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worked out why STA seemed an exception

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James Tauber 2011-08-14 21:37:34 -04:00
parent 2caed7b36d
commit 12671d81cb
1 changed files with 11 additions and 28 deletions
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cycle_notes.txt
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@ -17,43 +17,28 @@ given on http://www.6502.org/tutorials/6502opcodes.html )
NOTE: there appears to be an error in AND and ORA zero page timings on that 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. webpage given above. I've now corrected this below.
There are 11 classes of instructions when it comes to cycle times: There are 10 classes of instructions when it comes to cycle times:
Class Ia Class I
(followed by ADC, AND, BIT, CMP, CPX, CPY, EOR, LDA, LDX, LDY, ORA, SBC, STX, (followed by ADC, AND, BIT, CMP, CPX, CPY, EOR, LDA, LDX, LDY, ORA, SBC, STA,
STY) STX, STY)
immediate 2 immediate 2
zero page 3 zero page 3
zero page, x 4 zero page, x 4
zero page, y 4 zero page, y 4
absolute 4 absolute 4
absolute, x 4 (+1 if page crossed) absolute, x 4 (+1 if page crossed or writing)
absolute, y 4 (+1 if page crossed) absolute, y 4 (+1 if page crossed or writing)
indirect, x 6 indirect, x 6
indirect, y 5 (+1 if page crossed) indirect, y 5 (+1 if page crossed or writing)
Note 1: the zero page indexed and x-index indirect don't have the page cross Note 1: the zero page indexed and x-index indirect don't have the page cross
addition because they wrap. addition because they wrap.
Note 2: writes to indexed non-zero-page memory (e.g. STA) have the +1 even
Class Ib if not page crossing.
(followed by STA)
zero page 3
zero page, x 4
zero page, y 4
absolute 4
absolute, x 5
absolute, y 5
indirect, x 6
indirect, y 6
Note 2: just like Class Ia BUT takes the cycles it would take if there's a
page cross even if there isn't.
Question 1: why is this?
Class II Class II
@ -65,10 +50,8 @@ zero page, x 6
absolute 6 absolute 6
absolute, x 7 absolute, x 7
Note 4: looks like class Ib + 2 in the non-implied cases Note 3: looks like class I +2 (with the absolute, x +1 even if not page
crossing)
Question 2: why does absolute, x not have a page crossing addition? same
reason as for Ib?
Class IIIa Class IIIa