BBC Basic Line Numbers
----------------------

Within BBC Basic programs lines line numbers (eg after GOTO, GOSUB,
RESTORE) are encoded to make them easily found by RENUMBER, and to prevent
them appearing tp be control characters or tokens by forcing them into the
range &40-&7F.

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:
LDA #&8D:STA byte0    :\ &8D line number token
LDA lsb:AND #&C0
LSR A:LSR A
STA byte1
LDA msb:AND #&C0
LSR A:LSR A
LSR A:LSR A
ORA byte1:EOR #&54
STA byte1             :\ first encoded byte
LDA lsb:AND #&3F
ORA #&40:STA byte2    :\ second encoded byte
LDA msb:AND #&3F
ORA #&40:STA byte3    :\ third encoded byte



Detokenising byte 0,1,2,3:
lsb : byte 3 EOR (byte 1 * 16)
msb : byte 2 EOR ((byte 1 AND &30) * 4)

In 6502 code:
ASL byte1:ASL byte1
LDA byte1:AND #&C0
EOR byte2:STA lsb     :\ lsb of line number
ASL byte1:ASL byte1
LDA byte1
EOR byte3:STA msb     :\ msb of line number