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Format

by JGH, June 2006

BBC BASIC programs are stored in a tokenised format. Most of the tokens used by different versions of BBC BASIC are the same, but extended tokens are different, and different versions have different line headers. The exception is Brandy BASIC which does not tokenise programs and instead stores them as plain text.

Russell format BASIC is stored as:

  • “ {<len> <linelo> <linehi> <text> <cr>} <00> <ff> <ff>”


Acorn/Wilson format BASIC is stored as:

  • “ {<cr> <linehi> <linelo> <len> <text>} <cr> <ff>”


BASIC can also be stored as text, as:

  • “ {[<text>] [<cr>|<lf>|<cr><lf>|<lf><cr>]}”


It can be useful to be able to determine what format a BASIC file is stored in. Different file formats have defined RISC OS filetypes and DOS extensions:

Format
RISC OS filetype
File extension
Acorn/Wilson
&FFB “BASIC”
“.”
Russell
&1C7 “Basic8”
“.bbc”
Text/Brandy
&F1D “BasText”
“.bas”


However, you should never assume a file's format from its filetype or extension. Assuming a BASIC program has been saved normally, and there is no extra data appended to the end of the file, the format can be easily determined by looking at the final few bytes of the file:

  • “<cr><00><ff><ff> - Russell format (Z80, 80×86)”
  • <xx><xx><cr><ff> - Wilson/Acorn format (6502, 32000, ARM, PDP11)
  • <xx><xx><cr><lf> - text CR/LF terminated
  • <xx><xx><lf><cr> - text LF/CR terminated
  • <xx><xx><xx><lf> - text LF terminated (Brandy)
  • <xx><xx><xx><cr> - text CR terminated
  • <xx><xx><xx><xx> - unrecognised


The following code will examine the last four bytes of an open file and determine what format it is:

      PTR#in%=EXT#in%-4:FOR A%=0 TO 3:buffer%?A%=BGET#in%:NEXT
      type%=0                                       :REM unknown
      IFbuffer%?3=&0D                      :type%=7 :REM text cr
      IFbuffer%?3=&0A                      :type%=6 :REM text lf
      IF(!buffer% AND &FFFF0000)=&0D0A0000 :type%=5 :REM text lfcr
      IF(!buffer% AND &FFFF0000)=&0A0D0000 :type%=4 :REM text crlf
      IF(!buffer% AND &FFFF0000)=&FF0D0000 :type%=2 :REM 6502 format
      IF!buffer%=&FFFF000D                 :type%=1 :REM 80/86 format

Here in% is the handle of the file that has been opened, buffer% is a pointer to a four byte DIMensioned block of memory, A% is a temporary variable and on exit type% contains the file type.

Note that in some circumstances it is legitimate to append data to the end of a BBC BASIC program file. In that case this method of determining the format will not work, since the last four bytes of the file will not be the last four bytes of the program.

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format.txt · Last modified: 2018/04/17 16:21 by tbest3112