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iigs-sprite-compiler
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Update README.md

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Lucas Scharenbroich 2016-12-04 23:37:24 -06:00 committed by GitHub
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@ -61,7 +61,33 @@ Currently, the compile can only handle short, unmasked sequences, but it does co
PHA ; 4 cycles
; Total Cost = 24 cycles
```
## Limitations ##
The current state representation removes data from the sparse byte array whenever a store action is queued. This prevents certain optimization that redundently store the same byte more than once, in order to minimize other operations. For example, the byte sequence `$11 $22 $22` currently generated the following, sub-optimal code sequence
```
TCS ; 2 cycles
SEP #$10 ; 3 cycles
LDA #$11 ; 2 cycles
STA 00,s ; 4 cycles
REP #$10 ; 3 cycles
LDA #$2222 ; 3 cycles
STA 01,s ; 5 cycles
; Total Cost = 22 cycles
```
The optimal code sequence is
```
TCS ; 2 cycles
LDA #$2211 ; 3 cycles
STA 00,s ; 5 cycles
LDA #$2222 ; 3 cycles
STA 01,s ; 5 cycles
; Total Cost = 18 cycles
```
Notice that byte 1 ($22) is loaded redundently, which results in the 16-bit LDA/STA code being 2 cycles slower that the equivalent 8-bit code. However, this 2-cycle penalty is more than made up for by the saving gained from avoiding the 6-cycle SEP/REP pair in order to enter and exit 8-bit mode, resulting in a net savings of 4 cycles.
## License
MIT License