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{{#customtitle:OSWORD &7F (127) - Perform floppy disk operation}}



On entry: On exit:
Control block
XY?&00 Drive
XY!&01 Data address
XY?&05 Number of parameters (n)
XY?&06 Command
XY+7... Parameters
XY?(7+n) Result

No contents of the control block should be relied upon on exit other than the result byte. Some systems update the control block, some systems do not.

Command  Parameters Meaning
&40 <track> <sector> <size+count> <size> <field> Scan data **
&44 <track> <sector> <size+count> <size> <field> Scan data and deleted data multi-sector **
&4A <track> <sector> Write data 128 bytes
&4B <track> <sector> <size+count> Write data
&4E <track> <sector> Write deleted data 128 bytes
&4F <track> <sector> <size+count> Write deleted data
&52 <track> <sector> Read data 128 bytes
&53 <track> <sector> <size+count> Read data
&56 <track> <sector> Read data and deleted data 128 bytes
&57 <track> <sector> <size+count> Read data and deleted data
&5B <track> <&00> <count> Read IDs
&5E <track> <sector> Verify data and deleted data 128 bytes
&5F <track> <sector> <size+count> Verify data and deleted data
&63 <track> <gap3> <size+count> <gap5> <gap1>   Format track
&64 <track> (EDOS) Write track
&65 <track> (EDOS) Read track
&69 <track> Seek
&6C Read drive status and reset 'not ready'
&75 <&0D> <step> <settle> <load> Initialise
&75 <&1n> <bad 1> <bad 2> <current track> Specify bad tracks
&76 (EDOS) Force interrupt
&7A <register> <value> Write special register
&7D <register> Read special register
&E0 <track> <nine more bytes> (1770) Read track
&F0 <track> <nine more bytes> (1770) Write track

** Not possible on a BBC as needs DMA hardware.



The drive number specifies the drive to access. If bit 7 is set, then the previously-used drive is used, and the drive status check is skipped.

The drive number can be expressed as a binary number %pxDDDdsd.

  • b7  : p - If 1, selects the previously-used drive and density
  • b5-b3 : DDD - select the drive density:
 %xx0=Acorn single density
 %xx1=Acorn double density
 %0xx=Watford current density
 %10x=Watford single density
 %11x=Watford double density
  • b2,b0 : dd - physical drive number
  • b1  : s - side

Single density DFSs do not have any ability to select double density disk access. Acorn DFSs select the density with b3, Watford DFSs select the density with b4 and b5. To specify a density in a manner compatible with both Acorn and Watford DFSs specify the density with both methods:

       %001000sd - to specify single density
       %001110sd - to specify double density

Most DFSs do not implement physical drives numbers larger than 1 (logical drives greater than 3). Selecting the previously selected drive can only be relied upon if DFS is the currently selected filing system. A drive number of &FF should be passed to select the previously used drive.

DFS 2.xx in the Master MOS 3.20 ROM incorrectly passes the density bit in b3 to the drive control register b3 instead of b5, so OSWORD &7F cannot access double density disks without patching the DFS[1].

Data Address

This is the address to transfer data to or from.


This is the logical track to transfer data to or from. This may be different from the physical track number if the drive has previously been stepped to the correct physical track.


This is the logical sector number to start data transfer from. Sectors can be numbered anything from &00 to &FF, but single-density DFS-formatted disks use &00-&09 and double-density DFS and ADFS disks use &00-&0F.

Size + Count

Bits 0 to 4 specify the number of sectors to be read or written. Specifying zero sectors is undefined and may result in unpredictable behaviour.

Bits 5 to 7 specify the sector size as 128*2^size, as follows:

 b7   b6   b5   Sector size
0 0 0 128 bytes
0 0 1 256 bytes
0 1 0 512 bytes
0 1 1 1024 bytes
1 0 0 2048 bytes (8271 only)
1 0 1 4096 bytes (8271 only)
1 1 0 8192 bytes (8271 only)
1 1 1 16384 bytes (8271 only)

Special Registers

Command &7A and &7D write and read internal special registers. These registers are:

  • &00 40/80 flag (Opus) : reserved:dstep:0:0:0:0:0:0 0=single, 1=double
  • &03 Density (Opus) : reserved:densy:0:0:0:0:0:0 0=single, 1=double
  • &04 SROM (Opus) : &00-&0F
  • &06 Scan Sector (sector any error occured at)
  • &10 Bad Track Register 1 (drive 0/2)
  • &11 Bad Track Register 2 (drive 0/2)
  • &12 Track
  • &13 Scan Count LSB
  • &14 Scan Count MSB
  • &17 DMA Mode Register 1:1:0:0:0:0:<single head>:<no DMA>
  • &18 Bad Track 1 (drive 1/3)
  • &19 Bad Track 2 (drive 1/3)
  • &1A Track (drive 1/3)
  • &22 Drive Control Input
  • &23 Drive Control Output SEL1:SEL0:FAULT:LOW:LOAD:DIR:SEEK:WREN

What Special Registers are supported depends on the specific hardware and DFS firmware.


The result is returned to the byte after the last parameter. The location is addressed by XY?(7+XY?5). For command &6C this location contains the drive status; for command &7D it is the requested special register; and for command &7A the result is undefined. Values returned by the other commands are as made up as follows:

  • b7-b6: always zero (1770 DFS returns 11 if command unrecognised)
  • b5: deleted data found
  • b4-b3: completion type: 00=no error, 01=recoverable error, 10=unrecoverable error, 11=failure
  • b2-b1: completion code
  • b0: always zero

This gives the following result values:

 type   code   Result   Meaning
00 00 &00 Successful completion
00 01 &02 Success, scan met equal **
00 10 &04 Success, scan met not equal **
00 11 &06 unused
01 00 &08 Clock error
01 01 &0A Late DMA ** / Late ISR
01 10 &0C ID CRC error
01 11 &0E Data CRC error
10 00 &10 Drive not ready
10 01 &12 Disk write protected
10 10 &14 Track 0 not found
10 11 &16 Write error
11 00 &18 Sector not found
11 01 &1A unused
11 10 &1C unused
11 11 &1E Drive not present/drive empty

** Not possible on a BBC as needs DMA hardware.

The result byte will have &20 added to it if deleted data has been read.

