25-01-2005, 21:50
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מנהל פורום חומרה
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חבר מתאריך: 27.05.02
הודעות: 22,477
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זה 1T ו 2T.
בבקשה, היישר מגוגל:
http://www.anandtech.com/memory/showdoc.aspx?i=2223
The command rate specifies how many consecutive clock cycles that commands need to be presented to the DRAMs before the DRAMs sample the address and command bus wires. The package of the memory controller, the wires of the address and command buses, and the package of the DRAM all have some electrical capacitance. As electrical 1's and 0's in the commands are sent from the memory controller to the DRAMs, the capacitance of these (and other) elements of the memory system slow the rate at which an electrical transition between a 1 and a 0 (and vice versa) can occur. At ever-increasing memory bus clock speeds, the clock period shrinks, meaning that there is less time available for the transition between a 1 and a 0 (and vice versa) to occur. Because of the way that addresses and commands are routed to DRAMs on memory modules, the total capacitance on these wires may be so high that transitions between 1 and 0 cannot occur reliably in only one clock cycle. For this reason, commands may need to be sent for 2 consecutive clock cycles so that they can be assured of settling to their appropriate values before the DRAMs take action. A 2T command rate means that commands are presented for 2 consecutive clocks to the DRAMs. In some implementations, command rate is always 1T, while in others, it may be either 1T or 2T. On DDR/DDR2, for instance, using high-quality memory modules (which cost a little more) and/or reducing the number of memory modules on each channel can allow 1T command rates. If you are wondering how the command rate can impact performance, that explanation will have hopefully made it clear that CMD can be just as important as CL. Every memory access will incur the CMD and CL delays, so removing one memory clock cycle from each benefits every memory access.
המשמעות של כל זה היא פשוטה.
כאשר מידע עובר על באסים, הוא עובר בפעימות של שעון. כדי להיות בטוחים שעבר מספיק זמן כדי שהמידע שעבר בפעם שעברה על הבאס הספיק להיעלם, או יותר נכון שהמידע החדש הספיק להתייצב - צריכים לחכות איזשהו "זמן בטוח" כדי לתת לאות להתייצב לפני שהזיכרון יכול לגשת אל הבאס ולדלות משם את הנתונים שלו (שזה כתובת והוראה). לך יש את האפשרות לשנות את הזמן הזה ל 1T או 2T, כלומר קלוק אחד או שני קלוקים (פעימות שעון הכוונה).
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The only certainty in life is that there are no certainties.
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