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global start
section .text
bits 32
start:
mov esp, stack_top
call check_multiboot
call check_cpuid
call check_long_mode
; print 'OK'
mov dword [0xb8000], 0x2f4b2f4f
hlt
check_multiboot:
cmp eax, 0x36d76289
jne .no_multiboot
ret
.no_multiboot
mov al, "0"
jmp error
check_cpuid:
; Check if CPUID is supported by attempting to flip the ID bit (bit 21)
; in the FLAGS register. If we can flip it, CPUID is available.
; Copy EFLAGS in to EAX via stack
pushfd
pop eax
; Save the current flags
mov ecx, eax
; Flip the ID bit
xor eax, 1 << 21
; push eax to eflags
push eax
popfd
; Copy EAX to FLAGS via the Stack
pushfd
pop eax
; Restore FLAGS from the old version stored in ECX (i.e. flipping the
; ID bit back if it was ever flipped).
push ecx
popfd
; Check if the ID was changed
cmp eax, ecx
je .no_cpuid
ret
.no_cpuid:
mov al, "1"
jmp error
check_long_mode:
; test if extended processor info in available
mov eax, 0x80000000 ; implicit argument for cpuid
cpuid ; get highest supported argument
cmp eax, 0x80000001 ; it needs to be at least 0x80000001
jb .no_long_mode ; if it's less, the CPU is too old for long mode
; extended info about long mode
mov eax, 0x80000001 ; argument for the cpuid function
cpuid ; Cpu id
test edx, 1 << 29 ; check long mode availablity
jz .no_long_mode
ret
.no_long_mode
mov al, "2"
jmp error
; Prints 'ERR: ' and the given error code to the screen and halts
; parameter: error code letter (ascii) in al
error:
mov dword [0xb8000], 0x4f524f45
mov dword [0xb8004], 0x4f3a4f52
mov dword [0xb8008], 0x4f504f20
mov byte [0xb800a], al
hlt
section .bss
stack_bottom:
resb 64
stack_top:
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