Apple-2-SW/apple2_hgr_font_tutorial
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Fix order of HGR to TEXT address high byte

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michaelangel007 2016-01-10 17:29:06 -08:00
parent 04710ac998
commit 7083cdb977
1 changed files with 8 additions and 8 deletions
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@ -1388,14 +1388,14 @@ 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:
Technically, to convert 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 |
| HGR High Byte | Text High Byte |
|:----------------:|:---------------:|
| $20 = %0010_0000 | $4 = %0000_0100 |
| $21 = %0010_0001 | $5 = %0000_0101 |
| $22 = %0010_0010 | $6 = %0000_0110 |
| $23 = %0010_0011 | $7 = %0000_0111 |
Which we could do via:
@ -1415,7 +1415,7 @@ But we'll save a byte and use the normal subtraction instead:
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.
There is also the reverse conversion -- to convert a Text address to a HGR address which could be done with the same `AND #3, OR #20`, but since we don't have a Text Y table address and already have a HGR 16-bit address table we will re-use that.
Here's the Pseudo-code to copy the text screen to the HGR Screen: