2-Issue and In-Order: Intel's Version of the Cell's PPE

The Austin design team started with a single-issue in-order core but quickly expanded it to be a superscalar, 2-issue design, in other words it is capable of sending up to two instructions down the pipeline at the same time. By comparison most desktop x86 microprocessors are 3 or 4-issue designs.

 

In order to feed the 2-issue machine, Intel equipped Atom with two decoders. These decoders take instructions fetched from the L1 instruction cache and sequence the series of 1s and 0s into figuring out what the instructions are telling the CPU to do. While the decoders are equal in their ability to decode instructions, there are two paths that an instruction may take: slow and fast.

In the earlier days of the x86 ISA (Instruction Set Architecture) many complained about its support for variable length instructions.

If I tell you that I'm going to give you 2 oranges every 10 seconds, your job becomes much easier than if I tell you that I'm going to give you somewhere between 1 and 3 oranges every 10 seconds. The former would be an example of a fixed length instruction set and the latter a variable length instruction set, unfortunately x86 falls into the realm of the latter.

Atom's slow decoding path does not include any speculative decoding. The instructions are sequenced manually, meaning that each bit is looked at (which takes time) but the instruction is guaranteed to be decoded properly. The instruction is also tagged so that the next time it comes through it can be sent through the fast path.

The fast path obviously employs some speculative decoding and is aided by the tag bit that's set after an instruction goes through the slow path. The slow path yields 1 instruction every 3 clocks, while the fast path can produce 2 instructions every clock.

As Intel learned with Banias (Pentium M), the power penalty for incorrect speculation is unacceptable in a device running on a battery. You'll see a number of tradeoffs where speculative performance tricks are sacrificed in order to maintain low power operation with the Atom processor.

Intel's Atom: Changing Intel from the Inside Instructions Gone Wild: Safe Instruction Recognition
POST A COMMENT

46 Comments

View All Comments

  • 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

Log in

Don't have an account? Sign up now