Barcelona Architecture: AMD on the Counterattack
by Anand Lal Shimpi on March 1, 2007 12:05 AM EST- Posted in
- CPUs
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.
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.
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Amiteriver - Tuesday, March 27, 2007 - link
Sounds groooovyNow lets just hope they have something good to plug it into.
trisweb2 - Friday, March 16, 2007 - link
I just want to say how refreshing it is to read an article written by Anand. He is a master of the English language; he perfectly communicates and explains every technical detail and I come away with a better understanding of whatever he's talking about.Thank you, Anand, for being a good writer!
MrWizard6600 - Thursday, March 22, 2007 - link
I Agree, Outstanding.No other site I know of gives nearly as many in depth details, and while ill admit my knowlage of some of the terms is sketchy, I got through that one with a good understanding.
Sounds like AMD has something to fight Core 2 against.
I do have one criticism:
I would have loved to have heard what Intels equivilent to all of AMDs technologies would be, mind you this criticism corrects it self toward the end of the artical.
stance - Monday, March 5, 2007 - link
Remember AMD's old president and CEO Jerry Sanders with commentslike "We will see what we see" and "More bang for your buck" I
cannot wait to see duel socket motherboards with two four core
Barcelona's working their magic. reminds me of Carol shelby
when he brought the Cobra out for road test. exciting is not
the word, jaw droping performance? Don't take Richard's Statements
lightly
lordsnow - Sunday, March 4, 2007 - link
Does anyone have any idea how compatible the "Barcelona" CPU will be with current motherboards? When it comes out, does it need a new n-phase voltage regulator, for example?the reason I'm asking is, I want to upgrade and with the current state of affairs was going to go for a C2D CPU. But with these Barcelona CPU's due out I may stick with AMD - get a AM2 motherboard and cheap AM2 CPU and upgrade to the Barcelona CPU at a later date. But I have to be sure that whatever motherboard I buy now will be 100% Barcelona compatible.
Can anyone inform us about what the situation is in this regard?
coldpower27 - Sunday, March 4, 2007 - link
Barcelona being the server variant will be compatible with the Socket F infrastructure, while Agena will be a Socket AM2+ processor compatible with exisiting Socket AM2 infrastructure.lordsnow - Sunday, March 4, 2007 - link
Any ideas as to what kind of features a user will be missing by dropping a AM2+ "Agena" CPU into a AM2 socket? The enhanced Power Saving features, perhaps?chucky2 - Sunday, March 4, 2007 - link
I asked above and non-AnandTech folks like you and I said it would...but no one from AnandTech themselves jumped right in to give an affirmative.I asked for links from AMD's own website confirming that Agena and Kuma would work in current AM2 motherboards, and no one posted back.
Right now the AM2+ CPU's will work in current AM2 boards rumor is just that, a rumor...when AMD themselves confirm it, or a site such as AnandTech confirms it with AMD and reports on it, then I'll believe it.
Until then, it's <i>probable</i> that AM2+ will work in current AM2 motherboards...if you're willing to take the risk I say go for it, else, wait until we have an official answer one way or the other.
JMHO...
Chuck
Calin - Saturday, March 3, 2007 - link
"Intel regained the undisputed performance crown it hadn't seen ever since the debut of AMD's Athlon 64."Intel in fact lost the "undisputed performance king" title during the early lifetime of the K7 architecture. The Pentium !!! was faster at some tasks and slower at others (games) than the K7. Before that, the Pentium II was better than the K6-2 (the K6-3 had better IPC than Pentium3, but was slower in MHz)
coldpower27 - Sunday, March 4, 2007 - link
Intel had the undisputed performance crown again with the Athlon XP 3200+ vs the Pentium 4 3.0C/3.2C and higher processors.