DRAM Ratio (CPU:DRAM)
Common Options : 1:1, 3:2, 3:4, 4:5, 5:4
The choice of options in this BIOS feature depends entirely on the setting of the DRAM Ratio H/W Strap or N/B Strap CPU As BIOS feature.
When DRAM Ratio H/W Strap has been set to Low, the available options are 1:1 and 3:4.
When DRAM Ratio H/W Strap has been set to High, the available options are 1:1 and 4:5.
When N/B Strap CPU As has been set to PSB800, the available options are 1:1, 3.2 and 5:4.
When N/B Strap CPU As has been set to PSB533, the available options are 1:1 and 4:5.
When N/B Strap CPU As has been set to PSB400, the only available option is 3:4.
The options of 1:1, 3:2, 3:4 and 4:5 refer to the available CPU-to-DRAM (or CPU:DRAM) ratios.
Please note that while the Pentium 4 processor is said to have a 400 MHz or 533 MHz or 800 MHz FSB (front side bus), the front side bus (also known as CPU bus) is actually only running at 100 MHz or 133 MHz or 200 MHz respectively. This is because the Pentium 4 bus is a Quad Data Rate or QDR bus which transfers four times as much data as a single data rate bus.
For marketing reasons, the Pentium 4 bus is labeled as running at 400 MHz or 533 MHz or 800 MHz when it is actually running at only 100 MHz, 133 MHz and 200 MHz respectively. It is important to keep this in mind when setting this BIOS feature.
For example, if you set a 3:2 ratio with a 200 MHz (800 MHz QDR) CPU bus, the memory bus will run at (200 MHz / 3) x 2 = 133 MHz or 266 MHz DDR.
By default, this BIOS feature is set to By SPD. This allows the chipset to query the SPD (Serial Presence Detect) chip on every memory module and use the appropriate ratio.
It is recommended that you select the ratio that allows you to maximize your memory modules' capabilities. But bear in mind that synchronous operation using the 1:1 ratio is also highly desirable as it allows a high throughput.
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