终于到了保护模式了，这部分是自制操作系统的中的第一个难关。为了理解保护模式的切换，整整看了一周资料。切换保护模式是采用清华大学的xv6改版系统的代码。
#include <asm.h> # Start the CPU: switch to 32-bit protected mode, jump into C. # The BIOS loads this code from the first sector of the hard disk into # memory at physical address 0x7c00 and starts executing in real mode # with %cs=0 %ip=7c00. .set PROT_MODE_CSEG, 0x8 # kernel code segment selector .set PROT_MODE_DSEG, 0x10 # kernel data segment selector .set CR0_PE_ON, 0x1 # protected mode enable flag # start address should be 0:7c00, in real mode, the beginning address of the running bootloader .globl start start: .code16 # Assemble for 16-bit mode cli # Disable interrupts cld # String operations increment # Set up the important data segment registers (DS, ES, SS). xorw %ax, %ax # Segment number zero movw %ax, %ds # -> Data Segment movw %ax, %es # -> Extra Segment movw %ax, %ss # -> Stack Segment # Enable A20: # For backwards compatibility with the earliest PCs, physical # address line 20 is tied low, so that addresses higher than # 1MB wrap around to zero by default. This code undoes this. seta20.1: inb $0x64, %al # Wait for not busy(8042 input buffer empty). testb $0x2, %al jnz seta20.1 movb $0xd1, %al # 0xd1 -> port 0x64 outb %al, $0x64 # 0xd1 means: write data to 8042's P2 port seta20.2: inb $0x64, %al # Wait for not busy(8042 input buffer empty). testb $0x2, %al jnz seta20.2 movb $0xdf, %al # 0xdf -> port 0x60 outb %al, $0x60 # 0xdf = 11011111, means set P2's A20 bit(the 1 bit) to 1 # Switch from real to protected mode, using a bootstrap GDT # and segment translation that makes virtual addresses # identical to physical addresses, so that the # effective memory map does not change during the switch. lgdt gdtdesc movl %cr0, %eax orl $CR0_PE_ON, %eax movl %eax, %cr0 # Jump to next instruction, but in 32-bit code segment. # Switches processor into 32-bit mode. ljmp $PROT_MODE_CSEG, $protcseg .code32 # Assemble for 32-bit mode protcseg: # Set up the protected-mode data segment registers movw $PROT_MODE_DSEG, %ax # Our data segment selector movw %ax, %ds # -> DS: Data Segment movw %ax, %es # -> ES: Extra Segment movw %ax, %fs # -> FS movw %ax, %gs # -> GS movw %ax, %ss # -> SS: Stack Segment # Set up the stack pointer and call into C. The stack region is from 0--start(0x7c00) movl $0x0, %ebp movl $start, %esp call bootmain # If bootmain returns (it shouldn't), loop. spin: jmp spin # Bootstrap GDT .p2align 2 # force 4 byte alignment gdt: SEG_NULLASM # null seg SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg for bootloader and kernel SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg for bootloader and kernel gdtdesc: .word 0x17 # sizeof(gdt) - 1 .long gdt # address gdt
代码其实很容易理解，第一步是打开A20，配置gdt寄存器并跳转到bootmain函数。这样的话，就可以正是采用C语言来编写系统了。