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X-DDOS is a floppy disc and hard disc operating system from the german company Dobbertin Industrie-Elektronik. It was first released in 1988. It provides 0.7 MB floppy disc formats for B drives and allows to access the Dobbertin Harddisc HD20. X-DDOS is delivered as a 16 KB EPROM with a lot of software and a manual.

X-DDOS has the ROM name 'XDDOS ROM' but shows a sign-on message of "X-DDOS 2.10 (C)1990 Dobbertin GmbH"

  • X-DDOS can be used as a rom 7 replacement.
  • X-DDOS can autoboot CPM if it's ROM 0.
  • X-DDOS has a checksum check

RSX commands

(same operation as AMSDOS): |A, |B, |CPM, |DISC, |DISC.IN, |DISC.OUT, |TAPE, |TAPE.IN, |TAPE.OUT, |DRIVE, |USER, |ERA, |REN



X-DDOS 2 extras: |COPY (great command to copy files from any drive/partition/user-number to any drive/partition/user-number. That can be users from 0 to 15 and A, B, C, D, E, F and G where C = RAMdisc and D, E, F, G = Harddisc)

|HELP,<number> - display list of RSX commands for the given rom <number>

|X,|XDRIVE - swap logical A and B. Normally logical drive A accesses unit 0 and logical drive B accesses unit 1. Therefore using |X:|A will access unit 1 as logical drive A and unit 0 as logical drive B.

|CPM or |CPM,<drive> - Boot CPM from drive A, or chosen drive. A=0,B=1,D=3,E=4,F=5,G=6. Can boot from D,E,F,G if HD is detected. Can't boot from drive C.

Booting CPM

The process to boot cpm is the same on floppy discs and hard disk and uses a common read sector function that re-directs to low-level functions based on the chosen drive.

Using |CPM,<drive> you can define which drive to boot from.

When |CPM is executed:

  • on 2.10 and later, the hard disc is detected and be4b is set to ff if detected, 00 if not detected.
  • the bios jumpblock is copied to be80 from c07b.
  • iobyte (0003) is set to 81
  • the current drive is written to &0004. A = 0, B = 1, D=3 etc.
  • 'enter firmware' (&ad33) jp is setup.
  • the XDPB is queried to find the location of the boot sector
  • the boot sector is loaded to &100.
  • if the boot sector has code, then the boot sector is executed from &100.

Floppy Discs

X-DDOS supports B drives on all CPCs with 80 tracks and 2 sides. It's compatible to the 704 KB Vortex format of the Vortex drives. Additionally it provides the Dobbertin Data- and System format (about 0.7 MB).

X-DDOS supports CP/M and CP/M Plus on 0.7 MB B-drives and can boot from either disc drive.

Hard Disc HD 20

  • XD-DOS 2.0 and 2.10 support the HD-20 hard disc drive.
  • XD-DOS auto detects if the HD20 is connected. A message "* Harddisc not active *" is displayed if it is not. This is shown below the ROM sign on message.
  • XD-DOS maps the HD20 to four partitions with 5120 KB each. Each partition is accessed by using the logical drive letters D, E, F and G.
  • The integration of X-DDOS with the firmware allows to load some games and demos from hard disc.
  • CP/M and CP/M Plus can be booted from hard disc.