Even More Tweaks

Translation Lookaside Buffers, TLBs for short, are used to cache what virtual addresses map to physical memory locations in a system. TLB hit rates are usually quite high but as programs get larger and more robust with their memory footprint, microprocessor designers generally have to tinker with TLB sizes to accommodate. With K8 AMD increased the size of its TLBs over K7, and with Barcelona AMD is repeating the process once more.

Barcelona's TLBs are slightly larger than K8's, but they now include support for 1G pages which are useful for database applications and virtualized workloads. AMD also introduced a 128 entry 2M L2 TLB with Barcelona, once again to help cope with newer programs using larger page sizes. The TLB improvements to Barcelona won't make any sort of tangible impact on desktop applications, but enterprise performance should improve in server applications with large memory footprints.

When Intel introduced its second Pentium M, codenamed Dothan, one of the enhancements made was a lower integer divide latency. Although details at the time are slim, AMD has indicated that it has moved to reduce integer divide latency in Barcelona as well. We're not sure if the changes implemented are similar in any way to what Intel did with Dothan, but don't expect the performance improvement to be vastly noticeable in real world applications. It's one of those tweaks that will add up to overall more efficient execution but not one that's going to give you double digit performance gains across the board.

In another attempt to effectively "widen" Barcelona without committing a significant amount of transistors to doing so, AMD took a couple of instructions that were microcoded and turned them into fastpath decode instructions. A microcoded instruction takes significantly longer to decode than an instruction able to go through one of the core's fastpath decoders. CALL and RET-Imm instructions are now fastpath, which is a part of Barcelona's sideband stack optimization enhancements. MOVs from SSE registers to integer registers are now fastpath as well.

While on the topic of instructions, AMD also introduced a few new extensions to its ISA with Barcelona. There are two new bit manipulation instructions: LZCNT and POPCNT. Leading Zero Count (LZCNT) counts the number of leading zeros in an op, while Pop Count counts the leading 1s in an op. Both of these instructions are targeted at cryptography applications.

AMD also introduced four new SSE extensions: EXTRQ/INSERTQ, MOVNTSD/MOVNTSS. The first two extensions are mask and shift operations combined into a single instruction, while the latter two are scalar streaming stores (streaming stores that can be done on scalar operands). We may see some of these same instructions included in Penryn and other future Intel processors.

Stacks and Loads of Optimizations A Faster Memory Controller
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  • JarredWalton - Thursday, March 01, 2007 - link

    Games have quite a lot of LOAD instructions, like most programs, as well as plenty of branches (esp. in the AI routines). Most likely the boost that Core 2 gets is due in a large part to the better instruction reordering and branch prediction, although the cache and prefetchers probably help as well. Given AMD was better than NetBurst due to memory latency, through in better OOE (Out of Order Execution) logic and keep the improved latency and they should do pretty well.

    Naturally, everything at this point is purely speculation, but in the next few months we should start to get a better idea of what's in store and how it will perform. One problem that still remains is that even if AMD can be competitive clock-for-clock, Intel looks primed to be able to go up to at least 3.6 GHz dual core and 3.46 GHz quad core if necessary. AMD has traditionally not reached clock speeds nearly as high as Intel, possibly due in part to having more metal layers (speculation again - process tech and other features naturally play a role), so if they release 2.9GHz Barcelona at $1000 you can pretty much guarantee Intel will launch 3.2 and/or 3.46 GHz Kentsfield (and/or FSB1333 3.33 GHz).

    On the bright side, at least things should stay interesting in the CPU world. :D
    Reply
  • yyrkoon - Thursday, March 01, 2007 - link

    Yes, interresting indeed, but from experience, AMD has always been too vocal in what they plan on doing, especially during the times they are in a 'rut'.

    What this usually means to me, is that AMD is trying to blow smoke up our backsides, we'll see though.

    Keep in mind, my main desktop system, and my backup server for that matter, both are AMD systems. The phrase "cost effective" applies here.
    Reply
  • kilkennycat - Thursday, March 01, 2007 - link

    Yesterday, Intel announced that they were converting a fourth fab to 45nm. A great deal of confidence in that process. And a few days ago they announced desktop shipments of Penryn-based CPUs pulled forward into 2007. Looks as if AMDs 'window of opportunity' is likely to be very small. IBM has not yet announced a successful implementation of a RAM on their 45nm process. Intel had their RAM design on 45nm up and running late 2005. Reply
  • archcommus - Thursday, March 01, 2007 - link

    True but the move to 45 nm might not make a huge difference in real world performance, just like the move to 65 nm didn't for AMD. Their next full blown architecture will still be a ways off. Reply
  • Roy2001 - Thursday, March 01, 2007 - link

    Dislike AMD's move to 65nm process, move to 45nm has shown that Penryn would eats less power and runs faster thanks to its high K material and metal gate. Reply
  • smitty3268 - Thursday, March 01, 2007 - link

    Every process shows that in theory before chips are actually being made on it. We'll see what actually happens when Penryn is released, not before. Reply
  • chucky2 - Thursday, March 01, 2007 - link

    Has AMD given any indication of how probable dropping an Agena or Kuma CPU into an existing AM2 motherboard will go?

    Especially AMD's own newly released 690G or the upcoming nVidia MCP68?

    Chuck
    Reply
  • mamisano - Thursday, March 01, 2007 - link

    It has been stated in the past that AM2+ based products will run in AM2 based boards. The limitation, if I understand it correctly, will be the lack of support of the new power features.

    Someone correct me if I am wrong :)
    Reply
  • chucky2 - Thursday, March 01, 2007 - link

    Then it should be no problem for AMD to confirm through AnandTech that this is the case.

    Surely if Barcelona is this close to shipping (only a few months away), AMD must know if Agena and/or Kuma will work in current AM2 motherboards, especially their own 690 series their just about to release.

    All I'm asking for is a definite either way, it shouldn't be that hard for AMD to do at this point.

    Chuck
    Reply
  • mino - Friday, March 02, 2007 - link

    AMD stated PUBLICLY to anyone who listened that AM2+ stuff will plug into AM2, just BIOS update needed.

    Why should they react to any consumer who ask on some forum the same question every second week ?

    Most important is they said it WILL(not "may") work with AM2-spec boards to big Tier 1 OEM's.
    They can not make it incompatible therefore. They would be out of bussines in no time.
    Reply

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