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OSBYTE &00 (0) - Identify Host/Operating System

On entry, X=0  - Generate error number 247 giving host and OS type
          X<>0 - Return host/OS in X

On exit,  X=host/OS type:
   :d.dir.file/ext       dir/file.ext            :d.dir.file/ext        d:file.ext
 0 Electron            8 UNIX-type system
 1 BBC                 9 6809/6309 system     17 6809/6309 system
 2 BBC B+             10 Atom/System          18 Atom/System
 3 Master 128
 4 Master ET                                                         28 Commodore 64/128
 5 Master Compact                                                    29 Texas Instruments calculator
 6 Arthur or RISC OS                                                 30 Amstrad CPC
 7 Springboard                                                       31 Sinclair ZX Spectrum

32 IBM PC-type system (DOS, Windows, etc.)
39 6809/6309 system

The Atom/System entries are tentative values for an Atom or System with a BBC API and OSBYTE entry.


Early documentation refers to OSBYTE 0 as returning the OS version. As more systems were developed, it became more correct to refer to it returning a value indicating the host. For example, code running on a second processor will always be told what the I/O host is regardless of what the host is and what the second processor is.

Platform capabilities

Over time the OSBYTE 0 return value has evolved to become a bitmap representing the capabilities of the host system, primarily of the file system structure.

  • %000x0xxx Filenames are directory.filename/extension, eg BBC, RISC OS
  • %000x1xxx Filenames are directory/filename.extension, eg Unix, CP/M
  • %nnnxxxxx Filenames are directory\filename.extension, eg DOS, Windows

This can also be represented as:

  • %000x0xxx Directory seperator is '.'
  • %000x1xxx Directory seperator is '/'
  • %nnnxxxxx Directory seperator is '\'
  • %000x0xxx Extension seperator is '/'
  • %nnnxnxxx Extension seperator is '.'

This allow programs to use code similar to the following:


See also

Jgharston 22:41, 6 September 2007 (BST)