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 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 * 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 | | HGR High Byte | Text High Byte |
|:----------:|:----------------:| |:----------------:|:---------------:|
| $4 = %0100 | $20 = %0010_0000 | | $20 = %0010_0000 | $4 = %0000_0100 |
| $5 = %0101 | $21 = %0010_0001 | | $21 = %0010_0001 | $5 = %0000_0101 |
| $6 = %0110 | $22 = %0010_0010 | | $22 = %0010_0010 | $6 = %0000_0110 |
| $7 = %0111 | $23 = %0010_0011 | | $23 = %0010_0011 | $7 = %0000_0111 |
Which we could do via: 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. 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: Here's the Pseudo-code to copy the text screen to the HGR Screen: