=Electronics Hardware Questions=

Q:
Your system has CPU, ALU and two 8bit registers. There is no external memory. Can you swap the contents of the registers ?

A: solution by hbegin
XOR has the unique property of giving you the original value if you XOR it twice: ((A XOR B) XOR B) = A
So... the answer:
Note: values on the left side, showing contents of the registers, happen after the step listed at right.
REG1       REG2
======================================
A       B
A XOR B     B Step 1:"XOR R1,R2" #R1 <= R1 XOR R2
A XOR B      B XOR (A XOR B) = A Step 2:"XOR R2,R1" #R2 <= R2 XOR R1
(A XOR B) XOR A = B A Step 3:"XOR R1,R2" #R1 <= R1 XOR R2