xv6-cs450/bootasm.S

94 lines
3.1 KiB
ArmAsm
Raw Normal View History

#include "asm.h"
2006-09-06 19:27:19 +02:00
# 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.
2006-09-06 19:27:19 +02:00
2009-05-31 03:00:38 +02:00
#define SEG_KCODE 1 // kernel code
#define SEG_KDATA 2 // kernel data+stack
#define CR0_PE 1 // protected mode enable bit
2006-09-06 19:27:19 +02:00
.code16 # Assemble for 16-bit mode
.globl start
start:
cli # Disable interrupts
2006-06-12 17:22:12 +02:00
# 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
2006-06-12 17:22:12 +02:00
2006-09-06 21:08:14 +02:00
# Enable A20:
2007-08-22 16:21:05 +02:00
# 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
testb $0x2,%al
jnz seta20.1
movb $0xd1,%al # 0xd1 -> port 0x64
outb %al,$0x64
2006-06-12 17:22:12 +02:00
seta20.2:
inb $0x64,%al # Wait for not busy
testb $0x2,%al
jnz seta20.2
movb $0xdf,%al # 0xdf -> port 0x60
outb %al,$0x60
//PAGEBREAK!
# Switch from real to protected mode, using a bootstrap GDT
# and segment translation that makes virtual addresses
2007-09-15 22:05:47 +02:00
# identical to physical addresses, so that the
# effective memory map does not change during the switch.
lgdt gdtdesc
movl %cr0, %eax
orl $CR0_PE, %eax
movl %eax, %cr0
# Jump to next instruction, but in 32-bit code segment.
# Switches processor into 32-bit mode.
2009-05-31 03:00:38 +02:00
ljmp $(SEG_KCODE<<3), $start32
2006-09-06 19:27:19 +02:00
.code32 # Assemble for 32-bit mode
start32:
# Set up the protected-mode data segment registers
2009-05-31 03:00:38 +02:00
movw $(SEG_KDATA<<3), %ax # Our data segment selector
movw %ax, %ds # -> DS: Data Segment
movw %ax, %es # -> ES: Extra Segment
2009-05-31 03:00:38 +02:00
movw %ax, %ss # -> SS: Stack Segment
2009-05-31 03:12:08 +02:00
movw $0, %ax # Zero segments not ready for use
movw %ax, %fs # -> FS
movw %ax, %gs # -> GS
2009-05-31 03:00:38 +02:00
# Set up the stack pointer and call into C.
movl $start, %esp
2007-08-28 21:25:04 +02:00
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 $0x8e00, %ax # 0x8e00 -> port 0x8a00
outw %ax, %dx
spin:
2007-08-28 21:25:04 +02:00
jmp spin
# Bootstrap GDT
.p2align 2 # force 4 byte alignment
2006-06-12 17:22:12 +02:00
gdt:
SEG_NULLASM # null seg
SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg
SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg
2009-05-31 03:00:38 +02:00
SEG_ASM(STA_W, 0x100, 0xffffffff) # per-cpu data seg; 0x100 is okay for now
2006-06-12 17:22:12 +02:00
gdtdesc:
2009-05-31 03:00:38 +02:00
.word 0x1f # sizeof(gdt) - 1
.long gdt # address gdt