diff --git a/ponydos.asm b/ponydos.asm
index 3934af8..a5a71e3 100644
--- a/ponydos.asm
+++ b/ponydos.asm
@@ -195,8 +195,13 @@ modify_sector:
 	push ax
 	push cx
 	push dx
+	push ds
 
-	mov cx, 18
+	xor cx, cx
+	mov ds, cx
+
+	;mov cx, 18
+	mov cl, 18
 	div cx
 
 	; cl = sector (1…18)
@@ -212,7 +217,7 @@ modify_sector:
 	mov ch, al
 
 	; dl = drive number
-	mov dl, [boot_disk]
+	mov dl, [ds:boot_disk]
 
 	.retry:
 	mov ax, 0x0201 ; read/write one sector
@@ -220,6 +225,7 @@ modify_sector:
 	int 0x13
 	jc .error
 
+	pop ds
 	pop dx
 	pop cx
 	pop ax
@@ -240,7 +246,7 @@ modify_sector:
 wallpaper_name: db 'wallpaper.bin', 0
 
 ; in:
-;  es:si = file name
+;  ds:si = file name
 ; out:
 ;  ax = LBA of first sector, 0 if no space left
 ;  cx = length in sectors
@@ -248,8 +254,11 @@ open_file:
 	push si
 	push di
 	push bx
+	push es
 
 	; Stolen from https://stackoverflow.com/a/72746473, get strlen in cx
+	mov cx, ds
+	mov es, cx
 	mov di, si
 	mov cx, -1
 	xor ax, ax
@@ -257,6 +266,7 @@ open_file:
 	not cx
 	dec cx
 
+	mov es, ax
 	;mov ax, 1
 	mov al, 1
 	mov bx, DIRENTS
@@ -266,7 +276,7 @@ open_file:
 	mov ax, 2
 	mov di, bx
 	.loop:
-		cmp word [di], 0
+		cmp word [es:di], 0
 		je .create_file
 
 		inc di
@@ -290,8 +300,9 @@ open_file:
 	xor ax, ax
 	; Return with mangled cx
 	.success:
-	mov cx, [di - 2]
+	mov cx, [es:di - 2]
 	.return:
+	pop es
 	pop bx
 	pop di
 	pop si
@@ -299,7 +310,7 @@ open_file:
 
 	.create_file:
 		; TODO: zero out the sector for this file?
-		inc word [di]
+		inc word [es:di]
 		inc di
 		inc di
 		rep movsb