One of the most difficult problems in computer science is reversing a secure hash (finding an input text for a given output, the digital signature). Let me explain this problem in simple terms. Let’s assume the wealthy but terminally ill Alice wrote her will and stored it on her computer. Knowing that a computer can be hacked and the will can be altered, Alice digitally signed her will with the secure hash algorithm SHA-256. She then emailed the digital signature to all her friends, allowing them to check the validity of the document. Bob wants to hack into the computer and change Alice’s will so that he becomes the sole beneficiary, but he faces a problem: he needs to change the will in such a way that the widely distributed SHA-256 signature stays the same. Otherwise, everybody realizes that the will has been forged. This is the computationally difficult problem of reversing or brute-forcing SHA-256, or finding an input that matches a predefined output. Satoshi famously decided that in order to find a new block, people all over the world need to compete in reversing SHA-256, turning block creation into a global lottery.
Once very popular among Bitcoin miners, but now somewhat dated, the Radeon HD 5830 card boasts 1120 stream processing units. But that doesn’t mean it literally has 1120 separate cores. Rather, the GPU employs 224 SIMD cores, each of which sports five ALUs operating in parallel (VLIW5).