1#include "asm.h" 2#include "memlayout.h" 3#include "mmu.h" 4 5# Start the first CPU: switch to 32-bit protected mode, jump into C. 6# The BIOS loads this code from the first sector of the hard disk into 7# memory at physical address 0x7c00 and starts executing in real mode 8# with %cs=0 %ip=7c00. 9 10.code16 # Assemble for 16-bit mode 11.globl start 12start: 13 cli # BIOS enabled interrupts; disable 14 15 # Zero data segment registers DS, ES, and SS. 16 xorw %ax,%ax # Set %ax to zero 17 movw %ax,%ds # -> Data Segment 18 movw %ax,%es # -> Extra Segment 19 movw %ax,%ss # -> Stack Segment 20 21 # Physical address line A20 is tied to zero so that the first PCs 22 # with 2 MB would run software that assumed 1 MB. Undo that. 23seta20.1: 24 inb $0x64,%al # Wait for not busy 25 testb $0x2,%al 26 jnz seta20.1 27 28 movb $0xd1,%al # 0xd1 -> port 0x64 29 outb %al,$0x64 30 31seta20.2: 32 inb $0x64,%al # Wait for not busy 33 testb $0x2,%al 34 jnz seta20.2 35 36 movb $0xdf,%al # 0xdf -> port 0x60 37 outb %al,$0x60 38 39 # Switch from real to protected mode. Use a bootstrap GDT that makes 40 # virtual addresses map directly to physical addresses so that the 41 # effective memory map doesn't change during the transition. 42 lgdt gdtdesc 43 movl %cr0, %eax 44 orl $CR0_PE, %eax 45 movl %eax, %cr0 46 47//PAGEBREAK! 48 # Complete the transition to 32-bit protected mode by using a long jmp 49 # to reload %cs and %eip. The segment descriptors are set up with no 50 # translation, so that the mapping is still the identity mapping. 51 ljmp $(SEG_KCODE<<3), $start32 52 53.code32 # Tell assembler to generate 32-bit code now. 54start32: 55 # Set up the protected-mode data segment registers 56 movw $(SEG_KDATA<<3), %ax # Our data segment selector 57 movw %ax, %ds # -> DS: Data Segment 58 movw %ax, %es # -> ES: Extra Segment 59 movw %ax, %ss # -> SS: Stack Segment 60 movw $0, %ax # Zero segments not ready for use 61 movw %ax, %fs # -> FS 62 movw %ax, %gs # -> GS 63 64 # Set up the stack pointer and call into C. 65 movl $start, %esp 66 call bootmain 67 68 # If bootmain returns (it shouldn't), trigger a Bochs 69 # breakpoint if running under Bochs, then loop. 70 movw $0x8a00, %ax # 0x8a00 -> port 0x8a00 71 movw %ax, %dx 72 outw %ax, %dx 73 movw $0x8ae0, %ax # 0x8ae0 -> port 0x8a00 74 outw %ax, %dx 75spin: 76 jmp spin 77 78# Bootstrap GDT 79.p2align 2 # force 4 byte alignment 80gdt: 81 SEG_NULLASM # null seg 82 SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg 83 SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg 84 85gdtdesc: 86 .word (gdtdesc - gdt - 1) # sizeof(gdt) - 1 87 .long gdt # address gdt 88 89