From 12671d81cb3304676fe2f062941b8c9486f7e75d Mon Sep 17 00:00:00 2001
From: James Tauber <jtauber@jtauber.com>
Date: Sun, 14 Aug 2011 21:37:34 -0400
Subject: [PATCH] worked out why STA seemed an exception

---
 cycle_notes.txt | 39 +++++++++++----------------------------
 1 file changed, 11 insertions(+), 28 deletions(-)

diff --git a/cycle_notes.txt b/cycle_notes.txt
index 740eb18..15ab7aa 100644
--- a/cycle_notes.txt
+++ b/cycle_notes.txt
@@ -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
 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
-(followed by ADC, AND, BIT, CMP, CPX, CPY, EOR, LDA, LDX, LDY, ORA, SBC, STX,
-STY)
+Class I
+(followed by ADC, AND, BIT, CMP, CPX, CPY, EOR, LDA, LDX, LDY, ORA, SBC, STA,
+STX, STY)
 
 immediate       2
 zero page       3
 zero page, x    4
 zero page, y    4
 absolute        4
-absolute, x     4 (+1 if page crossed)
-absolute, y     4 (+1 if page crossed)
+absolute, x     4 (+1 if page crossed or writing)
+absolute, y     4 (+1 if page crossed or writing)
 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
 addition because they wrap.
 
-
-Class Ib
-(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?
+Note 2: writes to indexed non-zero-page memory (e.g. STA) have the +1 even
+if not page crossing.
 
 
 Class II
@@ -65,10 +50,8 @@ zero page, x    6
 absolute        6
 absolute, x     7
 
-Note 4: looks like class Ib + 2 in the non-implied cases
-
-Question 2: why does absolute, x not have a page crossing addition? same
-reason as for Ib?
+Note 3: looks like class I +2 (with the absolute, x +1 even if not page
+crossing)
 
 
 Class IIIa