Section
03 Part 04 – The EOR Instruction (XOR) 
“An action doesn't have to be wrong just because it is not logical. It doesn't have to be right just because it has its logic.” ~Lion Feuchtwanger 
Introduction
EOR – Exclusive OR logical
This instruction
will perform XOR logical between the source operand and
destination
operand, the
result is saved to the destination operand.
So
what is XOR logical?
Let
us take two bits, one from the source operand and one from the destination
operand, and perform
XOR logical:
source operand 

destination operand 

Result 





0 
XOR 
0 
= 
0 
0 
XOR 
1 
= 
1 
1 
XOR 
0 
= 
1 
1 
XOR 
1 
= 
0 
The
idea is that only one has to be 1 for the result to be 1. To understand further, once again, pretend 0
is false, and 1 is true:
source operand 

destination operand 

Result 





False 
XOR 
False 
= 
False 
False 
XOR 
True 
= 
True 
True 
XOR 
False 
= 
True 
True 
XOR 
True 
= 
False 
They
must be the opposite of each other for the result to be true.
The name
that most other instruction sets use is XOR, the 68k uses the mnemonic EOR
instead. They have the same meaning and
are the same thing, but I’m writing it here just so we’re clear on the
situation.
Examples
So, here
is an example (we’ll pretend that d0 contains 01234567 again):
eori.b #$E2,d0 
This instruction will “XOR” the byte E2
with the byte 67 inside d0. E2
and 67 in binary is 1110 0010 and 0110 0111.
E2 
1 
1 
1 
0 
0 
0 
1 
0 









XOR 
XOR 
XOR 
XOR 
XOR 
XOR 
XOR 
XOR 
XOR 









67 
0 
1 
1 
0 
0 
1 
1 
1 









= 
= 
= 
= 
= 
= 
= 
= 
= 









85 
1 
0 
0 
0 
0 
1 
0 
1 
As
you see above, the source and the destination must not match for the result
to be 1, if they match, the result is 0.
E2 XOR 67 = 85
85 is
saved into d0, d0 now contains 01234585.
Just like
the “AND” and “OR” instructions, EOR has the same abilities, with an
exception. You can EOR one register to
another:
eor.w d0,d1 
You
can EOR on memory in various ways:
eori.w #$07FF,$00004000 eor.w d0,$00004000 
And you
can EOR on memory using the address registers:
eori.w #$07FF,(a1) eor.w d0,(a3) 
For
EOR, the source
operand cannot be a memory address, or memory using an address register:
eor.w $00020000,d0 eor.b (a0),d0 
Additionally,
you cannot EOR onto address
registers directly:
eori.w #$0042,a0 
EOR
Immediate
If
the source
operand is
“immediate”, you must use the instruction “eori”
instead of “eor”:
eori.w #$E003,d0 
The
assembler will turn “eor” into “eori”
for you, when it assembles your code, if you use “eor”
instead by mistake.