bounds check gpt starting lba

boot0.s

@@ -86,14 +86,22 @@ bits 16
   repe cmpsb
   jne .panic
 
-  ; mov word fs:[0x0000], 0xc048
-  ; mov word fs:[0x0002], 0xc069
-  mov al, [0x7e00]
-  mov byte fs:[0x0000], al
-  mov al, [0x7e01]
-  mov byte fs:[0x0002], al
-  mov al, [0x7e02]
-  mov byte fs:[0x0004], al
+  ; Ensure the 8-byte starting LBA fits in 16 bits
+  mov di, 0x7e00 ; the rep increments di so we need to reset it
+  xor bx, bx
+  mov ax, [di + 0x4a]
+  or bx, ax
+  mov ax, [di + 0x4c]
+  or bx, ax
+  mov ax, [di + 0x4e]
+  or bx, ax
+  jnz .panic
+
+  mov ax, [di + 0x48]
+
+  mov word fs:[0x0000], 0xc048
+  mov word fs:[0x0002], 0xc069
+  
   hlt
 
 .panic: