--- z80pack-1.24/iodevices/imsai-fif.c	2014-08-25 21:50:21.000000000 +0200
+++ z80pack/iodevices/imsai-fif.c	2014-09-17 20:10:49.000000000 +0200
@@ -16,6 +16,7 @@
  * 02-MAR-14 improvements
  * 23-MAR-14 got all 4 disk drives working
  *    AUG-14 some improvements after seeing the original IMSAI CP/M 1.3 BIOS
+ * 17-SEP-14 FDC error 9 for DMA overrun, as reported by Alan Cox for cpmsim
  */
 
 #include <unistd.h>
@@ -135,6 +136,7 @@
  *	6 - seek error
  *	7 - read error
  *	8 - write error
+ *	9 - DMA overrun > 0xffff
  *	15 - invalid command
  *
  *	These error codes will abort disk I/O, but this are not the ones
@@ -178,6 +180,12 @@
 	sector = *(ram + addr + DD_SECTOR);
 	dma_addr = (*(ram + addr + DD_DMAH) * 256) + *(ram + addr + DD_DMAL);
 
+	/* check for DMA overrun */
+	if (dma_addr > 0xff80) {
+		*(ram + addr + DD_RESULT) = 9;
+		return;
+	}
+
 	/* convert IMSAI unit bit to internal disk no */
 	switch (unit) {
 	case 1:	/* IMDOS drive A: */