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<html><body><a href="doc089.html"><img src=../icons/next.gif alt="Next"></a><a href="doc000.html"><img src=../icons/up.gif alt="Up"></a><a href="doc087.html"><img src=../icons/previous.gif alt="Previous"></a><a href="doc000.html"><img src=../icons/contents.gif alt="Contents"></a><a href="doc123.html"><img src=../icons/index.gif alt="Index"></a><hr><h2><a name="sC.5">C.5 A Few Notes On Clocking</a></h2><title>A Few Notes On Clocking</title><p> All of the current Alpha CPUs use high-speed clocks, because theirmicroarchitectures have been designed as so-called short-tickdesigns. None of the sytem busses have to run at horrendous speeds asa result though:<p><ul><p><li> on the 21066(A), 21064(A), 21164 the off-chip cache (Bcache)timing is completely programmable, to the resolution of the CPUclock. For example, on a 275MHz CPU, the Bcache read access time canbe controller with a resolution of 3.6ns<p><li> on the 21066(A), the DRAM timing is completely programmable, tothe resolution of the CPU clock (<em>not</em> the PCI clock, the CPU clock).<p><li> on the 21064(A), 21164(A), the system bus frequency is asub-multiple of the CPU clock frequency. Most of the 21064motherboards use a 33MHz system bus clock.<p><li> Systems that use the 21066 can run the PCI at any frequencyrelative to the CPU. Generally, the PCI runs at 33MHz.<p><li> Systems that use the APECs chipset (see Section <a href="doc089.html#sC.6">C.6</a>) always have their CPU system bus equal to their PCI busfrequency. This means that both busses tends to run at either 25MHz or33MHz (since these are the frequencies that scale up to match the CPUfrequencies). On APECs systems, the DRAM controller timings aresoftware programmable in terms of the CPU system bus frequency<p></ul><p><b>Aside:</b> someone suggested that they were getting bad performanceon a 21066 because the 21066 memory controller was only running at33MHz. Actually, it's the superfast 21064A systems that have memorycontrollers that 'only' run at 33MHz.<p><p><hr><a href="doc089.html"><img src=../icons/next.gif alt="Next"></a><a href="doc000.html"><img src=../icons/up.gif alt="Up"></a><a href="doc087.html"><img src=../icons/previous.gif alt="Previous"></a><a href="doc000.html"><img src=../icons/contents.gif alt="Contents"></a><a href="doc123.html"><img src=../icons/index.gif alt="Index"></a><hr></body></html>⌨️ 快捷键说明
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