K8: Athlon64, AthlonFX, Opteron

Overclocking setting for various bus frequencies

In A64 system, there is no external FSB per se, there are the CPU, the memory bus, the HT bus. The FSB may still be there internally to the CPU. The FSB still exists in motherboard bios to set various frequencies.

Some bios, CPUID utilities call FSB as HTT or internal bus, so FSB is used interchangeably with HTT as a setting to set CPU, memory, HT bus frequencies until everybody agree and use the same terms.

1. CPU_frequency = FSB x CPU_multiplier

CPU mulitplier is CPU specific, some can be adjusted from below up to the stock multiplier of the CPU.
E.g. A64 754 3200+, stock 2GHz = 200x10, multiplier X10, X9, X8, X7, ....

CPU multiplier

The FX are unlocked.

But the A64 754 are unlocked up to the default stock multiplier.
E.g. for a A64 754 3200+, 2 GHz stock frequency, default multiplier is x10, so it is unlocked for x10, x9, x8, ....
This is good since one can still overclocking the FSB which is related to the HT bus, memory bus via their respective multipliers.


2. AGP_frequency

Can be set independently if motherboard/chipset have PCI/AGP lock such as w/ chipset Nforce3 250 GB. K8T800 Pro may still have PCI/AGP lock problem.
E.g. AGP can be adjusted to hold constant at 66, 67, 68, ... MHz independent of the FSB overclocking.


3. HT_bus_frequency = FSB x HT_multiplier

HT_multiplier (also called LDT multiplier), most bios has it ranged between 1X and 5X.
E.g. FSB = 200 MHz, 1X to 5X for HT from 200 MHz to 1000 MHz


4. memory_bus_frequency = FSB x memory_FSB_ratio

In terms of CPU_frequency
memory_bus_frequency = CPU_frequency / CPU_memory_divider
or
memory_bus_frequency = CPU_multiplier x FSB / CPU_memory_divider
where
CPU_memory_divider = ceiling_0.5(CPU_multiplier / memory_FSB_ratio)

Using this formula, the CPU_memory_divider can be calculated based on CPU_multiplier and memory_FSB_ratio.
Some common cpu_memory_dividers generated by spreadsheet are listed in this table.
Relationship between CPU_memory_divider and CPU_multiplier, memory_FSB_ratio
How to determine memory bus frequency
From which the memory bus frequency can be determined
(memory_bus_frequency = CPU_multiplier x FSB / CPU_memory_divider).

Some planning/estimation based on the CPU multiplier, memory rating to ensure the CPU_memory_divider is an integer. Otherwise some round down of memory bus speed would occur as the divider would round up to next integer.

E.g,
HTT = 270 MHz
CPU_multiplier = 9
memory_FSB_ratio = 5/6 (= 166/200 bios setting)
So
CPU_frequency = 270 x 9 = 2430 MHz
CPU_memory_divider = ceiling(9/(5/6)) = 11 or from table in link
memory_bus_frequency = 2430 /11 = 221 MHz
HT_bus_frequency = 270 x 3 = 810 MHz (using x4 would be to high)
AGP/PCI is locked (AGP = 66, 67 MHz)

- Additional settings are the RAS/CAS timings of tRAS, tRCD, tRP, CAS.
- Can run any speed (ASYNC) using the CPU_memory_divider, to match memory module speed.
- Slow or faster memory can be used to get whatever bandwidth allowed in the memory modules.
- ASYNC has minimum impact on memory bandwidth efficiency
- 754 efficiency 95%+, 939 efficiency 86%+

For desirable overclocking control, one should be able to set all these frequencies of CPU, HT, AGP/PCI, memory independently as shown above. Exact bios naming may vary.


Appendix: Memory SYNC vs ASYNC

SYNC: FSB = memory_bus_frequency (or memory_FSB_ratio = 1/1)

Without a higher ratio of memory_FSB_ratio > 1, running SYNC is the easiest way to max out a memory module that can take high bus frequency.
E.g. with PC4000 or above, by running the FSB (HTT) as high as possible, to 250 - 300+ MHz. Many boards are able to support 300 MHz FSB (HTT) stable.

If HT bus is limited to around 1000 MHz,
- when FSB (HTT) at 250 MHz, HTT_multiplier can be set at x4,
- when FSB (HTT) is far beyond 250 MHz towards 300 MHz, HTT_multiplier may have to be dropped to x3 so the HT bus is stable under 1000 MHz.
This is a tradeoff between memory bus frequency and the HT bus frequency.

E.g. assume CPU maxed around 2500 MHz, with PC4000-4400 memory
Case 1 (PC4000):
FSB = 250 MHz, CPU_multiplier = 10, CPU_frequency = 2500 MHz
HT_bus_frequency = 1000 MHz (w/ x4 HT_mulltiplier)
CPU_memory_divider = 10/1 = 10
memory_bus_frequency = 250 MHz
Case 2 (~PC4400):
FSB = 280 MHz, CPU_multiplier = 9, CPU_frequency = 2520 MHz
HT_bus_frequency = 840 MHz (w/ x3 HT_mulltiplier)
CPU_memory_divider = 9/1 = 9
memory_bus_frequency = 280 MHz

ASYNC: In general, FSB != memory_bus_frequency.

memory_bus_frequency = FSB x memory_FSB_ratio
where memory_FSB_ratio = ..., 6/5, 5/4, 4/3, 3/2, 2/1, 1/2, 2/3, 3/4, 4/5, 5/6, ...
In some bios, only a few can be selected.
- FSB = 200, memory = 200, memory_FSB_ratio = 1/1
- FSB = 200, memory = 166, memory_FSB_ratio = 5/6
- FSB = 200, memory = 160, memory_FSB_ratio = 4/5
- FSB = 200, memory = 150, memory_FSB_ratio = 3/4
- FSB = 200, memory = 133, memory_FSB_ratio = 2/3
- FSB = 200, memory = 100, memory_FSB_ratio = 1/2

E.g. assume CPU maxed at around 2500 MHz
Case 1 (CPU x10 max, ~ PC3200):
FSB = 250 MHz, CPU_multiplier = 10, CPU_frequency = 2500 MHz
memory_FSB_ratio = 5/6
CPU_memory_divider = 10/(5/6) = 12
memory_bus_frequency = 208 MHz
Case 2 (CPU x9 max, ~ PC3200):
FSB = 280 MHz, CPU_multiplier = 9, CPU_frequency = 2520 MHz
memory_FSB_ratio = 3/4
CPU_memory_divider = 9/(3/4) = 12
memory_bus_frequency = 210 MHz
Case 3 (CPU x8 max, ~ PC4000):
FSB = 310 MHz, CPU_multiplier = 8, CPU_frequency = 2480 MHz
memory_FSB_ratio = 4/5 (may not be available, may need 3/4 or 5/6)
CPU_memory_divider = 8/(4/5) = 10
memory_bus_frequency = 248 MHz (assuming 4/5 ratio)


Memory modules (for 754 and 939 platforms) (post 14)

200MB/s na kanal pa nič ne pomaga če jih ne rabiš...

Ena gigabit kartica in en RAID/SCSI/SATA kontroller že skupaj porabita precej več kot 133MB/s, tako da bi se že takoj poznala menjava na PCI-E.