Talk:RTS Trick: Difference between revisions
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* 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. | * 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). | * PHA, PHA, RTS requires less bytes than STA, STA, JMP (3 vs. 9). | ||
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 | |||
Revision as of 16:01, 21 May 2013
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.
MetalSlime: 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).
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
