Indicate multiboot version on boot

src/kernel/boot/boot.S

@@ -7,11 +7,27 @@
 _start:
 cli
 
-# Save the multiboot magic number
 mov ecx, eax
 
-// Print a red exclamation mark in top left corner
+// Print a red digit in the upper left corner to indicate the multiboot version
-mov eax, 0x0421
+// If not loaded by multiboot, print an exclamation mark
-mov short [0xB8000], ax
+cmp eax, MBOOT1_REPLY
-// and loop infinitely
+je mboot1
+cmp eax, MBOOT2_REPLY
+je mboot2
+mov eax, 0x0421 // Red exclamation point
+jmp print
+
+mboot1:
+mov eax, 0x0431 // Red 1
+jmp print
+
+mboot2:
+mov eax, 0x0432 // Red 2
+jmp print
+
+print:
+mov short [0xB8000], ax // Place in upper left corner
+
+// loop infinitely
 jmp $