Setting up CPU, memory bus, HT bus frequencies for A64
AXP has only FSB to adjust for CPU, memory frequency.
For A64, there are CPU, memory bus (CPU to memory), HT system bus (HyperTransport from CPU to chipset) to adjust. All three frequency are based on a clock signal called HTT (different than HT).
1. CPU_frequency = HTT * CPU_multiplier
2. memory_bus_frequency = CPU_frequency / CPU_memory_divider
where
CPU_memory_divider = ceiling(CPU_multiplier / memory_HTT_ratio)
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)
3. HT_bus_frequency = HTT * LDT_multiplier
Many programs mix up the terms FSB, HT, HTT and this is where the confusion arises since the beginning of A64.
Typically, HTT = 200 - 300+ MHz, multiplying by CPU_multiplier gives the CPU frequency, multiplying by LDT_multiplier gives the HT system bus frequency.
E.g.
Winchester 3000+ with x9 multiplier:
HTT = 200 - 300+ will give 1800 - 2700+ MHz CPU frequency
With memory_HTT_ratio = 1:1 (max memoryclock = 200 MHz), memory_divider = 9
memory_bus_frequency = 200 - 300+ MHz (= CPU_frequency / 9)
With memory_HTT_ratio = 5:6 (max memoryclock = 166 MHz), memory_divider = 11
memory_bus_frequency = 164 - 245+ MHz (= CPU_frequency / 11)
Winchester 3200+ with x10 multiplier:
HTT = 200 - 300+ will give 2000 - 3000+ MHz CPU frequency
With memory_HTT_ratio = 1:1 (max memoryclock = 200 MHz), memory_divider = 10
memory_bus_frequency = 200 - 300+ MHz (= CPU_frequency / 10)
With memory_HTT_ratio = 5:6 (max memoryclock = 166 MHz), memory_divider = 12
memory_bus_frequency = 167 - 250+ MHz (= CPU_frequency / 12)
(for memory_divider, refer to table below)
LDT_multiplier is typically set to x3, x4 with a HTT of 200 - 300+ MHz to keep the HT bus around specification of 1000 MHz for 939, 800 MHz for 754. It is usually kept under 1000 MHz to avoid stability uncertainty, higher HT bus frequency may be used.
E.g. HTT = 300 MHz, LDT_multiplier = 3, HT_bus_frequency = 900 MHz
For more details,
Overclocking setting for various bus frequencies (post 8)
