Posted By
bubis
on 2012-11-23
05:19:22
|  Re: Draw a single pixel in assembler
If performance is not critical, you can simply use ROM routines.
Like you can use JSR $C1A5 that reads the x coordinate from $02ad-$02ae and y from $02af.
You can find plenty of examples using this ROM routine in
http://plus4world.powweb.com/software/Your_Daily_128_Byte_Intro
faster routines work like this:
XR: x coord lo
$d0: x coord hi
YR: y coord
lda row_addr_lo,y
sta $d2
clc
lda $d0
adc row_addr_hi,y
sta $d3
txa
and #$f8
tay
lda ($d2),y
ora bits,x
sta ($d2),y
rts
You have to prepare three tables for this to work:
* row_addr_lo/row_addr_hi needs to point to the first byte on the bitmap for the respective y coordinate, so it's like (bitmap_base+(y/8)*320+y&7).
* bits is a 256 bytes long periodic sequence of 128,64,32,16,8,4,2,1, 128, ....
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