Re: ZBasic

["Michael J. Mahon" <mjmahon@aol.com>]

Mark Percival wrote:
>   To: magnusfalkirk
> magnusfalkirk wrote:
>
>
> > Has anyone ever used this to
> > write a program for the Apple II and if so what do you think of it.
>
>
> The first program that comes to mind is Jim Royal's, Star Trek: First
> Contact.  Excellent game.
>
> http://www.apple2.org.za/mirrors/ground.icaen.uiowa.edu/apple8/Games/Startrek.fc2/STARTREK.FC.SHK

Interesting.

Looking at the compiled code, ZBASIC uses "JSR $082A / xx", where
xx is an interpretive opcode (followed by optional additional parameter
bytes), for all operations except simple 16-bit integer ops.

The interpreter is in the file RT.AUX.OBJ0.

So a typical program is:

  JSR $082A
  DB  $28
  JSR $082A
  DB  $2D
  JSR $082A
  DB  $40
  JSR $082A
  DB  $28
  JSR $082A
  DB  $55
  ...

Since these JSR sequences often occur in long sequences, it would
have saved a lot of memory to use an interpretive loop to scan
through the ops instead of the JSR linkage (except to *enter* the
interpreter.  One interpretive op would exit the interpreter and
give control to in-line code.

This is the Sweet-16 approach, and it could have saved as much as
75% of the program memory in ZBASIC.  It would cost one extra byte
(say, 00) and slightly more time when "exiting" a string of
interpretive ops, but they appear seldom to be isolated.

Of course, time would be saved by not exiting and re-entering the
interpreter for each op.

So the code above could have been:

  JSR $082A
  DB  $28
  DB  $2D
  DB  $40
  DB  $28
  DB  $55
  DB  $00
  ...

-michael

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