bootother.S

#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

//PAGEBREAK!
  # 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