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 tracks the state of the 65816 CPU registers in order to find an optimal sequence of operations to generated the sprite data. The space of possible actions are defined by the subclasses of the CodeSequence class.

Currently, the compiler can only handle short, unmasked sequences, but it does correctly find 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 optimizations that redundently store the same byte more than once, in order to minimize other operations. For example, the byte sequence $11 $22 $22 currently generates 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 than the equivalent 8-bit code. However, this 2-cycle penalty is more than made up for by the savings 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