Introduction

Wide Dynamic Execution, Advanced Digital Media Boost, Smart Memory Access and Advanced Smart Cache; those are the technologies that according to the marketing people at Intel enable Intel to build the high performance, low energy CPUs using the new Core architecture.

Of course, as an AnandTech Reader, you couldn't care less about which Hyper Super Advanced Label the marketing folks glue on their CPUs. "Extend the digital lifestyle by combining robust performance with low power consumption" could have been another marketing claim for the new Core architecture, but VIA already cornered that sentence for its C7 CPUs. The marketing slogans for Intel's Core and VIA's C7 are almost the same; the architectures are however vastly different.

No, let us find out what is really behind all this marketing hyper-talk, and preferably compare it with the AMD "K8" (Athlon 64, Opteron) architecture of Intel's NetBurst and Pentium M processors. That is what this article is all about. We talked to Jack Doweck, the engineer who designed the completely new Memory Reorder Buffer and Memory disambiguation system. Jack Doweck is one of the Intel Israel Development Center (IDC) architects.

The Intel "P8"

Intel marketing states that Core is a blend of P-M techniques and NetBurst architecture. However, Core is clearly a descendant of the Pentium Pro, or the P6 architecture. It is very hard to find anything "Pentium 4" or "NetBurst" in the Core architecture. While talking to Jack Doweck, it became clear that only the prefetching was inspired by experiences with the Pentium 4. Everything else is an evolution of "Yonah" (Core Duo), which was itself an improvement of Dothan and Banias. Those CPUs inherited the bus of the Pentium 4, but are still clearly children of the hugely successful P6 architecture. In a sense, you could call Core the "P8" architecture, with Banias/Dothan being based on the "P7" architecture. (Note that the architecture of Banias/Dothan was never given an official name, so we will refer to it as "P-M" for simplicity's sake.)

Of course this doesn't mean that Intel's engineers just bolted a few functional units and a few decoders on Yonah and called it a day. Jack told us that Woodcrest/Conroe/Merom are indeed based on Yonah, but that almost 80% of both the architecture and circuit design had to be redone.

CPU architecture in a nutshell

For those of you who are not so familiar with CPUs, we'll start with a crash course in CPU architectures. To understand CPU design, you must first look at the instructions that are sent to the CPU, and thus we start with the software.

Typical x86 software code consists of about 50% stores and loads, and there are about twice as many loads as there are stores. Of the remainder, about 15 to 20% of the instructions are branches (If, Then, Else), and the rest are mostly "ADD" (addition) and "MUL" (multiply) instructions. Only a very small percentage of code consists of more exotic instructions such as DIV (divisions), SQRT (square root), or other higher order math (e.g. trigonometric functions).

All these instructions are processed in a typical "Von Neuman" pipeline: Fetch, Decode, Operand Fetch, Execute, Retire.

Instructions are fetched based on the instruction pointer register, and initially they are nothing but long bit patterns to the CPU. It's only after the CPU starts decoding the bits that the instructions "start to make sense" to the CPU. Addresses and opcodes are decoded out of the instructions, and the addresses are used for the next step: the operand fetch. As you don't want the CPU to perform calculations with the addresses but rather on the content of these addresses - the "operands" - the CPU has to fetch the right data out of the data cache. Once these operands are put in the registers, the ALU is steered by the "opcode" (which has been decoded) to perform the right calculation on the operands in the registers.

The results are written to the architecture register file, the registers which can be used by the compiler. The results must also be written to the caches and the main memory, so that these are also up to date. That is the final phase, the retire phase. That is the basically how processing works in all CPUs.

The main challenge for the CPU designer today is the average memory latency the CPU sees. A Pentium 4 3.6 GHz with DDR-400 runs no less than 18 times faster than the base clock of the RAM (200 MHz). Every cycle the memory is being accessed, a minimum of 18 cycles pass on the CPU. At the same time, it takes several cycles to even send a request, and it takes a few cycles to send a request back. (We discussed this in the past in our overview of memory technology article.) The result is that wait times of 200 to 300 cycles are not uncommon on the Pentium 4. The goal of CPU cache is to avoid accessing RAM, but even if the CPU only has to go to system memory 4% of the time, that 4% of the time can lower performance significantly.

Memory Subsystem
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  • Betwon - Tuesday, May 02, 2006 - link

    P6: sub [mem],eax decodes to three micro-ops
    Core duo: sub [mem],eax decodes to two micro-ops
    K8: sub [mem],eax decodes to one macro-op

    P6: sub eax,[mem] decodes to two micro-ops
    Core duo: sub eax,[mem] decodes to one micro-op
    K8: sub eax,[mem] decodes to one macro-op

    Intel's micro-fusion is different with the K7/K8's macro-op.

    P4 has 2X2 int ALU and 2 AGU.
    K7/K8 has 3 int ALUand 3 AGU.
    But Core duo has only 2 int ALU and 2 AGU.

    The integer performance:
    Core duo>K7/K8

    Why?
    Because Core duo's length of the depenency chain of the critical path is the shorest.
    The most integer program asm codes can be thought as a high and thin tree of the depenency chain, (the longest depenency chain is called the critical path)
    The critical path determines the performance. The length of the depenency chain of the critical path is the cycles needed to complete this critical path.
    Core duo(2ALU/2AGU) spends less cycles than K7/K8(3ALU/3AGU) -- Because more INT funtions can not accelerate the true dependency atoms-operations.

    The P-M/Core duo's special ability of anti true dependency atoms-operations is the real reason of it's INT outperformance, which is different with old P6(such as Pentium 3).

