置的 • 系统总是始于实模式 • 实模式下 • 地址总线：20位 • 内存范围：0~1MB • 逻辑地址 = 段地址 + 段内偏移 • 段地址 = 段寄存器中的值*16 （或左移4位） • 段寄存器长度：16bit Embedded Operating Systems

  4. I386实模式下的内存布局图 RAM ROM-BIOS 0xF0000 0xC0000 VIDEO-BIOS VRAM 0xA0000 1-MB 0x00000 Embedded Operating Systems

  5. 软盘启动，bootsect.S 0x7c000x90000 0x7c00, BIOS 0x90000, lilo 堆栈，0x3ff4(0x4000-12), 向下增长 磁盘参数表，12Bytes，0x3ff4~0x4000 显示“Loading” Setup0x90200 系统 小内核，0x10000（64KB处），低装载 大内核，0x100000（1MB处），高装载 setup 硬盘启动，两阶段引导 装载LILO（LInuxLOader） 第一个扇区 … 装载LINUX Bootsect.S0x90000 Setup.S0x90200 系统 0x10000 0x100000 跳转到setup 软盘和硬盘启动 Embedded Operating Systems

  6. 初始化硬件设备并为内核程序的执行建立环境 内存检测 键盘 视频 磁盘控制器 IBM微通道总线MCA PS/2设备（总线鼠标） APM BIOS 若低装载，将系统移动到0x1000处（4KB处）否则，不必 临时IDT和临时GDT FPU PIC, 16个硬件中断中断向量32~47 实模式保护模式 Startup_32 Setup Embedded Operating Systems

  7. Compressed/head.S • Startup_32 • 初始化段寄存器和一个临时堆栈 • 初始化BSS段 • 解压缩 • 高装载或低装载解压缩0x100000（1MB） • 跳转到0x100000处 Embedded Operating Systems

  8. Head.S • Startup_32 • 初始化段寄存器 • 建立进程0的内核堆栈 • Setup_idt • 拷贝系统参数 • 识别处理器 • GDT、IDT • Start_kernel Embedded Operating Systems

  9. The system-segment registers • In protected-mode the CPU needs quick access to some important data-structures, such as: • Memory-Segment Descriptors • Interrupt-Gate Descriptors • Call-Gate Descriptors • Task-State Descriptors • Page-Directory and Page-Table Descriptors • Special CPU registers locate these items Embedded Operating Systems

  10. 47 16 15 0 Segment Base-Address Segment Limit 32 bits 16 bits GDT and IDT • Two most vital system registers for protected-mode execution are: • GDTR (Global Descriptor Table Register) • IDTR (Interrupt Descriptor Table Register) • 48-bits widebase-address + segment-limit (an array of descriptors, the GDT & the IDT) • Special instructions • SGDT/LGDT, SIDT/LIDT Embedded Operating Systems

  11. Interrupt Descriptor Table descriptor Global Descriptor Table descriptor descriptor descriptor descriptor descriptor descriptor descriptor descriptor descriptor descriptor descriptor descriptor descriptor descriptor descriptor GDTR descriptor descriptor descriptor descriptor IDTR System Relationships Embedded Operating Systems

  12. LDT and TSS • For protected-mode multitasking, the CPU needs to access two other data-structures: • The current Local Descriptor Table (LDT) • The current Task-State Segment (TSS) • Again, special registers tell the CPU where to find these data-structures in memory (assuming protected-mode is enabled) • Instructions: • SLDT/LLDT, STR/LTR Embedded Operating Systems

  13. Control Registers • CR0 • CR1 • CR2 • CR3 Embedded Operating Systems

  14. CR0 • CR0, MSW register (Machine Status Word, 32-bit version) • Contains system control flags that control operating mode and states of the processor • PE-bit (Protection Enabled) 0 CPU is in real-mode, 1 CPU is in protected-mode • Instruction: lmsw • LINUX’ setup.S: • movw $1, %ax • lmsw %ax • jmp flush_instr // why? • flush_instr: • To turn on the PE-bit (enables protected-mode), Embedded Operating Systems

