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ORG 0x7c00
BITS 16
CODE_SEG equ gdt_code - gdt_start ; 0x8
DATA_SEG equ gdt_data - gdt_start ; 0x10
_start:
jmp short start
nop
; Setup dummy BIOS Parameter block so a dumb BIOS does not corrupt us
; if it starts messing up with the parameter block
times 33 db 0
start:
; Set CS to 0 with label step2 as its beginning offset
jmp 0:step2
step2:
; Setup segments
cli ; Disable interrupts
mov ax, 0
mov ds, ax
mov es, ax
mov ss, ax
mov sp, 0x7c00 ; Stack segment starts right before ds
; and grows down to 0x0. This works because
; Intel implements a full descending stack
sti ; Enable interrupts
.load_protected:
cli
lgdt[gdt_table]
mov eax, cr0
or eax, 0x1
mov cr0, eax
jmp CODE_SEG:start_32
; GDT table description
gdt_start:
gdt_null: ; First segment Descriptor is always 0
dd 0x0
dd 0x0
;Second segment descriptor... Code segment - 0x8
gdt_code:
dw 0xffff ; Segment limit 0-15
dw 0 ; Base address 0-15
db 0 ; Base 16-23
db 0x9a ; Access byte field 0b10011010
; We don't have a 4 bits type, so use a byte type
; to set both Segment limit high bits and flags
db 11001111b ; Segment limit 16-19
; Flags 0-3
db 0 ; Base address high bits 24-31
;Second segment descriptor... Data segment - 0x10
gdt_data:
dw 0xffff ; Segment limit 0-15
dw 0 ; Base address 0-15
db 0 ; Base 16-23
db 0x92 ; Access byte field 0b10010010
db 11001111b ; Segment limit 16-19
; Flags 0-3
db 0 ; Base address high bits 24-31
gdt_end:
gdt_table:
dw gdt_end - gdt_start - 1
dd gdt_start
[BITS 32]
; No access to BIOS from now on....
start_32:
; Set all segments to the same as the DATA_SEG
mov ax, DATA_SEG
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
; Set the stack pointer and base pointer further in mem
mov ebp, 0x00200000
mov esp, ebp
jmp $
; Fill in to the end and add bootloader signature
times 510 - ($ - $$) db 0
dw 0xAA55
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