Intel documentation states that result type 01 should be retried up to a total of ten times before abandoning. In practice:

  • results types 01 and 10, other than &10 - Drive not ready - and &12 - Disk write protected, should be retried up to ten times
  • result &10 should be retried indefinitely until some other result occurs
  • result &18 - Sector not found - could be followed by a seek to track zero then one additional attempt.
  • While result &1E - Drive not present/empty - is not mentioned in official documentation, if left trying to access an empty drive this result will eventually occur. As such, emulators such as BeebEm will give Disk fault 1E if trying to access an empty drive. 1770 DFSs usually return Disk error 18 (Sector not found) instead.

The result byte can be preloaded with a value to return if no OSWORD &7F routine exists. For instance, preloading the result with &00 makes all such failed calls appear to succeed, preloading with &18 makes all calls appear to return 'Sector not found'. HADFS preloads the result byte with &1E to give a default result of 'Drive not present'.

Read Drive Status (&6C) resets any 'not ready' error and returns the drive status as the result byte:

  • b7  : unused
  • b6  : READY1
  • b5  : FAULT
  • b4  : INDEX
  • b3  : WR PROTECT
  • b2  : READY0
  • b1  : TRACK0
  • b0  : COUNT

8271 Commands

The 8271 command passed in the control block is actually a bitmap made up as follows:

  | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 
    |   |   |   ||  |   |   |   |
    |   |   +---+---+   |   +---+----Size:  00 multiple scan data
    |   |       ||      |                   01 no transfer
    |   |       ||      |                   10 128 bytes
    |   |       ||      |                   11 multiple sectors
    |   |       ||      |
    |   |       ||      +------------Data:   0 ignore delete data
    |   |       ||                           1 deleted and undeleted data
    |   |       ||
    |   |       |+--------------Direction:   0 Write to FDC
    |   |       |                            1 Read from FDC
    |   |       |
    |   |       +-----------------Command: 000 Scan
    |   |                                  001 Write
    |   |                                  010 Read
    |   |                                  011 Verify
    |   |                                  100 Format
    |   |                                  101 Seek
    |   |                                  110 Initialise
    |   |                                  111 Special Registers
    |   |
    |   +---------------------------------- Drive
    +-------------------------------------- Side

DFS copies the drive and side bits from the drive number passed in XY?0, and this is the origin of the DFS drive numbering scheme. Bit 4 is used to select the appropriate NMI and Tube code to transfer to or from the floppy disk controller.

Obviously, some combinations result in commands that are not sensible or are ineffective. For instance, it makes no sense to combine "Read" with "no transfer".


Some DFSs, such as DFS 2.45 supplied in the Master MOS 3.50 ROM, only respond to OSWORD &7F if DFS is the current filing system and fail silently if any other filing system is selected.

Acorn DFS

This is the original implementation of OSWORD &7F. It is a fairly simple interface to the Intel 8271 floppy disc controller; it passes the command and parameters to the controller, supplies the appropriate NMI service routine to fetch or store the disc data, and returns the result.

Acorn 1770 DFS

This emulates key commands of the 8271. Software double-stepping is available. Double density access can be made by setting bit 3 of the drive number. Unfortunately, the programmers didn't realise that the double density bit was moved in the BBC Master hardware, so double density access is not possible without a patched filing system[2].

Watford DDFS

Density selectable with b4-b5 of the drive number.


This emulates key commands of the 8271, and operates on each drive in the density that was detected at the last *CAT (single or double). Software double-stepping is enabled if *OPT 8,255 is entered and *CAT detects a 40 track disc.

The Format Track command formats 256-byte sectors only in single density, or double density if 16 or more sectors are specified. The density of operation cannot be directly specified to other commands.

The sector size parameter is ignored in favour of the sectors' N records.


The following routine can be used to perform an OSWORD &7F call that takes three parameters, such as Write (&4B) and Read (&53), or Track Read (&E0) and Track Write (&F0) which take ten parameters. It requires X%=>15-byte control block, Y%=X%DIV256. den%=1 for single density and, if supported by the hardware, den%=2 for double density. It returns a result of &1E (not present) if no OSWORD &7F call is available.

   DEFFNdisk(addr%,cmd%,drv%,trk%,sec%,num%,den%):LOCAL fs%
   fs%=FNfs:IF fs%<>4:*FX143,18,4
   UNTIL A%<>&10:IF fs%<>4:OSCLI"FX143,18,"+STR$ fs%

Tube note

The Tube MOS copies a 16-byte control block over the Tube, so on a read operation, any data read to immediately after the control block will be overwitten by the returned control block if it is within 16 bytes of the start of the control block.

If code is to run properly, the destination address of any reads must be at least 16 bytes away from the start of the control block, for example:

   DIM X% 15, T% 255
   Y% = X% DIV 256
   X%!1 = T%

If blocks are declared with variables in the heap, the variable definitions count towards the 16-byte distance, so the following is ok:

   DIM ctrl% 12, data% 255
   X%=ctrl%:Y%=X% DIV 256
   X%!1 = data%

As the variable data% is assigned space between the two buffers, the value of data% - ctrl% exceeds 15.

See Also


  1. http://mdfs.net/ROMs/Filing/Disk
  2. http://mdfs.net/ROMs/Filing/Disk