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OSFILE: File and directory information


6502 Z80 6809 PDP11 80x86 32016 ARM On entry: On exit:
A A A R0 AL R1 R0 = function code object type or preserved if function unsupported
YX HL X R1 BX =>control block undefined, control block updated
Control block
&00 R2 R1 =>filename undefined
&02 R3 R2 Load address Load address
&06 R4 R3 Execution address Execution address
&0A R5 R4 Start address Length
&0E R6 R5 End address or Attributes  Attributes
Function summary 
&FB &00 Save file &05 Read information
&FC Write system information &01 Write information &06 Delete object
&FD Read system information &02 Write load address &07 Create file
&FE Verify file &03 Write exec address &08 Create directory
&FF Load file &04 Write attributes &09


&FC   Write file system information.
&FD   Read file system information (System Internal Name). The attributes field is returned holding the sector start address.
&FE   Verify file. Control block is as for A=&FF, Load. If the verification is successful, A is returned holding &01 and the control block holds the file information. If the verification fails, A is returned holding &00 and the load address is returned holding the address where the verification failed. If the file does not exist or is a directory or does not have read access, then an error is generated.
&FF   Load a file into memory. If the low byte of the execution address is zero the file is loaded to the supplied load address, otherwise the file is loaded to the file's own load address. If the filename does not exist, or is a directory, or an execute-only file, or does not have read access, then an error is generated.
&00 Save a file. If a file already exists with the same name, it is overwritten, with the file access staying the same. If the file is locked, or a directory exists with the same name, then an error is generated.
&01 Write a object's reload address, execution address and attributes.
&02 Write a object's reload address.
&03 Write a object's execution address.
&04 Write a object's attributes.
&05 Read object's catalogue information into the control block.
&06 Delete object. If the object does not exist, A returned as &00. If the object is locked, or is not owned, or is a directory that is not empty, or is open, then an error is generated.
&07 Create an empty file of defined length. Block as for SAVE. The supplied start address is usually passed as &0 and the end address as the required length. No data is transfered, and the file does not necessarily contain zeros. Some file systems may deliberately overwrite any existing data in the file. If a file already exists with the same name, it is overwritten, with the file access and the case of the name staying the same. If the file is locked, or a directory exists with the same name, or the file is open, then an error is generated.
&08 Create a directory. The length field is passed holding a suggested minimum number of entries required, or zero for a default number. If a directory already exists, there is no error. The created directory is locked. If a file already exists, an error is generated.
Object types returned in A are:
&FF   Execute-only file
&00   Object not found
&01   File found
&02   Directory found
&03   Image file found (file accessible as a directory)
&04   Symbolic link found
The returned object type is also a bitmap:
bit 0: file
bit 1: directory
bit 2: symbolic link

Note: The Tube protocol masks off bit 7 of the return value from OSFILE, so A=&FF is returned as A=&7F.

LOAD and VERIFY generate an error if the specified file does not exist. All other functions return A=0 if the specified object does not exist.

File attributes: These are held in four bytes as follows:
Byte &0E bit 7 Private bit 3   Locked
bit 6 Executable by others bit 2   Executable by owner
bit 5 Writable by others bit 1   Writable by owner
bit 4 Readable by others bit 0   Readable by owner
Byte &0F bits 0‑4 Date: day of month bits 5‑7 Date: year since 1981, bits 4-6
Byte &10 bits 0-3 Date: month of year bits 4-7 Date: year since 1981, bits 0-3
Byte &11 bits 0-7 Undefined (returned as zero)

Note that lots and lots of documentation incorrectly describe the access byte as being not readable, etc. This is further propagated by DFS using a restricted and mangled access byte. Each bit being not readable, etc., is incorrect. Each bit is set if the corresponding access permission is present.

The normal access of WR/WR is an access byte of &33. See also File access.

Calling from BBC BASIC

  • LOAD fn$ calls OSFILE &FF
  • CHAIN fn$ calls OSFILE &FF
  • SAVE fn$ calls OSFILE &00

Entry points

  • BBC BASIC Entry Address: &FFDD
  • 6502 Entry Address: &FFDD, vectors via &0212
  • Z80 Entry Address: &FFDD, vectors via &FFDE
  • 6809 Entry Address: &FFDD, vectors via &FFDE
  • 80x86 Entry Address: INT &45, vectors via &0000:0114
  • 32000 Entry Address: SVC &0A
  • PDP-11 Entry Address: EMT 7, vector &07
  • ARM Entry Address: SWI &08 "OS_File", vector &08


Some implementations generate an error if an object does not exist instead of the correct action of returning object=&00. LOAD and VERIFY are the only functions that should generate an error if the file is not found.

Some implementations generate an error when writing load or execution addresses to directories. The correct action is to not generate an error; either actually write the information, or - if not possible - just return.

Some implementations generate an error when creating a directory if a directory already exists. The correct action is to just return, "create directory" is functionally "ensure the specified directory exists".

Some implementations return the wrong return values. All functions should return the relevent object type, regardless of the call.


CFS implements functions &00 (Save) and &FF (Load), and returns with any other value with A unchanged.


ROMFS gives an error for &00 (Save), implements &FF (Load), and returns with any other value with A unchanged.


Acorn DFS and contemporary filing systems implement functions &00 to &06 and &FF, and return with A unchanged if called with any other value.

As Acorn DFS and its clones allocate a single bit for attribute storage when reading the attributes the byte at &0E is returned as &08 if the file is Locked, and as &00 otherwise. When setting attributes the file is Locked if bits 1 or 3 of byte &0E are set (ie Locked or Writable). See also File access.

Some clones of Acorn DFS never return A=0, causing an error instead if no file is found.


ADFS implements &FF to &08.

When called with an unsupported action some versions of ADFS return A preserved, some versions return A corrupted.


Andrew Duggan's Hierarchial DFS implements &FF to &06 and &08. It also implements functions &07 and &09, but with different parameters.

  • A=7 Create an object, but with the control block set up as with OSFILE &05, not OSFILE &00.
  • A=9 Changes the length of an existing file.
  • A=10 Changes the attributes, load and execution addresses, and length of a file. This is like OSFILE 1, but the length is also changed.

HDFS also inverts the File access byte.


HADFS implements &FC, &FD and &FF to &08.


Acorn file servers give an error if an attempt is made to write the load and execution address of a directory. MDFS servers allow writing of a directory's load and execution addresses, but the action is ignored.

Filing System Calls
  • OSFILE : File and directory operations
  • OSARGS : Information on open objects
  • OSBGET : Read (get) a byte
  • OSBPUT : Write (put) a byte
  • OSGBPB : Read or write multiple bytes of data
  • OSFIND : Open or close an object
  • FSCV : Filing system control

Jgharston (talk) 22:30, 9 March 2015 (UTC) Jgharston (talk) 21:01, 15 January 2017 (UTC)