Add a small stack

src/arch/x86_64/boot.asm

@@ -17,6 +17,8 @@ global start
 section .text
 bits 32
 start:
+    mov esp, stack_top
+
     ; print `OK` to screen
     mov dword [0xb8000], 0x2f4b2f4f
     hlt
@@ -29,3 +31,8 @@ error:
     mov dword [0xb8008], 0x4f204f20
     mov byte  [0xb800a], al
     hlt
+
+section .bss
+stack_bottom:
+    resb 64
+stack_top: