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updated notes, fixing what seems to a mistake on the webpage I referenced

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James Tauber 2011-08-14 20:57:22 -04:00
parent 8e1b71dbca
commit 2caed7b36d
1 changed files with 7 additions and 20 deletions
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27
cycle_notes.txt
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@ -14,11 +14,15 @@ document some of the "why".
What follows is an attempt to "find the patterns" in the cycle times (as What follows is an attempt to "find the patterns" in the cycle times (as
given on http://www.6502.org/tutorials/6502opcodes.html ) given on http://www.6502.org/tutorials/6502opcodes.html )
There are 12 classes of instructions when it comes to cycle times: NOTE: there appears to be an error in AND and ORA zero page timings on that
webpage given above. I've now corrected this below.
There are 11 classes of instructions when it comes to cycle times:
Class Ia Class Ia
(followed by ADC, BIT, CMP, CPX, CPY, EOR, LDA, LDX, LDY, SBC, STX, STY) (followed by ADC, AND, BIT, CMP, CPX, CPY, EOR, LDA, LDX, LDY, ORA, SBC, STX,
STY)
immediate 2 immediate 2
zero page 3 zero page 3
@ -52,23 +56,6 @@ page cross even if there isn't.
Question 1: why is this? Question 1: why is this?
Class Ic
(followed by AND, ORA)
immediate 2
zero page 2
zero page, x 3
absolute 4
absolute, x 4 (+1 if page crossed)
absolute, y 4 (+1 if page crossed)
indirect, x 6
indirect, y 5 (+1 if page crossed)
Note 3: just like class Ia except the zero page are a cycle faster
Question 2: why is this? is it a bug on the webpage?
Class II Class II
(followed by ASL, DEC, INC, LSR, ROL, ROR) (followed by ASL, DEC, INC, LSR, ROL, ROR)
@ -80,7 +67,7 @@ absolute, x 7
Note 4: looks like class Ib + 2 in the non-implied cases Note 4: looks like class Ib + 2 in the non-implied cases
Question 3: why does absolute, x not have a page crossing addition? same Question 2: why does absolute, x not have a page crossing addition? same
reason as for Ib? reason as for Ib?