A peer of mine approached me with this challenge and I wasn't able to come up with an elegant solution. After discussing my approach, we began to wonder what the people at Code Golf would come up with.

Given

A processor that implements only two instructions:
1. STO <register>, <address> (store from this register to this address)
2. SUB <register>, <address> (subtract the value at this address from this register)
It also only contains 2, 32-bit registers: A and B
As well as 2, 32-bit RAM address: 0x00 and 0x01
The values of A and B are unknown

Challenge

Write a program to copy RAM address 0x00 into 0x01 optimised to use the fewest number of instructions possible.

Brett

Posted 2015-06-12T19:21:33.193

Reputation: 63

Welcome to PCG! Note that this isn't a very good challenge. It's simple enough to likely only have one good solution. Try using the Sandbox for Proposed Challenges first next time to get feedback about proposed challenges. Thanks.

– mbomb007 – 2015-06-12T21:15:56.157

@mbomb007 Thank you, I'm new and figured there was a better way to do this. I'm sorry for the poor question and I greatly appreciate the help. Thank you! – Brett – 2015-06-12T21:18:18.717

Answers

Assuming registers are signed and/or wrapping

8 instructions:

STO A 0x01
SUB A 0x01
SUB A 0x00
STO A 0x00
SUB A 0x00
SUB A 0x00
STO A 0x00
STO A 0x01

A 7-instruction version provided by lrn:

STO A 0x01
SUB A 0x01
SUB A 0x00
STO A 0x01
SUB A 0x01
SUB A 0x01
STO A 0x01

SuperJedi224

Posted 2015-06-12T19:21:33.193

Reputation: 11 342

That and the next instruction serve to zero register A. Then we do a what amounts to [00]=-[00], then some more stuff to restore [00] to its original value while simultaneously setting A to that value, then we write that value to [01]. The requirement was that [01] and [00] both end up having the value that [00] initially had. – SuperJedi224 – 2015-06-12T20:50:34.497

Oops, I read the challenge as being to swap the memory locations. – feersum – 2015-06-12T21:00:55.733

1@feersum: I don't think you could do that with this command set without adding a third address. – SuperJedi224 – 2015-06-12T21:02:04.550

All without using register B. Nice! This is probably the optimal solution. – mbomb007 – 2015-06-12T21:10:06.240

A 3rd address or maybe an XOR instruction. I don't see a way for there to be an answer any simpler than this. It was definitely better than mine, so I marked it as the answer. Congratulations! – Brett – 2015-06-12T21:14:45.630

1@Brett Can you post yours, please? I'm interested. – mbomb007 – 2015-06-12T21:17:09.470

2Why store A to 0x00 in the second to last step? If you use 0x01 as temporary in step 4 and forward, you don't need to store back into 0x00. That is: STO A 0x01; SUB A 0x01; SUB A 0x00; STO A 0x01; SUB A 0x01; SUB A 0x01; STO A 0x01 should copy 0x00 to 0x01 and keep 0x00 unchanged in one instruction less. – lrn – 2015-06-12T22:58:15.743

Copy Memory Only Using Store And Subtraction

Given

Challenge

Answers