diff --git a/src/arch/x86_64/boot.asm b/src/arch/x86_64/boot.asm
index 58bbed04..64d0c0db 100644
--- a/src/arch/x86_64/boot.asm
+++ b/src/arch/x86_64/boot.asm
@@ -148,14 +148,17 @@ check_cpuid:
 
 ; Throw error 2 if the CPU doesn't support Long Mode.
 check_long_mode:
-    mov eax, 0x80000000    ; Set the A-register to 0x80000000.
-    cpuid                  ; CPU identification.
-    cmp eax, 0x80000001    ; Compare the A-register with 0x80000001.
-    jb .no_long_mode       ; It is less, there is no long mode.
-    mov eax, 0x80000001    ; Set the A-register to 0x80000001.
-    cpuid                  ; CPU identification.
-    test edx, 1 << 29      ; Test if the LM-bit, which is bit 29, is set in the D-register.
-    jz .no_long_mode       ; They aren't, there is no 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
+
+    ; use extended info to test if long mode is available
+    mov eax, 0x80000001    ; argument for extended processor info
+    cpuid                  ; returns various feature bits in ecx and edx
+    test edx, 1 << 29      ; test if the LM-bit is set in the D-register
+    jz .no_long_mode       ; If it's not set, there is no long mode
     ret
 .no_long_mode:
     mov al, "2"