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2.7 KiB

## Synopsis

A sprite compiler that targets 16-bit 65816 assembly code on the Apple IIgs computer. The sprite compiler uses informed search techniques to generate optimal code for whole-sprite rendering.

## Example

The compiler takes a simple masked, sparse byte sequence which are represented by (data, mask, offset) tuples. During the search, it tracked the state of the 65816 CPU registers in order to find an optimal sequence if operations to generated the sprite data. The space of possible actions are defined by the subclasses of the CodeSequence class.

Currently, the compile can only handle short, unmasked sequences, but it does correctly find the optimal code sequences. Here is a sample of the code that the compiler generates

### Data = $11

```
TCS ; 2 cycles
SEP #$10 ; 3 cycles
LDA #$11 ; 2 cycles
STA 00,s ; 4 cycles
REP #$10 ; 3 cycles
; Total Cost = 14 cycles
```

### Data = $11 $22

```
TCS ; 2 cycles
LDA #$2211 ; 3 cycles
STA 00,s ; 5 cycles
; Total Cost = 10 cycles
```

### Data = $11 $22 $11 $22

```
TCS ; 2 cycles
LDA #$2211 ; 3 cycles
STA 00,s ; 5 cycles
STA 02,s ; 5 cycles
; Total Cost = 15 cycles
```

### Data = $11 $22 $33 $44 $55 $66

```
ADC #5 ; 3 cycles
TCS ; 2 cycles
PEA $6655 ; 5 cycles
PEA $4433 ; 5 cycles
PEA $2211 ; 5 cycles
; Total Cost = 20 cycles
```

### Data = $11 $22 $11 $22 $11 $22 $11 $22

```
ADC #7 ; 3 cycles
TCS ; 2 cycles
LDA #$2211 ; 3 cycles
PHA ; 4 cycles
PHA ; 4 cycles
PHA ; 4 cycles
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