Apple-2-SW/apple2_hgr_font_tutorial
1
0
Fork 0
mirror of https://github.com/Michaelangel007/apple2_hgr_font_tutorial.git synced 2024-05-28 23:41:30 +00:00
Code Issues Projects Releases Wiki Activity

Added table to convert text to HGR high byte

Browse Source
This commit is contained in:
michaelangel007 2016-01-10 17:23:16 -08:00
parent 7e82890cd5
commit 04710ac998
1 changed files with 28 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

29
README.md
View File

@ -1,6 +1,6 @@
#Apple ]\[ HGR Font Tutorial
Revision: 13, Jan 10, 2016.
Revision: 14, Jan 10, 2016.
# Table of Contents
@ -1388,6 +1388,33 @@ While the Apple's memory layout seems esoteric it has beautiful symmetry. For an
* the low byte of the text address is the same low byte of the HGR address
* the high byte of the text address is 0x1C less then the high byte of the HGR address
Technically, to conver the HGR high byte address to a Text high byte address, we only need to map these 4 high bytes:
| Text High Byte | HGR High Byte |
|:----------:|:----------------:|
| $4 = %0100 | $20 = %0010_0000 |
| $5 = %0101 | $21 = %0010_0001 |
| $6 = %0110 | $22 = %0010_0010 |
| $7 = %0111 | $23 = %0010_0011 |
Which we could do via:
```assembly
LDA HgrHi, Y ; Y is row
AND #7 ; strip off top 6 bits
OR #4 ; Set text page 1 = $0400
````
But we'll save a byte and use the normal subtraction instead:
```assembly
LDA HgrHi, Y ; Y is row
CLC ; Convert HgrHi to TextHi byte
SBC #$1B ; A -= 0x1C
```
If we care about absolute speed we could see which one takes the fewer clock cycles.
But since we already have a HGR 16-bit address table we can re-use it.
Here's the Pseudo-code to copy the text screen to the HGR Screen: