BBC Basic Line Numbers
----------------------
Within BBC BASIC programs line numbers (eg after GOTO, GOSUB, RESTORE)
are 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.

Encoding line number
--------------------
Tokenising line number lsb, msb:
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

In 6502 code:
\ On entry  num     16-bit line number
\           (ptr),Y => insertion point
\ On exit   (ptr),Y => after inserted bytes
LDA #&8D:STA (ptr),Y     :\ &8D line number token
LDA num+0:AND #&C0
LSR A:LSR A
INY:STA (ptr),Y
LDA num+1:AND #&C0
LSR A:LSR A
LSR A:LSR A
ORA (ptr),Y:EOR #&54
STA (ptr),Y:INY          :\ first encoded byte
LDA num+0:AND #&3F
ORA #&40:STA (ptr),Y:INY :\ second encoded byte
LDA num+1:AND #&3F
ORA #&40:STA (ptr),Y:INY :\ third encoded byte


Decoding line number
--------------------
Detokenising byte 0,1,2,3:
lsb : byte 2 EOR ((byte 1 * 4) AND &C0)
msb : byte 3 EOR (byte 1 * 16)

In 6502 code:
\ On entry  (ptr),Y => 141 byte
\ On exit   num     =  16-bit line number
\           (ptr),Y => after encoded number
INY:LDA (ptr),Y
ASL A:ASL A:TAX:AND #&C0
INY:EOR (ptr),Y:STA num+0  :\ b0-b7
TXA:ASL A:ASL A
INY:EOR (ptr),Y:STA num+1  :\ b8-b15
INY                        :\ step past last byte


In 6502 code:
\ On entry  byte1/2/3=encoded number
\ On exit   num      = 16-line number
\           byte1/2/3=corrupted
ASL byte1:ASL byte1
LDA byte1:AND #&C0
EOR byte2:STA num+0   :\ lsb of line number
ASL byte1:ASL byte1
LDA byte1
EOR byte3:STA num+1   :\ msb of line number


In BASIC:
REM On entry  ptr% => &8D byte
REM Returns   16-bit line number
DEFFNline(ptr%)
=((ptr%?3 EOR (ptr%?1 * 16))*256 + (ptr%?2 EOR ((ptr%?1 * 4) AND &C0))) AND &FFFF