# 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 0x7c00and starts executing in real mode # with %cs=0 %ip=7c00.
# Zero data segment registers DS, ES, andSS. 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
# Bootstrap GDT .p2align2 # force 4byte alignment gdt: SEG_NULLASM # null seg SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg
# 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
// 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); }
/* Adjust the address for the data segment to the next page */ . = ALIGN(0x1000);
/* Conventionally, Unix linkers provide pseudo-symbols * etext, edata, and end, at the end of the text, data, and bss. * For the kernel mapping, we need the address at the beginning * of the data section, but that's not one of the conventional * symbols, because the convention started before there was a * read-only rodata section between text and data. */ PROVIDE(data = .);
# By convention, the _start symbol specifies the ELF entry point. # Since we haven't set up virtual memory yet, our entry point is # the physical address of 'entry'. .globl _start _start = V2P_WO(entry)
到这个地方 xv6 的 boot loader 就基本上结束了,嗯,xv6 的设计的话,还是进行了简化,但是确实是可行的,其中段机制的使用很少,然后就是由于页机制迟迟没有打开,先是在物理内存上,把整个操作系统安排好了,再做页机制的映射,后续对照一下 linux 看一下是怎么做到的。