Overclocking the A64


Overclocking an A64 is a balance between getting the maximum memory speed and maximum cpu speed at the same time by utilizing the vitually unlimited HTT bus. Dropping the LDT multiplier to a low number like 2x can allow one to clock generally over 300 and sometimes over 400mhz HTT on an nforce3 board. With this ability, you have to find the ideal HTT and cpu multiplier combination that maximized your cpu speed, while using the right divider on the RAM to get the best blend of memory timings and frequency. Maximizing memory speed can be tricky on the A64 because of the way the integrated memory controller determines the memory frequency:

Memory frequency is determined by both CPU multiplier and RAM divider, unlike any other system.

The Memory Frequency (f) = CPU Frequency (c) / [Multiplier (m) + Divider (d)]

f = c / (m + d)

where d is 0, 2, or 5 (1:1, 6:5, 3:2)
m is an integer, always rounded up (therefore .5 multi's will round up)

so if you run 2.2ghz 1:1 on a 10x multiplier your memory frequency is 220mhz

220 = 2200 / (10 + 0)

but if you run 2.4ghz on a 9x multiplier (267x9) 6:5 your memory frequency is NOT 222 (267 x .83), but rather 218.

218.18 = 2400 / (9 + 2)

EXCEPT in 1:1 with an integer multiplier, then it works like always:

memory frequency = bus speed.


The easiest solution when overclocking an Athlon 64 is simply to get RAM that is well paired with your processor so that you can simply run 1:1 while maximizing both. If you get a newcastle cpu, plan on speeds of 2.5-2.6ghz with conventional cooling and accordingly buy RAM that is capable of about 266mhz or thereabout with good timings. If you are buying a clawhammer, planning on speeds of 2.4-2.5ghz is more reasonable and therefore the memory you need would not have to go quite as fast to get the most out of the processor. With winchesters or higher speed clawhammers and newcastles, you may have to play the divider game or get some very good RAM.

Athlon 64 chips are also very sensitive to the ras:cas latency of the memory. while other timings are not as important as they are on other platforms, maintaining a low ras:cas value is crucial to getting maximum performance. Therefore, it may be neccessary to test your memory and system performance at various speeds with various ras:cas latencies to determine the best setting for you.

This guide was written by eshbach