From 5584f6adcf704cc0dd4ffa390935a4afc2545d45 Mon Sep 17 00:00:00 2001
From: tkchia <tkchia@users.noreply.github.com>
Date: Sun, 14 Aug 2022 23:32:58 +0800
Subject: [PATCH] Improve detection of boot media on bare metal (#535)

hello.com now starts up correctly on QEMU when run as a hard
disk image.
---
 ape/ape.S | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/ape/ape.S b/ape/ape.S
index e0cc12183..6a9a08b3e 100644
--- a/ape/ape.S
+++ b/ape/ape.S
@@ -301,7 +301,21 @@ pc:	cld
 //	@since	IBM Personal Computer XT
 dsknfo:	push	%bx
 1:	push	%dx
-	mov	$0x08,%ah		# get disk params
+	mov	$0x16,%ah		# make sure there is disk in drive,
+	int	$0x13			# by querying change-line status
+	jnc	2f
+	xor	%ax,%ax			# if error or change line active,
+	int	$0x13			# do a reset...
+	jc	9f
+	mov	$0x0201,%ax		# ...then do a read, to confirm that
+	mov	$0x0001,%cx		# there is disk in drive
+	mov	$0,%dh			# (verify (%ah = 4) does not work
+	mov	$IMAGE_BASE_REAL>>4,%bx	# under QEMU, which always returns
+	mov	%bx,%es			# success)
+	xor	%bx,%bx
+	int	$0x13
+	jc	9f
+2:	mov	$0x08,%ah		# get disk params
 	int	$0x13
 	jc	9f
 	mov	%cl,%bh