  15. CR1、CR2、CR3 • CR1 • Reserved • CR2 • Contains the page-fault linear address (the linear address that caused a page fault) • CR3 • Contains the physical address of the base of the page directory and two flags (PCD & PWT) Embedded Operating Systems

  16. 31 16 Base[31..24] G D R S V A V L Limit [19..16] P D P L S X C / D R / W A Base[23..16] Base[15..0] Limit[15..0] 0 15 Segment Descriptor Format Embedded Operating Systems

  17. 15 2 1 0 index TI RPL Segment selector “Hidden” part of Segment Registers selector Segment base Segment limit Access rights The “invisible” parts of a segment-register The programmer-visible part of a segment-register Embedded Operating Systems

  18. COLOR TEXT 32-KB 0xB8000 MONOCHROME TEXT 32-KB 0xB0000 GRAPHICS 64-KB 0xA0000 Three VRAM zones Embedded Operating Systems

  19. Array of picture-elements • Text-mode VRAM is organized as an array • Each array-element occupies one word • Word’s LSB holds ascii character-code • Word’s MSB holds a color-number pair 0 12 11 8 7 15 bgcolor fgcolor ASCII character-code nybble nybble byte Embedded Operating Systems

  20. Blink R G B Intense G B R background color attribute foreground color attribute Color-Attribute Byte Embedded Operating Systems

  21. 80 columns characters 0..79 characters 80..159 25 rows characters 1920..1999 Video screen Screen-element locations Embedded Operating Systems

  22. 逻辑地址 线性地址 物理地址 分段单元 分页单元 内存寻址 • 逻辑地址 • 段：偏移量 • 线性地址 • 32位无符号整数，0~4G-1, 即0x00000000~0xFFFFFFFF • 物理地址 • 芯片级内存单元寻址 • 地址转换过程 Embedded Operating Systems

  23. 实模式？保护模式？分页？ • Real mode • Exists mostly to maintain processor compatibility with older models, & • To allow the OS to bootstrap Embedded Operating Systems

  24. 段选择子 2 1 0 15 index TI RPL Segment selector Embedded Operating Systems

  25. __KERNEL_CS 0x10 = 0000 0000 0001 0000b Index(0x2), TI(0), RPL(0, highest) limit(0xFFFFF), base(0), G(1, =4096), D(1,=32位), P(1, in main memory), DPL(0, hignest), S(1), type(0xA) E(1), executable, code C(0), not current R(1), readable A(0), unaccessed gdt: .word 0, 0, 0, 0 # dummy .word 0, 0, 0, 0 # unused .word 0xFFFF # 4Gb.word 0 # base address = 0 .word 0x9A00 # code read/exec.word 0x00CF # granularity(4096).word 0xFFFF # 4Gb .word 0 # base address = 0 .word 0x9200 # data read/write .word 0x00CF # granularity(4096) __KERNEL_CS Embedded Operating Systems

  26. 段描述符 Embedded Operating Systems

  27. Embedded Operating Systems

  28. Segment selector Segment-offset Logical Address: Segment Descriptor Table descriptor Validity is checked by CPU Segment Base-address descriptor + (also Segment-Limit and Access Rights) descriptor descriptor Physical Address: Operand’s effective address 逻辑地址的转换 Embedded Operating Systems

  29. 分页 • PG • Paging (bit 31 of CR0). • 1 Enables paging with PE=1, • Linearaddress space is divided into fixed-size pages (4KB, 2MB, or 4MB) that can be mapped into physical memory and/or diskstorage using paging mechanism. • 0 Disables paging, linear addresses = physical addresses. Embedded Operating Systems

  30. lss • lss a, %esp ; dses, aesp Embedded Operating Systems

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