    The most FP program asm codes can be thought as a boskage (there are many short depenency chains). The best ILP can be performed--The more FP FADD/FMUL funtions, the more performence.

    Double the FP funtions or double the speed of half-speed FP functions is a good idea for the most FP programs.
    But double INT funtions do not always enhence the INT preformence so greatly.
    Conroe only has three ALUs and two AGUs, but not 4 ALU and 4AGU.
    K7/K8 has three ALUs and three AGUs.
    Reply
  • Betwon - Tuesday, May 02, 2006 - link

    Sorry, I'm not one from a English-language nations. I just tell my idea with many spelling or syntax errors.
    funtions -- funtion units

    I want to tell why Conroe with only 3ALU/2AGU has the INT outperformance. Core has the superexcellent ability to process the true dependency chains.(much much better than K8 3ALU/3AGU).
    Even Core duo with 2ALU/2AGU has the INT outperformance(much better than K8 3ALU/3AGU).

    The length of the depenency chain of the critical path
    The true dependency atom-operations
    Reply
  • Starglider - Monday, May 01, 2006 - link

    A truly excellent article. I just have a couple of questions;

    quote:

    The second and more important advantage is the on die memory controller, which lowers the latency to the memory considerably. However, the lower clockspeeds of the Core CPUs (relative to NetBurst) and the faster FSB also lower latency significantly. With the numbers available to us now, we have reason to believe that the Athlon 64 X2's latency advantage will shrink to only 15 to 20%.


    It's clear that increasing FSB speed can reduce memory latency. However I'm not clear why lower CPU core speed will reduce absolute latency - sure it will reduce the number of CPU cycles that occur while waiting for memory, but how can it reduce the absolute delay?

    You don't seem to have included inter-instruction latency in your comparison tables. I know this data can be hard to get hold of, but it's critical to the performance of highly serial code (e.g. the pi calculation benchmarks that seem to be so popular at the moment). Is there any chance it could be included?

    Finally I'm wondering if Intel will revist some of the P4 clock speed enhancing tricks later on. Things like LVS and double pumped AUs would only have slowed down an already complex development process that Intel desperately needed to be completed quickly. But if AMD do come out with a new architecture that matches or exceeds Conroe on IPC, Intel might be able to respond quite quickly by bringing back some of their already well understood clock speed tricks to accelerate Conroe.
    Reply
  • Makaveli - Monday, May 01, 2006 - link

    You need to get away from thinking of increased clockspeed for extra performance, the future is multicore Cpu's and parellism. Reply
  • Starglider - Tuesday, May 02, 2006 - link

    I write heavily multithreaded applications for a living, but sometimes there is just no substitute for fast serial execution; a lot of things just can't be parallelised. Serial execution speed is effectively IPC * clock rate, so yes increasing clock rate is still very helpful as long as IPC doesn't suffer. Reply
  • saratoga - Monday, May 01, 2006 - link

    ^^ Did you read the post you replied to? His point is valid.

    Lower clock speed is not going to improve memory latency. It may mean that latency is less painful, but if you took two core chips, one at 2GHz and the other at 3GHz, the absolute latency is roughly if not exactly the same for each. Though the cost of each ns of latency is 50% more dear on the 3GHz chip.
    Reply
  • Spoonbender - Tuesday, May 02, 2006 - link

    It would be more accurate to say that each cycle of latency is more dear on the 2ghz chip, wouldn't it? ;)
    What matters is how *long* the latency is, in ns. A ns latency is a ns, and it forces the cpu to wait exactly one ns, no matter its clock speed... :)

    So yeah, definitely a valid point, and I wondered about that in the article as well.
    Reply
  • saratoga - Wednesday, May 03, 2006 - link

    quote:

    It would be more accurate to say that each cycle of latency is more dear on the 2ghz chip, wouldn't it? ;)


    No. Its 50% worse for the 3GHz chip (since the clock speed is 50% higer).

    quote:

    What matters is how *long* the latency is, in ns. A ns latency is a ns, and it forces the cpu to wait exactly one ns, no matter its clock speed... :)


    And one ns is how many clock cycles on a 2GHz chip? And how many of a 3GHz chip? Think this through . . .
    Reply
  • Spoonbender - Wednesday, May 03, 2006 - link

    quote:

    No. Its 50% worse for the 3GHz chip (since the clock speed is 50% higer).

    No it isn't. They both waste *exactly* one ns of execution per, well, ns of latency. How many cycles they can cram into that ns is irrelevant.

    [quote]
    And one ns is how many clock cycles on a 2GHz chip? And how many of a 3GHz chip? Think this through . .
    [/quote]
    Yeah, of course, with 1 ns latency, a 3ghz chip will waste more clock cycles than a 2ghz chip, yes. That's obvious. But they will both lose exactly 1 ns worth of execution. That's what matters. Not the number of clock cycles.
    If they both perform equally well, despite the clock speed difference, then adding 1 ns latency to both will have exactly the same impact on both. Yes, the 3ghz chip will lose more clock cycles, but the 2ghz will (if we stick with the assumption of similar performance), have a higher IPC, and so waste the same amount of actual work.

    If you like, look at Athlon 64 and P4.
    If both cpu's waste one clock cycle, then the A64 takes the biggest impact, because of its higher IPC.
    If both cpu's waste one ns, it doesn't change anything. True, the P4 loses the biggest amount of cycles, but as I said before, the A64 loses more *per* cycle. The net result is that they both lose, wait for it, *one* nanosecond's worth of execution.
    Reply
  • BigT383 - Monday, May 01, 2006 - link

    I loved this article. It's due to articles like these that I've been reading Anandtech since before the days of the K6-2. Reply

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