From f86617e750146f454fcca1d2b324c36f2dbc0020 Mon Sep 17 00:00:00 2001
From: pantonshire <tom@tomandtally.co.uk>
Date: Thu, 4 Jul 2024 09:54:17 +0100
Subject: [PATCH] bounds check gpt starting lba

---
 boot0.s | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/boot0.s b/boot0.s
index 79109c7..847b8c4 100644
--- a/boot0.s
+++ b/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: