Add compatibility checks

1 files changed, 65 insertions, 0 deletions

diff --git a/kernel/src/boot.asm b/kernel/src/boot.asm
index a10b14c..de832f8 100644
--- a/kernel/src/boot.asm
+++ b/kernel/src/boot.asm
@@ -5,10 +5,75 @@ 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: