# Start the first 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.
# Zero data segment registers DS, ES, and SS. xorw %ax,%ax # Set %ax to zero movw %ax,%ds # -> Data Segment movw %ax,%es # -> Extra Segment movw %ax,%ss # -> Stack Segment
# Physical address line A20 is tied to zero so that the first PCs # with 2 MB would run software that assumed 1 MB. Undo that. seta20.1: inb $0x64,%al # Wait for not busy testb $0x2,%al jnz seta20.1
movb $0xd1,%al # 0xd1 -> port 0x64 outb %al,$0x64
seta20.2: inb $0x64,%al # Wait for not busy testb $0x2,%al jnz seta20.2
movb $0xdf,%al # 0xdf -> port 0x60 outb %al,$0x60
# Switch from real to protected mode. Use a bootstrap GDT that makes # virtual addresses map directly to physical addresses so that the # effective memory map doesn't change during the transition. lgdt gdtdesc movl %cr0, %eax orl $CR0_PE, %eax movl %eax, %cr0
//PAGEBREAK! # Complete the transition to 32-bit protected mode by using a long jmp # to reload %cs and %eip. The segment descriptors are set up with no # translation, so that the mapping is still the identity mapping. ljmp $(SEG_KCODE<<3), $start32
.code32 # Tell assembler to generate 32-bit code now. start32: # Set up the protected-mode data segment registers movw $(SEG_KDATA<<3), %ax # Our data segment selector movw %ax, %ds # -> DS: Data Segment movw %ax, %es # -> ES: Extra Segment movw %ax, %ss # -> SS: Stack Segment movw $0, %ax # Zero segments not ready for use movw %ax, %fs # -> FS movw %ax, %gs # -> GS
# Set up the stack pointer and call into C. movl $start, %esp call bootmain
# If bootmain returns (it shouldn't), trigger a Bochs # breakpoint if running under Bochs, then loop. movw $0x8a00, %ax # 0x8a00 -> port 0x8a00 movw %ax, %dx outw %ax, %dx movw $0x8ae0, %ax # 0x8ae0 -> port 0x8a00 outw %ax, %dx 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 SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg
# Zero data segment registers DS, ES, and SS. xorw %ax,%ax # Set %ax to zero movw %ax,%ds # -> Data Segment movw %ax,%es # -> Extra Segment movw %ax,%ss # -> Stack Segment
// Boot loader. // // Part of the boot block, along with bootasm.S, which calls bootmain(). // bootasm.S has put the processor into protected 32-bit mode. // bootmain() loads an ELF kernel image from the disk starting at // sector 1 and then jumps to the kernel entry routine.
// Read 'count' bytes at 'offset' from kernel into physical address 'pa'. // Might copy more than asked. void readseg(uchar* pa, uint count, uint offset) { uchar* epa;
epa = pa + count;
// Round down to sector boundary. pa -= offset % SECTSIZE;
// Translate from bytes to sectors; kernel starts at sector 1. offset = (offset / SECTSIZE) + 1;
// If this is too slow, we could read lots of sectors at a time. // We'd write more to memory than asked, but it doesn't matter -- // we load in increasing order. for(; pa < epa; pa += SECTSIZE, offset++) readsect(pa, offset); }
下面就主要介绍bootmain这个函数。
先读入除了第一个扇区以外的前八个扇区。
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readseg((uchar*)elf, 4096, 0);
然后检查文件是否是elf格式。
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if(elf->magic != ELF_MAGIC) return; // let bootasm.S handle error
