From 92d5875f59c33bff46bcd74ff259a004255de559 Mon Sep 17 00:00:00 2001
From: Toni Wilen <twilen@winuae.net>
Date: Sun, 11 Oct 2020 20:17:55 +0300
Subject: [PATCH] Return random data if disk DMA started with selected drive
 but motor turned off.

---
 disk.cpp | 46 +++++++++++++++++++++++++++++++---------------
 1 file changed, 31 insertions(+), 15 deletions(-)

diff --git a/disk.cpp b/disk.cpp
index 3b31b695..cd7a517a 100644
--- a/disk.cpp
+++ b/disk.cpp
@@ -3633,21 +3633,6 @@ static int doreaddma (void)
 	return 0;
 }
 
-static void disk_doupdate_read_nothing(int floppybits)
-{
-	while (floppybits >= get_floppy_speed ()) {
-		word <<= 1;
-		doreaddma ();
-		if ((bitoffset & 7) == 7) {
-			dskbytr_val = word & 0xff;
-			dskbytr_val |= 0x8000;
-		}
-		bitoffset++;
-		bitoffset &= 15;
-		floppybits -= get_floppy_speed ();
-	}
-}
-
 static void wordsync_detected(bool startup)
 {
 	dsksync_cycles = get_cycles() + WORDSYNC_TIME * CYCLE_UNIT;
@@ -3674,6 +3659,37 @@ static void wordsync_detected(bool startup)
 	}
 }
 
+static void disk_doupdate_read_nothing(int floppybits)
+{
+	while (floppybits >= get_floppy_speed()) {
+		bool skipbit = false;
+		word <<= 1;
+		word |= (uaerand() & 0x1000) ? 1 : 0;
+		doreaddma();
+		// MSBSYNC
+		if (adkcon & 0x200) {
+			if ((word & 0x0001) == 0 && bitoffset == 0) {
+				word = 0;
+				skipbit = true;
+			}
+			if ((word & 0x0001) == 0 && bitoffset == 8) {
+				word >>= 1;
+				skipbit = true;
+			}
+		}
+		if (!skipbit && (bitoffset & 7) == 7) {
+			dskbytr_val = word & 0xff;
+			dskbytr_val |= 0x8000;
+		}
+		if (!(adkcon & 0x200) && word == dsksync) {
+			wordsync_detected(false);
+		}
+		bitoffset++;
+		bitoffset &= 15;
+		floppybits -= get_floppy_speed();
+	}
+}
+
 static void disk_doupdate_read (drive * drv, int floppybits)
 {
 	/*
-- 
2.47.3