Apple-2-SW/apple2_hgr_font_tutorial
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michaelangel007 2016-01-13 22:09:25 -08:00
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@ -1216,13 +1216,9 @@ Did you catch that **note** ? One popular trick on the 6502 was `self-modifying
However, there are still 2 more optimizations we can make:
1. If we assume our font is "page aligned", that is, starts at a multiple of 256 bytes -- we could remove the redundant `AddressLo += (FontLo + (c*8))` and replace with the direct `AddressLo = (c*8)`. Technically, in C you would keep the bottom 8-bits with `& 0xFF` but since the 6502 registers are only 8-bit and we're storing a byte the `& 0xFF` is not needed.
1\. If we assume our font is "page aligned", that is, starts at a multiple of 256 bytes -- we could remove the redundant `AddressLo += (FontLo + (c*8))` and replace with the direct `AddressLo = (c*8)`. Technically, in C you would keep only the bottom 8-bits by masking off the the other bits with `& 0xFF` but since the 6502 registers are only 8-bit and we're storing a byte the `& 0xFF` is not needed.
```assembly
033B:69 00 ADC #<Font ; += FontLo; Carry = 0 since R=0 from above
```
2. It is "funny" how we end up shifting and rotating in the **same** direction: Left! :-) It would nice to _leverage this work_ for both the high and low byte address calculation. That is the technical term for _"Don't do dumb (redundant) work"_ or in the immortal words of Back to the Future: "Think, McFly!" :-)
2\. It is "funny" how we end up shifting and rotating in the **same** direction: Left! :-) It would nice to _leverage this work_ for both the high and low byte address calculation. That is the technical term for _"Don't do dumb (redundant) work"_ or in the immortal words of Back to the Future: "Think, McFly!" :-)
Given: c