Apple-2-SW/apple2_hgr_font_tutorial
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Touch up a few misc. tables

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Michaelangel007 2016-01-15 05:10:08 -08:00
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@ -884,7 +884,7 @@ Since we only care about the high byte:
TmpHi = destination HGR address after drawing all 8 lines
Final = destination HGR address set back to initial value
|Y |Initial|TmpHi|Final| (T and $1F) | (T and $1F) or $20 |
|Y |Initial|TmpHi|Final| (TmpHi and $1F) | (TmpHi and $1F) or $20 |
|--:|:-----:|:---:|:---:|:---:|:---:|
| 0| $2000 | $40 | $20 | $00 | $20 |
| 8| $2080 | $40 | $20 | $00 | $20 |
@ -1053,6 +1053,9 @@ Recall that our font has this memory layout:
| D | $44 | $6220 |
| : | : | : |
| _ | $5F | $62F8 |
| : | : | : |
| ~ | $7E | $63F0 |
| | $7F | $63F8 |
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.
@ -1880,39 +1883,39 @@ For our final trick we are going to copy the characters off the text screen onto
The text screen, like the HGR screen, is also non-linear, and also broken up into a triad:
|Row|Text Address|HGR Address|
|--:|:----:|:-----:|
| 0 | $400 | $2000 |
| 1 | $480 | $2080 |
| 2 | $500 | $2100 |
| 3 | $580 | $2180 |
| 4 | $600 | $2200 |
| 5 | $600 | $2280 |
| 6 | $700 | $2300 |
| 7 | $780 | $2380 |
| - | ---- | ----- |
| 8 | $428 | $2028 |
| 9 | $4A8 | $20A8 |
|10 | $528 | $2128 |
|11 | $5A8 | $21A8 |
|12 | $628 | $2228 |
|13 | $6A8 | $22A8 |
|14 | $728 | $2328 |
|15 | $7A8 | $23A8 |
| - | ---- | ----- |
|16 | $450 | $2050 |
|17 | $4D0 | $20D0 |
|18 | $550 | $2150 |
|19 | $5D0 | $21D0 |
|20 | $650 | $2250 |
|21 | $6D0 | $22D0 |
|22 | $750 | $2350 |
|23 | $7D0 | $23D0 |
|Row|Text Address|HGR Address|HGR Scanline|
|--:|:----:|:-----:|---:|
| 0 | $400 | $2000 | 0|
| 1 | $480 | $2080 | 8|
| 2 | $500 | $2100 | 16|
| 3 | $580 | $2180 | 24|
| 4 | $600 | $2200 | 32|
| 5 | $600 | $2280 | 40|
| 6 | $700 | $2300 | 48|
| 7 | $780 | $2380 | 56|
| - | ---- | ----- | - |
| 8 | $428 | $2028 | 64|
| 9 | $4A8 | $20A8 | 72|
|10 | $528 | $2128 | 80|
|11 | $5A8 | $21A8 | 88|
|12 | $628 | $2228 | 96|
|13 | $6A8 | $22A8 | 104|
|14 | $728 | $2328 | 112|
|15 | $7A8 | $23A8 | 120|
| - | ---- | ----- | - |
|16 | $450 | $2050 | 128|
|17 | $4D0 | $20D0 | 136|
|18 | $550 | $2150 | 144|
|19 | $5D0 | $21D0 | 152|
|20 | $650 | $2250 | 160|
|21 | $6D0 | $22D0 | 168|
|22 | $750 | $2350 | 176|
|23 | $7D0 | $23D0 | 184|
While the Apple's memory layout seems esoteric it has beautiful symmetry. For any given text row notice that:
* 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 corresponding HGR scanline address, and
* the high byte of the text address is 0x1C less then the high byte of the HGR address
Technically, to convert the HGR high byte address to a Text high byte address, we only need to map these 4 high bytes: