Instructions Gone Wild: Safe Instruction Recognition

The biggest fear with conventional in-order architectures is what happens if you have a high latency instruction that needs a piece of data that isn't available in the caches.

Since in-order microprocessors have to execute the instructions in order, the execution units remain idle until the CPU is able to retrieve the data it needs from main memory - a process that could easily take over a hundred clock cycles. The problem is that during these clock cycles, power is expended but no work is getting done - which is the exact opposite of what we want in an ultra low power microprocessor.

Out of order processors would get around this problem by simply scheduling around the dependent instruction. The scheduler would simply select the next instruction that was ready for execution and work would progress while the data dependent instruction waited for data for main memory. We've already established that a full OoOE core would be too power hungry for Atom, but relying on a pure in-order design also has the potential to be inefficient. Intel's Austin team found a clever middle ground for Atom.

It's called the Safe Instruction Recognition (SIR) algorithm and it works like this. If Atom is executing a long latency floating point operation followed by a short latency integer op you would traditionally stall until the FP op is complete (as we described above). The SIR algorithm looks at the two instructions and determines whether or not there are any data dependencies between the two (e.g. C = A + B followed by D = C + F), if there aren't then Atom will allow the "younger", shorter latency operation to proceed ahead of the longer FP operation.

SIR addresses a very specific case but it sprinkles a little bit of out-of-order goodness into the Atom's otherwise very strict in-order design. I wouldn't be too surprised if future iterations of Atom expand the situations in which these sort of out-of-order tricks are allowed.

2-Issue and In-Order: Intel's Version of the Cell's PPE Return of the CISC: Macro-op Execution
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  • FlakeCannon - Wednesday, April 02, 2008 - link

    This was an absolutely fantastic article as far as I'm concerned. One of the best I've read from AnandTech. I'm truly impressed with the amount of effort and dedication that the engineers at Intel put into the Atom. Thought the consumer may not see its importance today the Atom will continue to develop one throughout the next 2 years and show why this is such a huge step in the right direction. I really think that this article outlines very well the architecture involved and where it intends to lead Intel and others in the future.

    I'm always impressed to see Intel take architecture that was revolutionary in its time 15 years ago in the Pentium and Pentium Pro and resurrect it in modern day fashion with help of the Dothan Pentium M architecture and even things borrowed from the miserable Netburst technology that 15 years later I believe will once again create a product revolutionary in nature. I was never able to appreciate it in the days of the Pentium but certainly can now.

    This is one product I think is deserving of being excited about.
    Reply
  • fitten - Wednesday, April 02, 2008 - link

    What does an on-die memory controller have to do with ILP? Reply
  • Anand Lal Shimpi - Wednesday, April 02, 2008 - link

    Woops, I've clarified the statement :)

    Take care,
    Anand
    Reply
  • erwos - Wednesday, April 02, 2008 - link

    I was thinking that this would be a fantastic platform for making a small, silent HTPC box for doing streaming media, but the lack of 1080p output kills that to a large extent. I know it's not a big priority for the first revision given the UMPC targeting, but I hope the "Atom 2" does try to squeeze that feature in. Reply
  • FITCamaro - Wednesday, April 02, 2008 - link

    It could always be paired with a different, more capable graphics core. Reply
  • ltcommanderdata - Wednesday, April 02, 2008 - link

    It;d be very interesting to see how the 1.86GHz Silverthorne stacks up against a 1.8GHz P4 Northwood, a 1.86GHz Dothan, a 1.8GHz Conroe-L based Celeron, and a 1.8GHz Athlon 64.

    I wonder if Apple is going to refresh AppleTV with Silverthorne since it seems ideal with replace the current 1GHz ULV Dothan in there.
    Reply
  • yyrkoon - Wednesday, April 02, 2008 - link

    Well at least Intel did not name their Atom CPUs the 'Atom Z80' . . . heh.

    Anyways, this is good for our future, as the mITX, and pITX 'systems' now days are still kind of large-ish, and cost quite a bit of money for what they are. Though, I think that putting a web browser on just any old appliance in the house would be way overkill, and possibly a very serious mistake. A TV with a web browser ? An Oven ? Please . . . this is why we have PCs, and micro mobile devices.

    Recently a friend and myself have been working on an embedded project, and I can see the potential here, but a 'problem' does exist. Some of the things you would want to do with such a processor . . . well lets just say there still would not be enough processing power. That being said, I do not see why these could not help make a TVs/HD-DVD player menu operate faster.


    Reply
  • pugster - Thursday, April 03, 2008 - link

    It certainly sounds nice, but the atom processor cost alot because some of the higher end models cost more than $100 each. I find it surprising that their Polosbo chipset is manufactured at 130mm. It probably came from one of their foundries that was due to upgrade to 32mm sometime next year anyways. They could've earily manufactured at 65mm.

    Somehow I don't see their product as mature and maybe the next gen product they would have a cpu and the north/south bridge in the same die.
    Reply
  • lopri - Wednesday, April 02, 2008 - link

    I honestly don't get the excitement. Should I? I mean, I wouldn't feel comfortable with one gigantic company controlling every single electronics in our life. If Intel opens up the X86 and everyone can compete on even end, then maybe. Since that won't happen, the future looks scary enough. Reply
  • clnee55 - Wednesday, April 02, 2008 - link

    NO, how can you get excitement. I am already bored with your conspiracy theory. Let's talk about tecnical issue here. Reply

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