EOR

Basic Info
Addressing Mode	Opcode	Length	Speed
Immediate	49	2/3 bytes	2 cycles*
Absolute	4D	3 bytes	4 cycles*
Absolute Long	4F	4 bytes	5 cycles*
Direct Page	45	2 bytes	3 cycles*
Direct Page Indirect	52	2 bytes	5 cycles*
Direct Page Indirect Long	47	2 bytes	6 cycles*
Absolute Indexed by X	5D	3 bytes	4 cycles*
Absolute Long Indexed by X	5F	4 bytes	5 cycles*
Absolute Indexed by Y	59	3 bytes	4 cycles*
Direct Page Indexed by X	55	2 bytes	4 cycles*
Direct Page Indexed Indirect by X	41	2 bytes	6 cycles*
Direct Page Indirect Indexed by Y	51	2 bytes	5 cycles*
Direct Page Indirect Long Indexed by Y	57	2 bytes	6 cycles*
Stack Relative	43	2 bytes	4 cycles*
Stack Relative Indirect Indexed by Y	53	2 bytes	7 cycles*

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

EOR is a 65x instruction that performs a bitwise exclusive or between its operand and the accumulator. The sum is stored in the accumulator. The size of the accumulator determines whether 8 or 16 bits are EORed.

Syntax

EOR #const
EOR addr
EOR long
EOR dp
EOR (dp)
EOR [dp]
EOR addr, X
EOR long, X
EOR addr, Y
EOR dp, X
EOR (dp, X)
EOR (dp), Y
EOR [dp], Y
EOR sr, S
EOR (sr, S), Y

One example usage of EOR is to flip all the bits of the accumulator:^[10]

EOR #$FFFF

Cycle Penalties

• In all addressing modes, EOR takes one extra cycle when the accumulator is 16 bits wide.
• In direct page addressing modes, EOR takes one additional cycle if the low byte of the direct page register is nonzero.
• In both Absolute Indexed and DP Indirect Indexed, Y admodes, EOR takes an extra cycle if adding the index crosses a page boundary.

See Also

• ORA
• AND
• EOR (SPC700)
• OR (SPC700)
• XOR (Super FX)

External Links

1. Eyes & Lichty, page 454 on EOR
2. Labiak, page 141 on EOR
3. 2.2.4.3 on MCS6500 Manual, page 21 on EOR
4. Carr, page 259 on EOR
5. Leventhal, page 3-63 on EOR
6. snes9x implementation of EOR: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L533
7. undisbeliever on EOR: https://undisbeliever.net/snesdev/65816-opcodes.html#eor-exclusive-or-accumulator-with-memory
8. Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#EOR
9. Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.1.2.1
10. Carr, page 260