[Coco] CCASM procedures

[Roger Taylor]

At 05:59 AM 12/9/2004, you wrote:
>Instead of passing the parameters in one or both of the stack registers, 
>you might consider that this should all be done by CCASM. In other words, 
>when CCASM reads a section of code defined as a "proc", it should store in 
>its own memory the variables and apply them within the procedure as the 
>source code calls procs. The assembled program won't know or see this and 
>all registers in a proc will behave in a normal fashion.

Macros work completely different because, as you know, the code is 
duplicated anytime you use the macro.  Btw, macros *are* on my agenda for 
CCASM very soon.

Procedures/subprograms are based on isolation.  A procedure or subprogram 
should not know anything (give or take) about the outside world or calling 
program, and there should be only one instance of a procedure.  You pass 
parameters into the procedure, it does its job, and you can optionally get 
something back (from a function).

The reason the procedures need to work the way they do is to also allow a 
library to be compiled into object code for linking in.  Stuff like this 
will come soon, but I have to start somewhere, so I think a source-code 
based library is great for now.

As of tonight, I have decided to use ,U as the means for accessing 
procedure parameters and local variables.  The S stack is unpredictable, 
especially since you can preserve your own registers across procedure calls 
by PSHS/PULS.  Add in the fact that you can call other procedures from 
within a procedure, and you just can't predict where the stack will be, so 
how can you use the precomputed offsets for parameters and local 
variables?  The only way is the copy S to U at a certain point, then use U.

Btw, U is also pushed to the S stack and preserved automatically, as well 
as D.  You must preserve any other registers you use, yourself.

Here's the latest working example....
I'm still studying whether the local variable offsets are correct since I'm 
dealing with negative offsets on the stack.

CCASM 2.93: assembling cross6809.asm to pro.bin

                                          org    3584

  0E00                  result            rmb    1

                       * pass-by-reference example

  0E01                  square            proc   param:byte,presult:word
               {$FFFE}  aa                var    2       reserve 1 local 
byte for no reason :)
               {$FFFC}  bb                var    2
  0E01      34401F43                      begin  square  procedure entry
  0E05      32E9FFFC
  0E09 A6   44                            lda    param,u access 
pass-by-value parameter
  0E0B 48                                 lsla
  0E0C A7   D8 05                         sta    [presult,u]     change 
pass-by-ref parameter
  0E0F ED   5E                            std    aa,u    access local variables
  0E11 ED   5C                            std    bb,u
  0E13      1F343540                      endproc
  0E17      39

  0E18      3406        start             call   square,2,result call main 
procedure
  0E1A      CC
  0E1B      0E003406
  0E1F      86023402
  0E23      BD
  0E24      0E01
  0E26      3506
  0E28      3263
  0E2A B6   0E00                          lda    result
  0E2D B7   0400                          sta    1024
  0E30 39                                 rts

               {$0E18}                    end    start



-- 
Roger Taylor