BBC BASIC program line format
                      =============================

BBC BASIC programs are usually stored in a tokenised format.

Acorn/Wilson format BASIC is stored as:
+----+------+------+------+----------------+----+      +----+-----+
| CR | line | line | line | tokenised line | CR | etc. | CR | &FF |
|    | high | low  |length|    contents    |    |      |    |     |
+----+------+------+------+----------------+----+      +----+-----+
  |                                                      ^
  +-------- start + length = next CR ------------->------+

Russell format BASIC is stored as:
+------+------+------+----------------+----+      +----+-----+-----+-----+
| line | line | line | tokenised line | CR | etc. | CR | &00 | &00 | &FF |
|length| low  | high |    contents    |    |      |    |     |     |     |
+------+------+------+----------------+----+      +----+-----+-----+-----+
  |                                                      ^
  +-------- start + length = next length byte ---->------+

Line numbers within a line are also tokenised, encoded to make them easily
found by RENUMBER, and to prevent them appearing to be control characters or
tokens by forcing them into the range &40-&7F.

  Byte 0 : 141
  Byte 1 :(((lsb AND &C0) DIV 4) OR ((msb AND &C0) DIV 16)) EOR &54
  Byte 2 :  (lsb AND &3F) OR &40
  Byte 3 :  (msb AND &3F) OR &40

6502 BASIC line numbers can go to a maximum of 32767 (&7FFF), other BASICs
can use the full range to 65279 (&FEFF).

Most of the tokens used by different versions of BBC BASIC are the same, but
extended tokens are different.

The differences are between:
* Acorn/Wilson BASIC descended from the 6502 version, the 32000, ARM, PDP11,
  6809 and others - uses 2-byte extended tokens
* Russell BASIC descended from the Z80 version, 80x86, Windows, SDL - uses
  1-byte extended tokens.

A running program can tell what format the program is in by examining TOP-3:
  IF ?(TOP-3)=0 THEN Russell format
  IF ?(TOP-3)>0 THEN Acorn format