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