Re: Introduction to to Graphics in C on the Apple II

[Bill Buckels <bbuckels@escape.ca>]

On Jan 7, 9:25 pm, "John B. Matthews" <nos...@nospam.invalid> wrote:
> And both optimize by using a base address lookup table. I wanted to
> compare your approach with an example of using static linking in cc65; I
> had the link, but forgot the example!

The lookup table trick avoided many calculations for me over the last
30 years. I highly recommend precalculating everything that one can.
This is probably just common-sense to most of us but I appreciate that
you have recognized how this technique can be applied to saving
precious instructions on these tiny processors:)

>
> Both give a single-load binary and yours is fast and cross-OS. I would
> say the results are exemplary. Thanks!
>

When one is using tiny processors (not only old C compilers) optimal
techniques seem to become honed razor-sharp as we also know. A good
example is the ballistic speed that cc65 works when compared to Aztec
C which was pretty damned good by the end of its life cycle.


I just wanted to post the alternative to that lookup table that you
refer to (see below)so I remember to make a note of this somewhere...
too many instructions... that's the thing for us to remember... this
is actually part of the code that I used in 1980-something to create
the table for the hi-res display:



/* provides base offset for hires scanlines         */
/* does not bother to check whether we are in range */
/* gets the address as quickly as possible          */
/* stays away from processor intensive mul and div  */

gethibase(currentline,currentbase)
int currentline;
int *currentbase;
{
  FILE *fp;
  int ybase=0,z,a;

	if(currentline >63)
	   {
	   if (currentline < 128)
	       {
		ybase+=0x28;
		currentline-=64;
	       }
	   else
	       {
	       ybase+=0x50;
	       currentline-=128;
	       }
	    }

	    z=(currentline>>3);
	    a = (z<<7)|ybase;
	    *currentbase = (currentline - (z<<3))<<10 | a;


/*
	    fp = fopen("hb.txt","a");
	    fprintf(fp,"%d,", *currentbase);
	    if (currentline%8 == 0) fprintf(fp,"\n");
	    fclose(fp);

	    */

}



Having Fun,

Bill