BBC BASIC Programming Techniques - Conditional Assembly Macros
      ==============================================================
                         J.G.Harston, 02-Nov-1996

The following assembly macros allow you to do conditional assembly with the
BASIC assembler.

  DEFFNif(A%):IFA%:z%=0:=opt% ELSE z%=P%:=opt%AND-2
  DEFFNendif:IFz%:z%=P%-z%:P%=P%-z%:O%=O%-z%:=opt% ELSE =opt%
  DEFFNelse:IFz%:z%=P%-z%:P%=P%-z%:O%=O%-z%:z%=0:=opt% ELSE z%=P%:=opt%AND-2

They are used in assembler source by calling them with OPT as with any other
assembler macro, as in the following example:

  DEBUG%=TRUE
  FOR opt%=4 TO 7 STEP 3
  P%=load%:O%=code%
  [OPT opt%
  ...
  OPT FNif(DEBUG%)
  JSR debug_code
  OPT FNendif
  ...
  ]NEXT

The functions rely on you using opt% to hold the assembly option, and uses
variable z%, which must not by changed by the rest of the source program.
Any assembly listing for the excluded section is turned off.


Programming considerations
--------------------------
The if/else/endif works by assembling the code, then backtracking if the
condition is FALSE. Consequently, there are some limitations which must
be born in mind.

The if/else/endif cannot be nested.

Labels within an if/else block must be different to those in an else/endif
block, otherwise the assembler will get confused with forward references.
For instance, use this type of coding:

  OPT FNif(PUSHWS%)
    LDY #0
    .CheckLp1
    LDA (zp),Y
    ...
    INY:BNE CheckLp1
  OPT FNelse
    .CheckLp2
    LDY #0:LDA (zp),Y:TAY
    ...
    INC zp+0:BNE CheckLp2
    INC zp+1:BNE CheckLp2
  OPT FNendif

If both conditional blocks used the same label for the loop, on the second
pass the first block would use the definition set in the second block.


Because unwanted code is assembled and then backtracked over, if there is a
conditional block right at the end of your assembly you must make sure there
is enough space for this in the memory you are assembling to. For example,
the following continues assembling, then moves back before finishing, so you
must ensure the memory at O% will allow this

  ...
  OPT FNif(TAILDATA%)
    EQUS "Data and end of code"
  OPT FNendif
  ]:NEXT opt%


Another way of calling macros is to use EQUS as this does not rely on the
assembly option being in opt%. The following are versions of the if/else/endif
functions that can be used this way. However, the assembly listing is not
turned off for the disabled section.

  DEFFNif(A%):IFA%:z%=0:="" ELSE z%=P%:=""
  DEFFNendif:IFz%:z%=P%-z%:P%=P%-z%:O%=O%-z%:="" ELSE =""
  DEFFNelse:IFz%:z%=P%-z%:P%=P%-z%:O%=O%-z%:z%=0:="" ELSE z%=P%:=""

You would use them in this manner:

  DEBUG%=TRUE
  FOR pass%=4 TO 7 STEP 3
  P%=load%:O%=code%
  [OPT pass%
  ...
  EQUS FNif(DEBUG%)
  JSR debug_code
  EQUS FNendif
  ...
  ]NEXT