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.global _start
.extern long_mode_start
.section .text
.code32
// Entry point. It puts the machine into a consistent state and starts long mode.
_start:
mov $0x80000, %esp // Setup the stack.
push %ebx // Pass multiboot info structure.
push %eax // Pass multiboot magic code.
call check_multiboot
call check_cpuid
call check_long_mode
call setup_page_tables
call enable_paging
lgdt (gdtdesc)
jmpl $0x8, $long_mode_start
//jmp gdt64.code_segment:long_mode_start
call halt
halt:
hlt
check_multiboot:
cmp $0x36d76289, %eax
jne .no_multiboot
ret
.no_multiboot:
mov $'M', %al
jmp error
check_cpuid:
pushfd
pop %eax
mov %eax, %ecx
xor $(1 << 21), %eax
push %eax
popfd
pushfd
pop %eax
push %ecx
popfd
cmp %eax, %ecx
je .no_cpuid
ret
.no_cpuid:
mov $'C', %al
jmp error
check_long_mode:
mov $0x80000000, %eax
cpuid
cmp $0x80000001, %eax
jb .no_long_mode
mov $0x80000001, %eax
cpuid
test $(1 << 29), %edx
jz .no_long_mode
ret
.no_long_mode:
mov $'L', %al
jmp error
setup_page_tables:
mov $page_table_l3, %eax
or $0b11 , %eax // present, writable
mov %eax, (page_table_l4)
mov $page_table_l2, %eax
or $0b11, %eax // present, writable
mov %eax, (page_table_l3)
mov $0, %ecx // counter
.loop:
mov $0x200000, %eax // 2MiB
mul %ecx
or $0b10000011, %eax // present, writable, huge page
mov %eax, +page_table_l2(,%ecx,8)
inc %ecx // increment counter
cmp $512, %ecx // checks if the whole table is mapped
jne .loop // if not, continue
ret
enable_paging:
// pass page table location to cpu
mov $page_table_l4, %eax
mov %eax, %cr3
// enable PAE
mov %cr4, %eax
or $(1 << 5), %eax
mov %eax, %cr4
// enable long mode
mov $0xC0000080, %ecx
rdmsr
or $(1 << 8), %eax
wrmsr
// enable paging
mov %cr0, %eax
or $(1 << 31), %eax
mov %eax, %cr0
ret
error:
// // print "ERR: X" where X is the error code
movl $0x4f524f45, 0xb8000
movl $0x4f3a4f52, 0xb8004
movl $0x4f204f20, 0xb8008
movb %al, 0xb800a
hlt
.section .bss
.align 4096
page_table_l4:
.skip 4096
page_table_l3:
.skip 4096
page_table_l2:
.skip 4096
stack_bottom:
.skip 4096 * 4
stack_top:
.section .rodata
.p2align 2 # force 4 byte alignment
gdt64:
.quad 0 // zero entry
.code_segment:
.quad (1 << 43) | (1 << 44) | (1 << 47) | (1 << 53) # code segment
gdtdesc:
.word (gdtdesc - gdt64 - 1) # sizeof(gdt) - 1
.long gdt64 # address gdt
//# Bootstrap GDT
//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
//
//section .rodata
//gdt64:
// dq 0 ; zero entry
//.code_segment: equ $ - gdt64
// dq (1 << 43) | (1 << 44) | (1 << 47) | (1 << 53) ; code segment
//.pointer:
// dw $ - gdt64 - 1 ; length
// dq gdt64 ; address
//
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