From c8bbd2e9eeecdd1160f0da7c2620c4b718eca915 Mon Sep 17 00:00:00 2001
From: "Chloe M." <chloe@aptel.org>
Date: Thu, 20 Aug 2026 06:55:41 +0000
Subject: boot/amd64: mbr: Relocate self to 0x7A00

Signed-off-by: Chloe M. <chloe@aptel.org>
---
 boot/arch/amd64/mbr/mbr.S | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/boot/arch/amd64/mbr/mbr.S b/boot/arch/amd64/mbr/mbr.S
index 236f9f1..4c8c993 100644
--- a/boot/arch/amd64/mbr/mbr.S
+++ b/boot/arch/amd64/mbr/mbr.S
@@ -1,19 +1,38 @@
 /* Segment constants */
-#define BOOT_SEG 0x7C0      /* Segment we reside on startup */
+#define BOOT_SEG       0x7C0    /* Segment we reside on startup */
+#define BOOT_RELOC_SEG 0x7A0    /* Relocation segment */
 
     .code16
 _start:
     /* Normalize CS + other selectors */
     ljmp $BOOT_SEG,$1f
 1:  xor %ax, %ax
-    mov %ax, %ds
     mov %ax, %ss
-    mov %ax, %es
     mov %ax, %gs
     mov %ax, %fs
-2:  cli
+
+    /*
+     * The second stage bootloader expects to be loaded to 0x7C00 but since we are
+     * already executing at this address we'll need to relocate ourselves first before
+     * we clobber it.
+     */
+    mov $BOOT_SEG, %ax
+    mov %ax, %ds
+    mov $BOOT_RELOC_SEG, %ax
+    mov %ax, %es
+    xor %si, %si
+    xor %di, %di
+    mov $0x200, %cx
+    cld
+    rep movsb
+
+    /* Jump to our relocated self */
+    ljmp $BOOT_RELOC_SEG,$reloc
+
+reloc:
+    cli
     hlt
-    jmp 2b
+    jmp reloc
 
     .org 510
     .byte 0x55
-- 
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