From bd11ed9a6f92d097972a4e3414ee03c2fc1bbaca Mon Sep 17 00:00:00 2001
From: Philipp Oppermann <dev@phil-opp.com>
Date: Thu, 13 Aug 2015 13:09:13 +0200
Subject: [PATCH] Check for Multiboot, CPUID, and Long Mode

---
 src/arch/x86_64/boot.asm | 47 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/src/arch/x86_64/boot.asm b/src/arch/x86_64/boot.asm
index c2f8dd7b..1b80840b 100644
--- a/src/arch/x86_64/boot.asm
+++ b/src/arch/x86_64/boot.asm
@@ -19,6 +19,10 @@ bits 32
 start:
     mov esp, stack_top
 
+    call check_multiboot
+    call check_cpuid
+    call check_long_mode
+
     ; print `OK` to screen
     mov dword [0xb8000], 0x2f4b2f4f
     hlt
@@ -32,6 +36,49 @@ error:
     mov byte  [0xb800a], al
     hlt
 
+; Throw error 0 if eax doesn't contain the Multiboot 2 magic value (0x36d76289).
+check_multiboot:
+    cmp eax, 0x36d76289
+    jne .no_multiboot
+    ret
+.no_multiboot:
+    mov al, "0"
+    jmp error
+
+; Throw error 1 if the CPU doesn't support the CPUID command.
+check_cpuid:
+    pushfd               ; Store the FLAGS-register.
+    pop eax              ; Restore the A-register.
+    mov ecx, eax         ; Set the C-register to the A-register.
+    xor eax, 1 << 21     ; Flip the ID-bit, which is bit 21.
+    push eax             ; Store the A-register.
+    popfd                ; Restore the FLAGS-register.
+    pushfd               ; Store the FLAGS-register.
+    pop eax              ; Restore the A-register.
+    push ecx             ; Store the C-register.
+    popfd                ; Restore the FLAGS-register.
+    xor eax, ecx         ; Do a XOR-operation on the A-register and the C-register.
+    jz .no_cpuid         ; The zero flag is set, no CPUID.
+    ret                  ; CPUID is available for use.
+.no_cpuid:
+    mov al, "1"
+    jmp error
+
+; 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, 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.
+    ret
+.no_long_mode:
+    mov al, "2"
+    jmp error
+
 section .bss
 stack_bottom:
     resb 64