ROR: Difference between revisions

Latest revision as of 03:51, 20 September 2024


Basic Info
Addressing Mode	Opcode	Length	Speed
Accumulator	6A	1 byte	2 cycles
Absolute	6E	3 bytes	6 cycles*
Direct Page	66	2 bytes	5 cycles*
Absolute Indexed by X	7E	3 bytes	7 cycles*
Direct Page Indexed by X	76	2 bytes	6 cycles*


Flags Affected
N	V	M	X	D	I	Z	C
N	.	.	.	.	.	Z	C

ROR (Rotate Right) is a 65x instruction that rotates a value and the carry flag right one bit. The least significant bit is shifted into the carry flag. The carry flag is shifted into the most significant bit.

When the accumulator is 8 bits wide, 9 bits are rotated.
When the accumulator is 16 bits wide, 17 bits are rotated.

Syntax

ROR
ROR A
ROR addr
ROR dp
ROR addr, X
ROR dp, X

Cycle Penalties

Except in accumulator addressing, ROR takes two extra cycles when the accumulator is 16 bits wide.
In direct page addressing modes, ROR takes another extra cycle if the low byte of the direct page register is nonzero.

ROR was originally broken and so was left out of the earliest 6502 datasheets before it was fixed. It is available on 65x processors manufactured after June 1976.

The arrow in the Carr textbook diagram does not circle back around.

External Links

Eyes & Lichty, page 491 on ROR
lbid, page 191, before & after diagram on ROR
Labiak, page 175 on ROR
10.4 on MCS6500 Manual, page 150 on ROR
Carr, page 269 on ROR
Leventhal, page 3-87 on ROR
snes9x implementation of ROR: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L1070
undisbeliever on ROR: https://undisbeliever.net/snesdev/65816-opcodes.html#ror-rotate-right
Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#ROR
Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.1.3

@@ Line 70: / Line 70: @@
 </pre>
-===== Cycle Penalties =====
+==== Cycle Penalties ====
 * Except in [[accumulator addressing]], ROR takes two extra cycles when the accumulator is 16 bits wide.
 * In [[direct page addressing]] modes, ROR takes another extra cycle if the low byte of the [[direct page register]] is nonzero.
@@ Line 76: / Line 76: @@
 [[File:816_ror.png]]
-ROR was originally broken and left out of the earliest 6502 datasheets before fixed.  It is available on 65x processors manufactured after June 1976.
+ROR was originally broken and so was left out of the earliest 6502 datasheets before it was fixed.  It is available on 65x processors manufactured after June 1976.
+The arrow in the Carr textbook diagram does not circle back around.
 === See Also ===
@@ Line 90: / Line 92: @@
 * 10.4 on [[MCS6500 Manual]], [https://archive.org/details/mos_microcomputers_programming_manual/page/n171 page 150] on ROR
 * [[Carr]], [https://archive.org/details/6502UsersManual/page/n282 page 269] on ROR
-* [[Leventhal]] page 3-87 on ROR: https://archive.org/details/6502-assembly-language-programming/page/n136
+* [[Leventhal]], [https://archive.org/details/6502-assembly-language-programming/page/n136 page 3-87] on ROR
 * snes9x implementation of ROR: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L1070
 * undisbeliever on ROR: https://undisbeliever.net/snesdev/65816-opcodes.html#ror-rotate-right