Meaning of Steve Wozniak's epiphany on register addition/subtraction in 1975? I just finished reading Walter Isaacson's Innovators and on page 350 there is a quote from Steve Wozniak recalling things he learned from the March 1975 Homebrew Computer Club meeting:


  A person at the meeting passed around the specification sheet for the new Intel microprocessor. "That night, I checked out the microprocessor data sheet and I saw it had an instruction for adding a location in memory to the A register," he recalled. "I thought, Wait a minute. Then it had another instruction you could use for subtracting memory from the A register. Whoa. Well, maybe this doesn't mean anything to you, but I knew exactly what these instructions meant, and it was the most exciting thing to discover ever."


Referencing the book, Intel started selling the 4004 microprocessor in 1971. By definition, I thought a microprocessor has to have registers, so did they just add functionality to the new 1975 microprocessor? Do you believe he was referencing the Intel 4040 microprocessor that came out in 1974? Also, does adding and subtracting memory allow arrays in RAM to be indexed in O(c) / constant time? Is that the significance or are there even more layers to this tale?

Thanks!
This was 1975. Microprocessors were not common knowledge. Wozniak had some experience with mainframe computers, but most likely hadn't seen a 4004 processor ever. 

Reading the instruction set of the 6502 processor showed him that this wasn't a useless toy but a real computer. The original price for a 6502 was 25 dollars plus 10 dollars for the documentation (which you were encouraged to copy), compared to a 6800 for $300 including added chips that were required. 

Wozniak's epiphany was realizing that you could take a $25 microprocessor and turn it into a real computer. Apple Inc. is now valued a bit over 700 billion dollars, a direct consequence of Wozniak's realisation, so this was a kind of important event. 

Not that it has much to do with computer science, except that eventually it put computers into everyone's hands.