RAM Specifications

[Alan Carlson]

I'm hoping someone can help me understand these numbers a bit better.

Everything I Know About RAM(TM):

- the FSB on a modern motherboard is 64 bits wide to facilitate the
processor reading 64 bits at a time

- SDRAM runs at the same speed of the motherboard

- DDR SDRAM performs two transfers per cycle thereby effectively
doubling the clock rate

- because the processor reads data 64 bits at a time, each DDR DIMM
consitutes a complete bank

Using a sampling of RAM types in use today, I came up with the
following:

Type Throughput
------ ---------------------------------
PC1600 100 MHz x 2 x 64 bits = 1600 MB/s
PC2100 133 MHz x 2 x 64 bits = 2133 MB/s
PC2700 166 MHz x 2 x 64 bits = 2666 MB/s
PC3200 200 MHz x 2 x 64 bits = 3200 MB/s

Ok, so far so good. My understanding of the phrase "same speed as the
motherboard" to mean "the speed of the FSB". Obviously, that isn't
quite right, because on newer PCs, the FSB speeds corresponding to the
above RAM types are:

400 MHz
533 MHz
666 MHz
800 Mhz

If someone someone clue me in as to what I'm missing, I'd be grateful.

 

[kony]

I'm hoping someone can help me understand these numbers a bit better.

Everything I Know About RAM(TM):

- the FSB on a modern motherboard is 64 bits wide to facilitate the
processor reading 64 bits at a time

- SDRAM runs at the same speed of the motherboard

- DDR SDRAM performs two transfers per cycle thereby effectively
doubling the clock rate

- because the processor reads data 64 bits at a time, each DDR DIMM
consitutes a complete bank

Using a sampling of RAM types in use today, I came up with the
following:

Type Throughput
------ ---------------------------------
PC1600 100 MHz x 2 x 64 bits = 1600 MB/s
PC2100 133 MHz x 2 x 64 bits = 2133 MB/s
PC2700 166 MHz x 2 x 64 bits = 2666 MB/s
PC3200 200 MHz x 2 x 64 bits = 3200 MB/s

Ok, so far so good. My understanding of the phrase "same speed as the
motherboard" to mean "the speed of the FSB". Obviously, that isn't
quite right, because on newer PCs, the FSB speeds corresponding to the
above RAM types are:

400 MHz
533 MHz
666 MHz
800 Mhz

If someone someone clue me in as to what I'm missing, I'd be grateful.

What you were missing is that you were only comparing
Pentium 4 chipset FSB rates which are 4X the clock rate
instead of 2X the clock rate like Athlon chipsets.

Given two 200MHz clock rates, one per each memory and FSB,
the memory is Double Data Rate, 2 x 200MHz, PC3200
The FSB is "quad-pumped", 4X the clock rate. 4 X 200MHz =
800MHz (in this context the "MHz" attached to "800" is
misleading as it's a data rate frequency instead of a clock
rate frequency).

 

[Alan Carlson]

I'm hoping someone can help me understand these numbers a bit better.

[snip]

What you were missing is that you were only comparing
Pentium 4 chipset FSB rates which are 4X the clock rate
instead of 2X the clock rate like Athlon chipsets.

Given two 200MHz clock rates, one per each memory and FSB,
the memory is Double Data Rate, 2 x 200MHz, PC3200
The FSB is "quad-pumped", 4X the clock rate. 4 X 200MHz =
800MHz (in this context the "MHz" attached to "800" is
misleading as it's a data rate frequency instead of a clock
rate frequency).

Sorry for the late reply, but thanks for your response. It helped.

Cheers.