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The time to read the first bit of memory from a DRAM without an active row is T RCD + CL. Row Precharge Time T RP: The minimum number of clock cycles required between issuing the precharge command and opening the next row. The time to read the first bit of memory from a DRAM with the wrong row open is T RP + T RCD + CL. Row Active Time T RAS
Although a normal read or write cycle refreshes a row of memory, normal memory accesses cannot be relied on to hit all the rows within the necessary time, necessitating a separate refresh process. Rather than use the normal read cycle in the refresh process, to save time, an abbreviated refresh cycle is used.
There are several limits on DRAM performance. Most noted is the read cycle time, the time between successive read operations to an open row. This time decreased from 10 ns for 100 MHz SDRAM (1 MHz = Hz) to 5 ns for DDR-400, but remained relatively unchanged through DDR2-800 and DDR3-1600 generations. However, by operating the interface ...
The time to read additional bits from an open page is much less, defined by the /CAS to /CAS cycle time. The quoted number is the clearest way to compare between the performance of different DRAM memories, as it sets the slower limit regardless of the row length or page size.
Likewise, a memory module which is underclocked could have its CAS latency cycle count reduced to preserve the same CAS latency time. [citation needed] Double data rate (DDR) RAM performs two transfers per clock cycle, and it is usually described by this transfer rate. Because the CAS latency is specified in clock cycles, and not transfers ...
The individual chips making up a 1 GB memory module are usually organized as 2 26 8-bit words, commonly expressed as 64M×8. Memory manufactured in this way is low-density RAM and is usually compatible with any motherboard specifying PC3200 DDR-400 memory. [18] [citation needed]
Early core memory systems had cycle times of about 6 μs, which had fallen to 1.2 μs by the early 1970s, and by the mid-70s it was down to 600 ns (0.6 μs). Some designs had substantially higher performance: the CDC 6600 had a memory cycle time of 1.0 μs in 1964, using cores that required a half-select current of 200 mA. [43]
It refers to the average time it takes to perform a memory access. It is the addition of the execution time for the memory instructions and the memory stall cycles. The execution time is the time for a cache access, and the memory stall cycles include the time to service a cache miss and access lower levels of memory. If the access latency ...