Keep empty floppy drives in no-media state

docs/internals/chipset.md

@@ -210,6 +210,10 @@ armed between disk-word boundaries; WORDSYNC is the explicit mode that
 realigns framing to a matched sync word before transfer. Supported image
 formats: ADF (read/write), gzip ADZ, single file ZIP, DMS (decompressed by
  `dms.rs`), UAE extended ADF, and read-only SCP flux images.
+Connected mechanisms with no media keep the active-low disk-change line
+asserted; a step pulse only clears that latch once media is actually
+present, so guest software sees a no-disk condition rather than unreadable
+track data.
 
 Standard ADF and AmigaDOS tracks are synthesized as one PAL-sized
 revolution: 11 sectors occupy 5984 MFM words, and the generated revolution

src/floppy.rs

@@ -203,6 +203,7 @@ pub struct FloppyController {
 impl Default for FloppyController {
     fn default() -> Self {
         let mut drives: [FloppyDrive; 4] = std::array::from_fn(|_| FloppyDrive::default());
+        drives[0].assert_no_media_change();
         for drive in drives.iter_mut().skip(1) {
             drive.external_id = 0;
         }
@@ -1454,6 +1455,9 @@ impl FloppyDrive {
         self.external_id_bit = 0;
         self.external_id_hold_deactivate = false;
         self.write_protected_sense = true;
+        if self.image.is_none() && self.external_id != 0 {
+            self.assert_no_media_change();
+        }
         self.status_settle_cck = DISK_STATUS_SETTLE_CCK;
     }
 
@@ -1572,6 +1576,11 @@ impl FloppyDrive {
         }
     }
 
+    fn assert_no_media_change(&mut self) {
+        self.disk_change = true;
+        self.disk_change_sense = true;
+    }
+
     /// Advance disk rotation by `cck` cycles, returning whether an index
     /// pulse occurred and whether any word crossed matched `sync_word` (the
     /// free-running DSKSYNC comparator). `sync_word` is `None` when the
@@ -3843,6 +3852,52 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn empty_internal_drive_keeps_disk_change_asserted_until_media_steps() -> Result<()> {
+        let media = temp_adf()?;
+        let mut ctrl = FloppyController::default();
+        let selected = !CIAB_DSKSEL0;
+        let inward_high = selected & !CIAB_DSKDIREC;
+
+        ctrl.write_prb(inward_high);
+        let status = ctrl.cia_a_status_bits();
+        assert_eq!(status & CIAA_DSKCHANGE, 0);
+        assert_ne!(status & CIAA_DSKRDY, 0);
+
+        ctrl.write_prb(inward_high & !CIAB_DSKSTEP);
+        ctrl.tick(DISK_STATUS_SETTLE_CCK, 0, &mut []);
+        assert_eq!(ctrl.cia_a_status_bits() & CIAA_DSKCHANGE, 0);
+
+        ctrl.insert_disk_image(0, media.clone(), true)?;
+        assert_eq!(ctrl.cia_a_status_bits() & CIAA_DSKCHANGE, 0);
+
+        ctrl.write_prb(inward_high);
+        wait_step_floor(&mut ctrl);
+        ctrl.write_prb(inward_high & !CIAB_DSKSTEP);
+        ctrl.tick(DISK_STATUS_SETTLE_CCK, 0, &mut []);
+        assert_ne!(ctrl.cia_a_status_bits() & CIAA_DSKCHANGE, 0);
+
+        let _ = fs::remove_file(media);
+        Ok(())
+    }
+
+    #[test]
+    fn connected_empty_external_drive_keeps_disk_change_asserted_through_step_poll() {
+        let mut ctrl = FloppyController::default();
+        ctrl.set_connected_drives([true, true, false, false]);
+
+        let selected = drive_select_prb(1, false);
+        let inward_high = selected & !CIAB_DSKDIREC;
+        ctrl.write_prb(inward_high);
+        let status = ctrl.cia_a_status_bits();
+        assert_eq!(status & CIAA_DSKCHANGE, 0);
+        assert_ne!(status & CIAA_DSKRDY, 0);
+
+        ctrl.write_prb(inward_high & !CIAB_DSKSTEP);
+        ctrl.tick(DISK_STATUS_SETTLE_CCK, 0, &mut []);
+        assert_eq!(ctrl.cia_a_status_bits() & CIAA_DSKCHANGE, 0);
+    }
+
     #[test]
     fn write_protect_line_settles_after_inserted_disk_change() -> Result<()> {
         let protected = temp_adf()?;