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#include "asm.h"
# Start an Application Processor. This must be placed on a 4KB boundary
# somewhere in the 1st MB of conventional memory (APBOOTSTRAP). However,
# due to some shortcuts below it's restricted further to within the 1st
# 64KB. The AP starts in real-mode, with
# CS selector set to the startup memory address/16;
# CS base set to startup memory address;
# CS limit set to 64KB;
# CPL and IP set to 0.
#
# Bootothers (in main.c) starts each non-boot CPU in turn.
# It puts the correct %esp in start-4,
# and the place to jump to in start-8.
#
# This code is identical to bootasm.S except:
# - it does not need to enable A20
# - it uses the address at start-4 for the %esp
# - it jumps to the address at start-8 instead of calling bootmain
.set PROT_MODE_CSEG, 0x8 # kernel code segment selector
.set PROT_MODE_DSEG, 0x10 # kernel data segment selector
.set CR0_PE_ON, 0x1 # protected mode enable flag
.globl start
start:
.code16 # Assemble for 16-bit mode
cli # Disable interrupts
cld # String operations increment
# 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
# Switch from real to protected mode, using a bootstrap GDT
# and segment translation that makes virtual addresses
# identical to their physical addresses, so that the
# effective memory map does not change during the switch.
lgdt gdtdesc
movl %cr0, %eax
orl $CR0_PE_ON, %eax
movl %eax, %cr0
# Jump to next instruction, but in 32-bit code segment.
# Switches processor into 32-bit mode.
ljmp $PROT_MODE_CSEG, $protcseg
.code32 # Assemble for 32-bit mode
protcseg:
# Set up the protected-mode data segment registers
movw $PROT_MODE_DSEG, %ax # Our data segment selector
movw %ax, %ds # -> DS: Data Segment
movw %ax, %es # -> ES: Extra Segment
movw %ax, %fs # -> FS
movw %ax, %gs # -> GS
movw %ax, %ss # -> SS: Stack Segment
movl start-4, %esp
movl start-8, %eax
jmp *%eax
# 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 0x17 # sizeof(gdt) - 1
.long gdt # address gdt
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