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Commodore PET
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The Commodore PET (Model : CBM 8096)

Disk Commands


The majority of personal computers equipped with floppy disk drives had disk interfaces based on the Shugart "standard", developed specifically for floppy disks. However, disk drives for Commodore PET/CBM computers are connected via the IEEE-488 interface, the notes on this page give a brief introduction to using disk drives with a PET/CBM computer, for more comprehensive instructions, refer to the Commodore DOS documentation.

The IEEE-488 interface was developed by Hewlett-Packard (HP) as the Hewlett-Packard Interface Bus (HP-IB), it was designed for the interconnection of test equipment, controllers and computers, allowing for the connection of up to 15 devices on a single bus segment. HP developed peripherals, including disk drives, for their own computers using HP-IB and Commodore chose the same bus design for use with PET/CBM computers.


CBM/PET Disk Concepts

IEEE-488 Device numbers allocated in PET/CBM computers are :-

Device No.

 Peripheral  Control / Interface
0  Keyboard  Kernel ROM (on system board)
1  Cassette Tape  Kernel ROM (on system board)
2  RS232 Interface  Kernel ROM (on system board)
3  Video Display  Kernel ROM (on system board)
4 to 15  Various I/O  Peripheral Bus

PET disk drive units are connected via the Peripheral bus and are equipped with either one or two floppy disk drives, the first disk drive unit is normally assigned device address 8, the device address is that of the controller inside the unit, it does not refer to a physical disk drive number.

The Commodore Disk Operating System (DOS) is somewhat unusual in that the DOS is not part of the computer operating system, rather, it is installed in the drive unit and executes from there in response to commands from the host system. The syntax used to communicate with the device is very much like file I/O operations used in other systems, e.g., "opening" a device, doing some operation and "closing" the device.

In order to execute commands in the drive unit, various parameters, including those below, must be set :

Parameter  Function Range
File number  A logical number used by the computer only, not by the drive unit 1 - 254*
Device number  Address number on the bus, the first drive unit is usually #8 4 - 15
Secondary address
  Communications channel to use for data transfer
  "Command Channel" for computer to controller commands
0 - 14*
Command string  Specifies disk device within a unit, filename, file type, access mode String

* The file number and communications channel are assigned by the user/programmer

CBM DOS supports the following file types :

File Type  Description
SEQ  Sequential file, must be read from start to finish
PRG  Similar to SEQ, used for all program files
REL  Relative file, supports indexing, individual 254 bytes records can be located individually
USR  User specified, identical to SEQ (limited use)
DEL  (Internal use)


Disk Commands - Examples

Using the information above, commands to the disk system can be sent from direct mode or programmatically, some examples are shown below :

File Access
Format OPEN <file number> , <device number> , <secondary address>, <command string>
Example OPEN 1, 8, 2, "0: FILENAME, S, W"
  open (action)  
   a sequential (S) file   (command string)
     called FILENAME   (command string)
       in Write (W) mode   (command string)
         from the first drive (0:)   (command string)
           in the device with address 8 (device number)  

           as stream 1

(file number)  

referencing its data as channel 2

(secondary address)  
Disk Status
Format OPEN <file number> , <device number> , <secondary address>, <command string>
  As noted above, when accessing the command channel, the secondary address is 15
Example OPEN 1, 8, 15 : INPUT#1, E, E$, T, S : PRINT E, E$, T, S : CLOSE 1
Result The command will open the command channel, read back the device status into the variables specified in the command string and close the command channel
  open (action)  
   the device with address 8 (device number)  
     using the command channel 15 (secondary address)  
       as stream 1   (command string)
         read the error number (E)   (command string)
           get its text description (E$)   (command string)

           report error track location (T)

  (command string)

and the sector on the track (S)

  (command string)
Program loading
Format LOAD "<file name>" [,<device number>]
Example LOAD "[x:] FILENAME", 8
  load (action)  
   a program (PRG) file   (command string)
     from disk number x (optional, 0 or 1)   (command string)
       called FILENAME   (command string)
         from the the device with address 8 (device number)  

These examples show how convoluted Commodore DOS commands can be, particularly when used with the earliest versions of Commodore BASIC. Some improvements were made by the time that BASIC 4.0 was available for the PET/CBM 4000/8000 series.

Program loading
Format DLOAD "<file name>"
Result   load (action)  
     a program (PRG) file   (command string)
       called FILENAME   (command string)
         from default disk (Unit=8, D=0)    
Disk Status
  System variables DS and DS$ were added for the disk error channel variables
Result Returns the error code from the last disk operation Outputs the status of last disk operation as a string, consisting of error code, text message, track and sector
Example 0 if there were no errors 00, OK, 00, 00  if there were no errors


DOS Wedges

Even with the improvements made in later versions of Commodore BASIC, PET DOS commands were not the most user friendly, "DOS wedges" were written for a range of Commodore systems to simplify disk commands and typically provided three main functions :

  • A simple command to read the device error channel to check the disk's error status
  • The ability to read the disk directory without overwriting the currently loaded program
  • Sending commands to the disk in a more user friendly form than using the normal BASIC syntax

A number of DOS wedges are available for the PET/CBM, but the one that I shall be using is Nils Eilers' version, available from his website,

The easiest way to use Nils' DOS wedge is to download the install program from his webpage and place a copy on your disks. When the program is run, it installs itself as a Terminate and Stay Resident (TSR) program and makes the wedge commands available from the command line.

Although requiring more initial effort, the wedge commands can be made available for immediate use when the system is started, they can be integrated with the PET Editor ROM. The Editor ROM is responsible for all video initialization, screen output, keyboard input, full-screen editor, and IRQ handling. Steve Gray has created a replacement Editor ROM which includes Nils'DOS wedge

 By putting all this in one ROM Commodore was able to customize the machines for various markets, with different options.




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