Relationship between CPU_memory_divider and CPU_multiplier, memory_HTT_ratio
Here FSB is used interchangeably with HTT, in the various settings for A64. In A64, FSB or HTT is a CPU frequency signal, NOT a physical bus. The system bus external to the CPU is the HT (HyperTransport bus).
memory_bus_frequency = CPU_frequency / CPU_memory_divider
or
memory_bus_frequency = CPU_multiplier x HTT / CPU_memory_divider
where
CPU_memory_divider = ceiling(CPU_multiplier / memory_HTT_ratio)
where memory_HTT_ratio = 1/1, 1/2, 2/3, 3/4, 4/5, 5/6, 7/8, 9/10, ... (availability of somel settings is bios/motherboard dependent)
Impact of CPU multiplier on memory divider flexibility
For A64, since the memory divider
available depends on the CPU multiplier, it is advantageous to know what memory modules to use in order to plan for what CPU and its multiplier to get.
In general, due to the discrete nature of CPU multiplier and memory divider, it may not be possible to have both the CPU and memory being topped out simultaneously (within 1 MHz), unless the ratio of the max CPU frequency to max memory frequency is an exact integer or half integer.
For example, refer to the memory_divider table below,
assume bios only has 1:1, 5:6, 2:3 memory_HTT_ratio
NC 2800+, max multiplier = 9, memory_divider available = 9, 11, 13.5
NC 3000+/CH 3200+, max multiplier = 10, memory_divider available = 9, 10, 11, 12, 13.5, 15
So unless knowing what exact memory_divider is needed and plan for the right CPU multiplier, in general, a CPU with higher max multiplier (such as 10) is more flexible (has more memory dividers) in setting up memory modules with various speed than a CPU with lower max multiplier (such as 9).
Note on bios with memory_HTT_ratio of 9:10 and 8:10
The additional offering of 9:10 and 8:10 (same as 4:5) (in DFI bios) to the other memory_HTT_ratios of 5:6, 2:3, 1:2, ... would allow a better chance to finer tune the memory and CPU speed during overclocking.
For example, for max CPU multiplier of 10 (3000+ NC. 3200+ CH), the 9:10 and 4:5 may give an additional memory_divider of 11.5 and 12.5 on top of the 9, 10, 11, 12, 13.5, 15.
So instead of about 20 MHz in tunning step in memory, it may reduce to about 10 MHz in step with the added 9:10 and 8:10 (4:5) ratio.
How to determine memory bus frequency
1. First, the memory_HTT_ratio is determined from the bios setting.
memory_HTT_ratio = ..., 6/5, 5/4, 4/3, 3/2, 2/1, 1/2, 2/3, 3/4, 4/5, 5/6, 9/10, ...
In some bios, only a few can be selected.
With reference to the rated HTT 200 MHz,
- HTT = 200, max memory frequency = 200, memory_HTT_ratio = 1/1
- HTT = 200, max memory frequency = 183, memory_HTT_ratio = 11/12
- HTT = 200, max memory frequency = 180, memory_HTT_ratio = 9/10
- HTT = 200, max memory frequency = 175, memory_HTT_ratio = 7/8
- HTT = 200, max memory frequency = 166, memory_HTT_ratio = 5/6
- HTT = 200, max memory frequency = 160, memory_HTT_ratio = 4/5
- HTT = 200, max memory frequency = 150, memory_HTT_ratio = 3/4
- HTT = 200, max memory frequency = 140, memory_HTT_ratio = 7/10
- HTT = 200, max memory frequency = 133, memory_HTT_ratio = 2/3
- HTT = 200, max memory frequency = 120, memory_HTT_ratio = 3/5
- HTT = 200, max memory frequency = 100, memory_HTT_ratio = 1/2
2. Using the above formula for CPU_memory_divider,
CPU_memory_divider = ceiling(CPU_multiplier / memory_HTT_ratio)
the CPU_memory_divider can be calculated based on only CPU_multiplier and memory_HTT_ratio.
Some common cpu_memory_dividers generated by spreadsheet are listed in this table.
In the table, for most motherboards and bios, 1/2 multipliers may not be supported. If that is the case, 1/2 multipliers should be round to the next integer. E.g. 13.5 should become 14.
3. Then the memory_bus_frequency can be determined
memory_bus_frequency = CPU_frequency / CPU_memory_divider
or
memory_bus_frequency = CPU_multiplier x HTT / CPU_memory_divider
Overclocking setting for various bus frequencies
How to choose memory divider and memory_HTT_ratio
Example to setup frequencies for CPU and memory
