Bus speed explained

Discussion in 'Mac Pro' started by corbin_a2, Feb 4, 2006.

  1. corbin_a2 macrumors regular

    Joined:
    Jan 7, 2002
    #1
    Ok, can anyone help me understand bus speed? What is it based on? Why can the G5 have either 1/2 or 1/3 bus speed based on the processor? Why was the G4 stuck at 167 MHz for it’s bus speed? Any outside articles would be great.

    Thanks!
     
  2. Lord Blackadder macrumors G5

    Lord Blackadder

    Joined:
    May 7, 2004
    Location:
    Sod off
    #2
    A good place to start is always Wikipedia - read their article and refer to the links on the page for further reading.

    As you will see, the system bus is pretty critical from a performance standpoint, as well as being an integral component of a system's architecture (and thus hard to change without totally redesigning a system).
     
  3. slooksterPSV macrumors 68030

    slooksterPSV

    Joined:
    Apr 17, 2004
    Location:
    Nowheresville
    #3
    Higher bus speed means more through-put for data, lower bus-speed = bottlenecks. With Apple Processors, bottle-necks = what is a bottleneck? Windows = Bottle-necks = computer crashing, freezing, can't move mouse.
    Help? Ok the bus speed puts data through at a certain speed, so you could have a 200GHz processor at 100MHz bus speed, but it'd be slower than a 10GHz processor at 5GHz bus speed. It is critical in computers.
     
  4. TheMonarch macrumors 65816

    TheMonarch

    Joined:
    May 6, 2005
    Location:
    Bay Area
    #4
    Hmm... Let see if I can use metaphors...

    In layman's terms, the bus speed is the "Straw" which is only way you, the "CPU" can get your drink from. As you, the CPU "get faster" [Grow], you want the "Straw" to get "Wider" [Faster], so you can drink more "Data"

    :) :p
     
  5. Laser47 macrumors 6502a

    Laser47

    Joined:
    Jan 8, 2004
    Location:
    Maryland
    #5
    Theres also a CPU multiplier in that mix also. It multiplies the bus speed by a multiplier which gives you your CPU speed. For instance My ibook has a 142Mhz system bus and a 1.42ghz CPU, which means my ibook has a multiplier of 10x (142x10=1420).
    Thats basically how overclocking works, you increase the system bus speed which makes the CPU run faster, or you could increase the multiplier.
    As for why the bus speed is slower than the cpu, i dont know for sure but i think the CPU cache has somthing to do with it.
     
  6. corbin_a2 thread starter macrumors regular

    Joined:
    Jan 7, 2002
  7. eXan macrumors 601

    eXan

    Joined:
    Jan 10, 2005
    Location:
    Russia
    #7
    I am more interested in why G5 is stuck at 1350! :D (per CPU)
     
  8. slooksterPSV macrumors 68030

    slooksterPSV

    Joined:
    Apr 17, 2004
    Location:
    Nowheresville
    #8
    It's 1/2 of the processors speed/core speed. With HyperTransport it can be exactly the speed of the processor. Your multiplier is 2.0, where I've seen an AMD multiplier of one of them 1.5 so that is large data through-put.
     
  9. eXan macrumors 601

    eXan

    Joined:
    Jan 10, 2005
    Location:
    Russia
    #9
    my question was equal to "Why G5 is stuck at 2.7 Ghz?", if you got it :D
     
  10. Catfish_Man macrumors 68030

    Catfish_Man

    Joined:
    Sep 13, 2001
    Location:
    Portland, OR
    #10
    Well, a 1GHz G5 bus is actually two DDR 250MHz buses, one each direction. If the G4 had DDR support, it would be marketed as a 333MHz bus chip. Supposedly it supports a 200MHz bus setting as well, but Apple hasn't used this (or possibly it hasn't been released yet).

    Technically speaking, the MPX bus protocol that the G4 uses is quite complex compared to the G5's. It's designed to allow multiple devices attached to the same bus, fairly long wires, etc... The G5's bus is a very short very simple 1 directional 1-1 link, using some fairly sophisticated signalling techniques. As a result, it's able to run very fast, although the northbridge chip it hooks up to ends up being quite hot due to having to run fast to keep up with it.

    The easiest upgrade (I think, anyway. I'm not a professional) would be to add DDR support to the MPXbus, and it's unclear why this hasn't been done. My guess is that they decided that the MPXbus was dead, and were moving on to an integrated memory controller, but it took a lot longer than they expected. A G4 with a high speed integrated memory controller and dual cores is expected to be released soon, which should be quite an interesting chip. It's kind of a pity that it's held back by being a Freescale chip :(

    The other annoying issue is latency, which is the amount of time it takes between the processor starting a memory read, and the first bit of the data reaching it. The G5's bus is rather high latency, which hurts it on certain types of code. Presumably it's designed with a bandwidth-uber-alles approach that sacrifices latency in places, but I don't really know.
     
  11. macgeek2005 macrumors 65816

    Joined:
    Jan 31, 2006
    #11
    The Buss speed is how fast the hard drive can transfer data to the proccesor.
     
  12. Laser47 macrumors 6502a

    Laser47

    Joined:
    Jan 8, 2004
    Location:
    Maryland
    #12
    Dont you mean the RAM?
     
  13. MRU macrumors demi-god

    MRU

    Joined:
    Aug 23, 2005
    Location:
    Ireland
    #13
    The most important thing to remember about bus speed, is that if it drops below 50kmph, it explodes......... oh wait that's a different 'speed'...
     
  14. CanadaRAM macrumors G5

    CanadaRAM

    Joined:
    Oct 11, 2004
    Location:
    On the Left Coast - Victoria BC Canada
    #14

Share This Page