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added notes on implementation that seems to give the right result

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James Tauber 2011-08-14 23:45:10 -04:00
parent e2211e4189
commit 1159cef81c
1 changed files with 31 additions and 1 deletions
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cycle_notes.txt
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@ -91,7 +91,7 @@ Class IV
(followed by BCC, BCS, BEQ, BMI, BNE, BPL, BVC, BVS) (followed by BCC, BCS, BEQ, BMI, BNE, BPL, BVC, BVS)
branch not taken 2 branch not taken 2
branch taken 3 (+1 is page crossed) branch taken 3 (+1 if page crossed)
Class V Class V
@ -106,3 +106,33 @@ Class VI
absolute 6 absolute 6
This seems a possible implementation (not yet considering page boundaries):
1. all instructions start with 2
2. absolute (including absolute indexed) adds 2
3. absolute indexed adds an additional 1 *if* instruction is of RMW type
4. zero page (including zero page indexed) adds 1
5. zero page indexed adds an addition 1
6. indirect (JMP) adds 4
7. indirect x adds 4
8. indirect y adds 3 plus an additional 1 *if* instruction is of RMW type
9. ASL, LSR, ROL, ROR add 2 cycles if not implied
10. JMP subtracts 1 cycle
11. JSR adds 2
12. RTS adds 4
13. branches add 1 if taken
14. DEC and INC add 2 cycles
15. PHA and PHP add 1 cycle
16. PLA and PLP add 2 cycles
17. BRK adds 5 cycles
18. RTI adds 4 cycles
RMW instructions are the absolute,x of ASL, DEC, INC, LSR ROL, ROR and STA
as well as indirect,y and absolute,y of STA
It may be possible to simplify even further given the particular functions
some of these share in command (where the cycle change could be placed
instead)