From eabb1d9fa9c444d7e29d410ce5560c868aae1575 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Mon, 30 Mar 2020 20:41:32 +0000
Subject: [PATCH] Properly set up registers when chainloading from boot sector

---
 boot/fat32.asm | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/boot/fat32.asm b/boot/fat32.asm
index 437c53b..60dad99 100644
--- a/boot/fat32.asm
+++ b/boot/fat32.asm
@@ -1,5 +1,6 @@
 	; Bootsector for FAT32 filesystems
 	org 0x0800
+	cpu 8086
 	jmp short init
 	nop
 
@@ -28,12 +29,17 @@ dn:	equ ( $$ + 0x040 )
 	times (0x5A - ($-$$)) db 0
 
 init:	xor ax, ax
-	mov ds, ax
-	mov es, ax
 	mov ss, ax
 	mov sp, 0x7C00
 
-	mov si, sp
+	; save potential partition table entry
+	push ds
+	push si
+
+	mov ds, ax
+	mov es, ax
+
+	mov si, 0x7C00
 	mov di, 0x0800
 	mov cx, 0x0100
 	rep movsw
@@ -101,7 +107,7 @@ read_:	; read sector DX:AX from disk
 
 	mov si, sp
 	mov ah, 0x42
-	mov dl, 0x80
+	mov dl, [dn]
 	stc
 	int 0x13
 	jc dskerr
@@ -213,7 +219,8 @@ loadf:	call search
 	mov dx, [si+0x14]
 	jmp load
 
-main:	; load root directory
+main:	mov [dn], dl
+	; load root directory
 	call loadr
 	; search for first system directory
 	mov ah, 0x14
@@ -222,6 +229,20 @@ main:	; load root directory
 	mov ah, 0x04
 	call loadf
 	; jump
+	xor ax, ax
+	xor cx, cx
+	xor dx, dx
+	xor bx, bx
+	xor di, di
+	xor bp, bp
+
+	; restore drive number
+	mov dl, [dn]
+
+	; restore potential partition table
+	pop si
+	pop ds
+
 	jmp 0:0x7C00
 
 end:	call msg