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src/boot.s

@@ -42,16 +42,53 @@ gdt64_end:
 .code32
 .global _start
 _start:
-	mov stack_top, esp
+	mov esp, stack_top
 
+    // Multiboot will place a magic value in eax
+    // If this magic value is not present, then we might not have multiboot info in ebx,
+    // therefore we throw an error
 	cmp eax, 0x2BADB002
 	jne error
 
+    // Check if cpuid is available by flipping the 22-nth youngest bit
+    // in the eflags register and checking if the cpu flipped it back
+    pushfd
+    pop eax
+    mov ecx, eax
+    xor eax, 1 << 21
+    push eax
+    popfd
+    pushfd
+    pop eax
+    // If cpuid is available, eax should be different than ecx
+    xor eax, ecx
+    jz error
+    // Finally restore eflags register to the original value
+    push ecx
+    popfd
+
+    // Check if extended cpuid is available by calling cpuid with 0x80000000,
+    // If cpuid is available, eax will be greater than 0x80000000 after the call
+    mov eax, 0x80000000
+    cpuid
+    cmp eax, 0x80000001
+    jb error
+
+    // Check if long mode is available by calling cpuid with 0x80000001
+    // this will place the extended features of the cpu in edx
+    // The 30-nth youngest bit tells us if long mode is supported or not
+    mov eax, 0x80000001
+    cpuid
+    test edx, 1 << 29
+    jz error
+
+    // todo(nub31): setup paging
+    // todo(nub31): enter long mode
+
+    // Load global descriptor table which is set up for 64 bit
     lgdt [gdt64_descriptor]
 
-	push ebx
 	call kernel_main
-	add esp, 4
 	jmp error
 
 error: