94 lines
3.1 KiB
ArmAsm
94 lines
3.1 KiB
ArmAsm
#include "asm.h"
|
|
|
|
# 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.
|
|
|
|
#define SEG_KCODE 1 // kernel code
|
|
#define SEG_KDATA 2 // kernel data+stack
|
|
|
|
#define CR0_PE 1 // protected mode enable bit
|
|
|
|
.code16 # Assemble for 16-bit mode
|
|
.globl start
|
|
start:
|
|
cli # Disable interrupts
|
|
|
|
# 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
|
|
|
|
# Enable A20:
|
|
# 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
|
|
|
|
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
|
|
# 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
|
|
|
|
# This ljmp is how you load the CS (Code Segment) register.
|
|
# SEG_ASM produces segment descriptors with the 32-bit mode
|
|
# flag set (the D flag), so addresses and word operands will
|
|
# default to 32 bits after this jump.
|
|
ljmp $(SEG_KCODE<<3), $start32
|
|
|
|
.code32 # Assemble for 32-bit mode
|
|
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
|
|
|
|
gdtdesc:
|
|
.word (gdtdesc - gdt - 1) # sizeof(gdt) - 1
|
|
.long gdt # address gdt
|