'{$STAMP BS2pe}
'{$PBASIC 2.5}
' (c) 2003 Tracy Allen, http://www.emesystems.com
' explores the PBASIC 2.5 commands 
' ON x GOTO label
' and
' ON x GOSUB label
'
' to use this program as a study guide,
' Enter alternative program cases after #DEFINE below
' then look at tokens using CTRL-M memory map window
' Scroll to the bottom of the window to the red area.


#DEFINE example=10

'  10  ON x GOTO
'  11  same tokens as 10, but using BRANCH in old PBASIC

'  20  ON x GOSUB
'  21  same numberof tokens as 20 using GOSUB/BRANCH in old PBASIC
'  22  a particulary inefficient way to accomplish the same thing as 20



#SELECT  example

' --- ON X GOTO ---
' under the hood this is exactly the same as BRANCH, see #CASE 11
' but note that the ON x GOTO does not require [] brackets

#CASE 10
x VAR Byte
ON x GOTO lab0,lab1,lab2
lab0:
  ' statements A
lab1:
  ' statements B
lab2:
  ' statements C
END


#CASE 11
x VAR Byte
BRANCH x,[lab0,lab1,lab2]
lab0:
  ' statements A
lab1:
  ' statements B
lab2:
  ' statements C
END




' --- ON X GOSUB ---
' This is apparently the only PBASIC 2.5 control structure that
' can NOT have an exact match in PBASIC 2.0
' Token coding for ON X GOSUB puts
' the RETURN target for the GOSUB after the
' BRANCH command,
' whereas in old PBASIC the return target is always
' the instruction right after the GOSUB.
' This difference is very minor and both old and new
' PBASIC can produce exactly the same length program.
'
' The ON x GOSUB instruction is very code-efficient
' It only uses one return target.


#CASE 20
x VAR Byte
ON x GOSUB lab0,lab1,lab2
END

lab0:
 ' statements A
RETURN
lab1:
 ' statements B
RETURN
lab2:
 ' statements C
RETURN
END



#CASE 21 		' like case 20, same program length tokenized
			' minor microcoding differences
x VAR Byte
GOSUB lab
END
lab:
BRANCH x,[lab0,lab1,lab2]
RETURN

lab0:
 ' statements A
RETURN
lab1:
 ' statements B
RETURN
lab2:
 ' statements C
RETURN
END




#CASE 22		' this is a very *inefficient* implementation
                  ' note that BRANCH comes first with separate GOSUB
                  ' then each GOSUB requires a RETURN target
                  ' and also a GOTO in order to exit the structure
x VAR Byte
BRANCH x,[lab0p,lab1p,lab2p]
GOTO lab9
lab0p:
GOSUB lab0
GOTO lab9
lab1p:
GOSUB lab1
GOTO lab9
lab2p:
GOSUB lab2
lab9:
END

lab0:
RETURN
lab1:
RETURN
lab2:
RETURN



#ENDSELECT