Apple-2-SW/apple2_hgr_font_tutorial
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Claried "C:/Program Files (x86)/Git/bin/sh.exe"400, and added note about non-standard assembly

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michaelangel007 2016-01-10 14:44:25 -08:00
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@ -559,7 +559,7 @@ Assuming we want to draw the `A` glyph at the top-left of the screen we would ne
For simplicity, we're going to "quantize" our destination Y so that we render font glyphs only on the start of every 8 rows and every 7 pixel columns. If we then had the starting address we simply could move to the next scan line by successively adding $0400 to our destination screen pointer.
How did I know $0400? One quirk of the HGR screen is that every 8 successive scan lines start this many bytes away.
How did I know to use $0400 when going to the next line? One quirk of the HGR screen is that every 8 successive scan lines start this many bytes away. Refer back to the `HGR Memory-mapped IO` table listed above.
## DrawChar() version 1
@ -1449,6 +1449,8 @@ Michael "AppleWin Debug Dev"
Figure it out ! You have all the tools and knowledge.
## Solution 2: ScrollHgrUpPixel()
There are many different ways to solve this depending if we want to prioritize space or speed.
@ -1675,11 +1677,11 @@ And let's try it out:
Sweet !
Here's the assembly to scroll the HGR screen up one pixel:
Here's the (non-standard) assembly to scroll the HGR screen up one pixel: (I'm using the non-conventional `:` as an assembler end-of-statement sepearator to logically group the byte copies together.)
```assembly
; FUNC: ScrollHgrUpPixel() = $1400
1400: LDX #27 ; 39 columns
1400: LDX #27 ; 39 columns ; Src Y Dst Y
1402: .1 LDA $2400,X : STA $2000,X ; [ 1] -> [ 0]
1408: LDA $2800,X : STA $2400,X ; [ 2] -> [ 1]
140E: LDA $2C00,X : STA $2800,X ; [ 3] -> [ 2]