TwinMOS PC-166

As you as see from the marking on the chip, it is a 6ns chip most commonly refer to the cycle time and it is confirmed by the ctSPD. There are so many memory timings and people are often confused by the timings stated on the chips. Most commonly, manufacturers would mark their chip according to the Cycle time or less common the Access Time. Mitsubishi always mark their PC-133 SDRAM as -6 which is often mistook as a "faster" RAM than the rest which are marked as -75. What Mitsubishi is refering to the Access Time not the Cycle Time while in actual fact is same speed as the rest. Now the Access Time for this TwinMOS PC-166 SDRAM is 5ns and Cycle Time is 6ns so it is indeed "faster" than the normal PC-133 SDRAM.

SDRAM is also rated by its CAS latency, the RAS to CAS latency, and RAS Precharge time, respectively. For a standard SDRAM, the latency will be the sum of the tRCD (RAS to CAS delay, or the amount of time to access the row and get ready for receiving the column address), CAS Latency tCAC (this is also called tCAC, or Column Access time. It is the amount of time from Column Address until data is on the output register) and tAC (Access Time, the amount of time from the output register until it appears on the bus) values.