Talk:RTS Trick

Indirect vs. RTS

Is there an advantage over using JMP ($0200), where $0200 has been loaded from the same kind of jump-table? that's what I wonder, but I'm not gonna count up the cpu cycles needed for either method right now. --Memblers 20:21, 25 Jun 2009 (PDT)

Not sure. Seems like a pick'em to me. Here are some things that come to mind:

• RTS Trick doesn't require any RAM.
• I personally think the RTS Trick is more readable. If I see a table of pointers in my (or somebody else's) code and they all have a "-1" after them, I immediately know their purpose and how they are used.
• PHA, PHA, RTS requires less bytes than STA, STA, JMP (3 vs. 9).

--MetalSlime 00:08, 26 Jun 2009 (PDT)

Self-modifying

If you use self modifying code and assure that the table has to start at a page border (and store pointers to the routines, without the -1) then you can use a smaller and faster code:

tb_opcode_launcher_smc:
	; bytes, cycles
	asl ; 1, 2
	sta smc+2 ; 3, 4
smc:
	jmp (tb_opcode_rts_table) ; 3, 5
	; total 7 bytes and 11 cycles

tb_opcode_launcher:
	; bytes, cycles
	asl ; 1, 2
	tax ; 1, 2
	lda tb_opcode_rts_table+1, x ; 3, 4
	pha ; 1, 3
	lda tb_opcode_rts_table, x ; 3, 4
	pha ; 1, 3
	rts ; 1, 6
	; total 11 bytes and 24 cycles

--212.8.208.194 (talk)

Assuming that by sta smc+2 you meant sta smc+1 because 6502 is little-endian. But if you're doing any sort of nontrivial work in the NMI or IRQ handler, you would need separate 7-byte self-modifying trampolines in RAM for the main, NMI, and possibly IRQ handlers. And with the NES's 2048 byte RAM, 21 bytes might be a lot, though it's still not as bad as it would be on the Atari 2600. --Tepples (talk) 11:18, 21 May 2013 (MDT)

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