Changeset 94649 in vbox for trunk/src/VBox/Devices/Storage

Timestamp:

Apr 20, 2022 10:30:03 AM (3 years ago)

Author:

vboxsync

svn:sync-xref-src-repo-rev:

151012

Message:

DevFdc: Only use the DOR to select drive, the drive selection bits in FDC command bytes have no real function (see bugref:6793).

File:

• trunk/src/VBox/Devices/Storage/DevFdc.cpp (modified) (6 diffs)

trunk/src/VBox/Devices/Storage/DevFdc.cpp

@@ -86 +86 @@
 /********************************************************/
 /* Floppy drive emulation                               */
+
+/* Drive selection note:
+ * For many commands, the FDC can select one of four drives through the
+ * second command byte. The Digital Output Register (DOR) can also select
+ * one of four drives. On PCs, the FDC drive selection is ignored, but
+ * should be reflected back in command status. Only the DOR drive selection
+ * is effective; on old PCs with a discrete NEC uPD765 or similar, the FDC
+ * drive selection signals (US0/US1) are not connected at all.
+ * NB: A drive is actually selected only when its motor on bit in the DOR
+ * is also set. It is possible to have no drive selected.
+ *
+ * The FDC cur_drv field tracks the drive the FDC thinks is selected, but
+ * the DOR is used for actual drive selection.
+ */
 
 #define GET_CUR_DRV(fdctrl) ((fdctrl)->cur_drv)
@@ -599 +613 @@
 enum {
 #if MAX_FD == 4
-    FD_DOR_SELMASK  = 0x03,
+    FD_DRV_SELMASK  = 0x03,
 #else
-    FD_DOR_SELMASK  = 0x01,
+    FD_DRV_SELMASK  = 0x01,
 #endif
+};
+
+enum {
+    FD_DOR_SELMASK  = 0x03, /* Always two bits regardless of FD_DRV_SELMASK. */
     FD_DOR_nRESET   = 0x04,
     FD_DOR_DMAEN    = 0x08,
@@ -888 +906 @@
 static fdrive_t *get_cur_drv(fdctrl_t *fdctrl)
 {
-    switch (fdctrl->cur_drv) {
+    switch (fdctrl->dor & FD_DRV_SELMASK) {
         case 0: return drv0(fdctrl);
         case 1: return drv1(fdctrl);
@@ -924 +942 @@
     uint32_t retval = fdctrl->dor;
 
-    /* Selected drive */
-    retval |= fdctrl->cur_drv;
     FLOPPY_DPRINTF("digital output register: 0x%02x\n", retval);
 
@@ -963 +979 @@
         }
     }
-    /* Selected drive */
-    fdctrl->cur_drv = value & FD_DOR_SELMASK;
 
     fdctrl->dor = value;
@@ -1057 +1071 @@
      */
     if (fdctrl_media_changed(get_cur_drv(fdctrl))
-     && (fdctrl->dor & (0x10 << fdctrl->cur_drv)))
+     && (fdctrl->dor & (0x10 << (fdctrl->dor & FD_DOR_SELMASK))))
         retval |= FD_DIR_DSKCHG;
     if (retval != 0)