Subject: Re: Hires calculations
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From: Charles Stephens <devnull@dobbs.eng.sun.com>
Newsgroups: comp.sys.apple2
Date: 04 May 1999 12:01:28 -0700
Organization: Sun Microsystems, Inc.
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>>>>> "BR" == bobryan9  <bobryan9@my-dejanews.com> writes:

 BR> How do I calculate the MEMORY LOCATION of a byte on the LEFT SIDE
 BR> of the Hires screen, given the Y location?

 BR> I would like something like:

 BR> MEMLOC=Y (insert math here)

To find a memory location for a given hi-res coordinate, use this
handy formula (remember everything is integer math):

location = base+(40 * cluster)+(128 * clusteroffset)+(1024 * line) + pixelgroup

base = 8192 (for page 1) or 16384 (for page 2)
pixelgroup = x / 7
row = y / 8
line = y mod 8
clusteroffset = row mod 8
cluster = row / 8 

Example, you want to find the address for 100,75 on Hi-Res page 1:

base = 8192
pixelgroup = 14
row = 9
line = 3
clusteroffset = 1
cluster = 1

Now, plug it all in:

location = 8192 + (40 * 1) + (128 * 1) + (1024 * 3) + 14
location = 11446 or $2CB6

cfs
-- 
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