PC Architecture. A book by Michael B. Karbo

Figure 135. Older RAM modules.
FPM RAM could not run any faster than 66 MHz, but that was fine for the system bus clock frequency in the original
Pentium processors.
After FPM came EDO RAM (Extended Data Out). EDO is a bit faster than FPM because the data paths to and from the
RAM cells have been optimised. The gain was a 3-5 % improvement in bandwidth. The clock frequency could be
increased to 75 MHz, but basically, EDO is not very different to FPM RAM.
When Intel launched the Pentium processor, there was a change to using the 64 bit wide RAM modules (with 168 pins,
as in Fig. 127, which are still used for SDRAM today.
Figure 136. An old motherboard with sockets for both 64-bit and 32-bit RAM modules. From the transition period
between EDO and SDRAM.
SDRAM
The big qualitative shift came in around 1997, when SDRAM (Synchronous DRAM) began to break in. This is a
completely new technology, which of course required new chipsets. SDRAM, in contrast to the earlier types of RAM,
operates synchronously with the system bus.
Data can (in burst mode) be fetched on every clock pulse. Thus the module can operate fully synchronised with (at the
same beat as) the bus – without so-called wait states (inactive clock pulses). Because they are linked synchronously
to the system bus, SDRAM modules can run at much higher clock frequencies.
The 100 MHz SDRAM (PC100) quickly became popular, and with new processors and chipsets, the speed was brought
up to 133 MHz (PC133).
Another innovation in SDRAM is the small EEPROM chip called the Serial Presence Detect chip, which is mounted on
the modules. It is a very small chip containing data on the modules speed, etc.