Synchronous DRAM (SDRAM) was originally a lower cost alternative
to VRAM. It is synchronized to the system clock (that is, the external CPU
frequency), taking memory access away from the CPU's control; internal registers
in the chips accept a request, and let the CPU do something else while the
data requested is assembled for the next time it talks to the memory, as
the memory knows when the next cycle is due because of the synchronization.
In other words, SDRAM works like standard DRAM, but includes
interleaving, synchronization and burst mode, so wait states are virtually
eliminated (SDRAM DIMMs also contain two cell banks which are automatically
interleaved). Itís not actually faster than DRAM, just more efficient; although
the chips are rated at 10 ns, they are not used at that speed - typically,
between 20-50 ns is more like it, since the smaller figure only refers to
reads from sequential locations in bursts - the larger one refers to the
initial data fetch.
Data bursts are twice as fast as with EDO (above),
but this is slightly offset by the organization required. The peak bandwidth
of 133 SDRAM is about 33% higher than that of 100.
Registered DIMMs contain registers on board, which re-drive
the signals, meaning you can have more chips. SLDRAM uses an even higher
bus speed and a packet system. However, with a CPU running at 4 or 5 times
the memory speed, even SDRAM is finding it hard to keep up, although DDR
(Double Data Rate) SDRAM doubles the memory speed by using the rising and
falling edges of the clock pulse, and has less latency than RAMBUS, giving
it a slight edge.
Performance wise, SDRAM only really comes into its own
with a memory bus above 75 MHz. Hitachi have developed a way of replacing
the capacitor in DRAM with a transistor attached to the MOSFET, where a 1
or 0 is represented by the presence (or not) of electrons between its insulating
layers. This means low power requirements, hence less heat, and speed.
This is an article from Phil Croucher, author of
"The BIOS Companion" Phil
has a way of explaining in "plain" English. The information is well presented and is
well above A+ standard.