And the fastest EDO I know so far is rated 50ns, which is miles away from the slowest SDRAM (PC-66) with 12/15ns.
Actually, no it's not "miles away." Sure, SDRAM can run at faster speeds, whereas EDO topped out around 75-83mhz... and SDRAM brought a whole lot of technical changes in the way the memory bus works, which allows a lot more bandwidth. But as far as real-world performance, clock-for-clock, EDO actually isn't all that much slower than SDRAM. The difference is only about 5%. None of the platforms from that era really needed the additional bandwidth that SDRAM could provide.
See, the thing that most people don't understand, is that the speed ratings on EDO/FPM describe something entirely different than the ones on SDRAM... the two numbers are not in any way comparable.
The rating on SDRAM describes only the minimum clock cycle time. For example, PC100 SDRAM is rated 10ns or better. That 10ns rating is derived very simply: 100mhz means 100 million cycles per second... if you take the reciprocal of that, one cycle takes 100 millionth of a second, i.e., 10ns.
EDO/FPM, on the other hand, is rated by the minimum access time, in other words, how long it actually takes to look up any particular bit in memory and then feed it back to the data bus. That rating takes a whole lot more steps into account than just the clock cycle time (which by itself doesn't mean much, since most memory operations take more than one clock cycle). Incidentally, the actual total access time on SDRAM at 66mhz, is somewhere in the range of 50-60ns.
