diff --git a/ponydos.asm b/ponydos.asm
index 6cf64ff..d58a2d4 100644
--- a/ponydos.asm
+++ b/ponydos.asm
@@ -283,11 +283,11 @@ modify_sectors:
 modify_sector:
 	pusha
 
-	mov cx, 18
-	div cx
+	mov cl, 18
+	div cl
 
 	; cl = sector (1…18)
-	mov cl, dl
+	mov cl, ah
 	inc cl
 
 	; dh = head (0…1)
@@ -295,7 +295,7 @@ modify_sector:
 	and dh, al
 
 	; ch = cylinder
-	shr ax, 1
+	shr al, 1
 	mov ch, al
 
 	; dl = drive number