Apple-2-SW/apple2_hgr_font_tutorial
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michaelangel007 2016-01-10 18:52:49 -08:00
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@ -1,6 +1,6 @@
#Apple ]\[ HGR Font Tutorial
Revision: 15, Jan 10, 2016.
Revision: 16, Jan 10, 2016.
# Table of Contents
@ -584,9 +584,9 @@ Another solution is to use a web browser that isn't "broken" such as Firefox, et
OK, so now that we have the font data, how do we draw a character "on screen" ?
Remember we need to transfer 8 consecutive bytes (1 byte / scanline) to 8 different scanlines.
Remember we need to transfer 8 consecutive bytes (1 byte / scanline) to 8 different scanlines scattered all over memory.
Assuming we want to draw the `A` glyph at the top-left of the screen we would need to transfer bytes from the (source) font glyph memory locations to the (destination) screen memory locations:
Assuming we want to draw the `A` glyph at the top-left of the screen we would need to transfer bytes from the (source) font glyph memory locations to these (destination) HGR screen memory locations:
($6208) -> $2000
($6209) -> $2400
@ -666,10 +666,13 @@ If we wanted to draw in columns 1 and 2 instead of column 0 then we need to set
Enter in:
```
306:1
300G
306:2
300G
```
![Screenshot 9](pics/hgrfont_09.png?raw=true)
@ -695,7 +698,7 @@ Since the Y-register controls the column we can inline this function and have th
LDY #column
After DrawChar() it is handy if we can advance both:
After drawing a character with `DrawChar()` it is handy if we can advance both:
* the column of the cursor
* the pointer to the screen where the next glyph will be drawn
@ -751,7 +754,7 @@ Recall that our font has this memory layout:
| : | : | : |
| _ | $5F | $62F8 |
The 6502 stores and loads 16-bit addresses in little-endian format so for glyph `D`, we need to store the bytes of the address `$6220` in reverse order.
The 6502 stores and loads 16-bit addresses in little-endian format so for glyph `D` we need to store the bytes of the address `$6220` in reverse order.
Enter in:
@ -1099,7 +1102,7 @@ The HGR screen address is broken up a triad. Every 64 scan lines the offset chan
|176| $2350 |$23|$50|
|184| $23D0 |$23|$D0|
We'll split the table of addresses into Low and High bytes for easier access. We'll also subtract off the hard-coded graphics page 1 high byte = $20 and instead use relative offsets to make it work with either graphics page 1 or 2.
We'll split this table of 16-bit addresses into Low and High bytes for easier access. We'll also subtract off the hard-coded graphics page 1 high byte = $20 and instead use relative offsets to make it work with either graphics page 1 or 2.
This is our mini HGR Y Address look-up table. "Funny" that it has 24 entries -- the same height as our text screen. :-)