Date   : Mon, 26 Mar 2001 15:12:23 -0800 (PST)
From   : Thomas Harte <t.harte@...>
Subject: Re: 2 requests

Sent my reply straight to the original poster, as I usually quite stupidly
do, but is this a good solution to 'decoding a BBC [80 column] screen
address' ?

int x, y;

/* gets x and y in either

	3 and's
	8 shifts
	7 adds
	1 conditional branch

or

	3 and's
	10 shifts
	8 adds
	1 conditional branch
*/

void get_addr(int offset)
{
	int yoffs;

	/* take away bottom 3 bits */
	yoffs = offset&7;
	offset >>= 3;

	/* in order to divide by 80, divide by 16 first, since 5*16=80 */
	y = offset >> 4;

	/*

	now divide by 5 using a numeric method based on the observation that

	x/5 = x/4 - x/10, and hence x/10 = x/8 - x/20, etc.

	a few fractional bits are used to maintain some accuracy - either 3
	or 5 depending on the low bit, since this produces the right result

	*/
	if(y&1)
	{
		y =	+ (y << 1) - (y >> 1)
			+ (y >> 3) - (y >> 5)
			+ (y >> 7);

		y &= ~7;
	}
	else
	{
		y =	+ (y << 3) - (y << 1)
			+ (y >> 1) - (y >> 3)
			+ (y >> 5) - (y >> 7);

		y = (y >> 2)& ~7;
	}

	/*

	get x just with

	offset - (y*60) = offset - y*64 - y*16

	*/
	x = offset - (y << 3) - (y << 1);

	/* add back low 3 bits */
	y += yoffs;
}

I made it up myself, and I've tested it and it works with 100% accuracy on
at least the range of valid screen addresses . . . just thought it might be
a little slow thats all.

-Thomas





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