From d76ad3e636bdcfab548e7cc7cb73ca809db6cb94 Mon Sep 17 00:00:00 2001
From: "Chloe M." <chloe@aptel.org>
Date: Thu, 20 Aug 2026 09:51:25 +0000
Subject: boot/amd64: mbr: Find second stage image in RDAR

Signed-off-by: Chloe M. <chloe@aptel.org>
---
 boot/arch/amd64/mbr/mbr.S | 40 ++++++++++++++++++++++++++++++++++++----
 1 file changed, 36 insertions(+), 4 deletions(-)

(limited to 'boot/arch/amd64/mbr/mbr.S')

diff --git a/boot/arch/amd64/mbr/mbr.S b/boot/arch/amd64/mbr/mbr.S
index e6c84fa..1ce1da1 100644
--- a/boot/arch/amd64/mbr/mbr.S
+++ b/boot/arch/amd64/mbr/mbr.S
@@ -5,6 +5,8 @@
 /* Constants */
 #define RDAR_MAGIC ">RDAR!"
 #define RDAR_MAGIC_LEN 6
+#define BOOT_PATH "boot/biosboot.bin"
+#define BOOT_PATH_LEN 17
 
     .code16
 _start:
@@ -54,16 +56,12 @@ reloc:
 
     /* Get stage2 */
     call find_stage2
-
 1:  cli
     hlt
     jmp 1b
 
 /*
  * Locate stage2 on the disk
- *
- * TODO: This only verifies the magic number is correct, this should
- *       be completed.
  */
 find_stage2:
     push %ds
@@ -72,6 +70,7 @@ find_stage2:
     /* Verify that magic is correct */
     mov $BOOT_SEG, %ax
     mov %ax, %ds
+    xor %bx, %bx    /* Tracks %SI */
     xor %si, %si
     mov $BOOT_RELOC_SEG, %ax
     mov %ax, %es
@@ -80,9 +79,40 @@ find_stage2:
     rep cmpsb
     jne .bad_magic
 
+    /* Iterate every data descriptor */
+    clc              /* Carry set if found */
+1:  movw %ds:6, %cx  /* Add data descriptor size */
+    mov %bx, %si
+    mov %si, %dx
+    add %cx, %bx
+    mov $rdar_magic, %di
+    mov $RDAR_MAGIC_LEN, %cx
+    rep cmpsb
+    jne .not_found
+
+    /* Does the path match? */
+    mov %dx, %si
+    add $8, %si
+    mov $stage2_path, %di
+    mov $BOOT_PATH_LEN, %cx
+    rep cmpsb
+    jne 1b
+
+    stc
     pop %es
     pop %ds
+    jnc .not_found
+
+    /* TODO: On found, return the offset to the data in %CX */
     ret
+.not_found:
+    pop %ds
+    mov $not_found_msg, %si
+    call putstr
+1:  cli
+    hlt
+    jmp 1b
+
 .bad_magic:
     /* Restore %DS */
     mov $BOOT_RELOC_SEG, %ax
@@ -157,8 +187,10 @@ dap_lba_high:
 drive_num: .byte 0x00
 boot_msg: .ascii "booting\r\n\0"
 bad_magic_msg: .ascii "bad RDAR magic\r\n\0"
+not_found_msg: .ascii "stage 2 not found\r\n\0"
 disk_err_msg: .ascii "\r\n!!!disk error!!!\r\n\0"
 rdar_magic: .ascii RDAR_MAGIC
+stage2_path: .ascii BOOT_PATH
 
     .org 510
     .byte 0x55
-- 
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