root/startup.s

.include "include/memory.inc"
OsCodeSeg   = 0x8
OsDataSeg   = 0x10
code64      = 0x18

#=========================================================================
# This file contains the code to switch the processor to 64-bit long mode
#=========================================================================

    .text

    .global _start

    .code32

#=====================
# Protected Mode code
#=====================
_start:
here:
    mov $OsDataSeg, %eax
    mov %eax, %ds
    mov %eax, %ss
    mov $tempstack, %esp
    call InitMemManagement
# Zero unused pages
    mov nPages, %edx
    mov $0, %ebx
    mov $PageMap, %eax
again:
    movw (%eax), %bx
    cmpw $0, %bx
    jne nextpage
    mov %eax, %edi
    sub $PageMap,%eax
    sal $11,%eax 
    mov $1000, %ecx
again2:
    movb $0, (%eax)
    add $1, %eax
    sub $1, %ecx
    cmp $0, %ecx
    jne again2
    mov %edi, %eax
nextpage:
    sub $1, %edx
    cmp $0, %edx
    je  finished
    add $2, %eax
    jmp again
finished:
    call HwSetup

    # create 64-bit page tables
    call CreatePageDir             # task1
    mov %eax, %cr3

    mov %cr4, %eax
    bts $5, %eax                   # Physical Address extension bit
    bts $7, %eax                   # Enable global pages
    mov %eax, %cr4

    mov $0xc0000080, %ecx          # These 2 instructions read the Extended Feature
    rdmsr                          # Enable register
    bts $0, %eax                   # This enables the syscall/sysret instructions
    bts $8, %eax                   # bit 8 of this register is Long Mode Enable (but we won't
    wrmsr                          # actually be in Long Mode until paging is enabled)

    mov %cr0, %eax
    bts $16, %eax                  # Enable paging
    bts $31, %eax                  # Enable write protection
    mov %eax, %cr0

    jmp $code64, $start64