To understand it, start with the fact that some instructions necessarily lead to a change in which instructions come next. Instead of waiting, the processor can move another instruction through the pipeline. For example, if an instruction requires data that is out in DRAM main memory rather than in the cache memory located in the CPU itself, it might take a few hundred clock cycles to get that data. That's helpful, because it allows the processor to keep working if an instruction stalls in the pipeline. If two instructions are independent of each other-that is, the output of one does not affect the input of another-they can be reordered and their result will still be correct. Since the 1990s, microprocessors have relied on two tricks to speed up the pipeline process: out-of-order execution and speculation.
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Pipelining basically brings parallelism down to the level of instruction execution: When one instruction is done using a stage, the next instruction is free to use it. Some typical stages for an Intel x86 processor include those that bring in the instruction from memory and decode it to understand what the instruction means.
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Like an assembly line, the pipeline is a series of stages, each of which is a step needed to complete an instruction. To speed execution, modern processors use an approach called pipelining.
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In modern computing systems, software programs written in human-understandable languages such as C++ are compiled into assembly-language instructions-fundamental operations that the computer processor can execute. If the prediction was wrong, the results of the speculation are thrown out.
The instructions are executed speculatively and become visible to the program only if the prediction was correct. Speculative Execution: To save time, a processor predicts what the next instruction will be. The first computers powered by chips that are intentionally designed to be resistant to even some of these vulnerabilities arrived only recently. Security patches were needed not just from the processor makers but from those further down the supply chain, such as Apple, Dell, Linux, and Microsoft. In fact, a year on, the job is far from over. Doing so without causing computing speeds to grind into low gear has made it even harder. In other words, nearly the whole world of computing was vulnerable.Īnd because speculative execution is largely baked into processor hardware, fixing these vulnerabilities has been no easy job. Spectre and its many variations added Advanced Micro Devices ( AMD) processors to that list. At the time it was discovered, Meltdown could hack all Intel x86 microprocessors and IBM Power processors, as well as some ARM-based processors. These types of attacks, called Meltdown and Spectre, were no ordinary bugs. Our own group had discovered the original vulnerability behind one of these attacks back in 2016, but we did not put all the pieces together. Throughout 2017, researchers at Cyberus Technology, Google Project Zero, Graz University of Technology, Rambus, University of Adelaide, and University of Pennsylvania, as well as independent researchers such as cryptographer Paul Kocher, separately worked out attacks that took advantage of speculative execution. But on 3 January 2018, the world learned that this trick, which had done so much for modern computing, was now one of its greatest vulnerabilities. So good in fact, that this ability, called speculative execution, has underpinned much of the improvement in computing power during the last 25 years or so. But the truth is, that for decades now, they've been doing their tasks out of order and just guessing at what should come next.
We're used to thinking of computer processors as orderly machines that proceed from one simple instruction to the next with complete regularity.
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