Known issues:
Fake cache
The motherboard may include a non-functional or counterfeit SRAM cache, which can adversely affect performance. Signs of fake cache often include soldered QFP or DIP-chips labeled "WRITE-BACK" with traces that do not connect to the chipset.

https://theretroweb.com/motherboards/s/pcchips-m912-v1.7-486

The CPUID and internal cache mode are determined by sensing the WB/WT# pin on the CPU at reset. Nothing you change in the BIOS affects that. It has to be something with how it is jumpered. Check the jumper settings for recommendations for a P24D configuration (that's a 486DX2WB which predates the DX4WB). You can use those settings aside from voltage (obviously) and clock multiplier.

The PCChips M912 is fairly infamous for having fake L2 cache.

Edit: After carefully looking at your SPEEDSYS results it looks like you do have real cache on your motherboard. I just saw the "This machine dose not seem to have any cache" message from your CACHECHK results and assumed the worst.

It is known for some motherboards to intentionally keep L1 cache in WT mode. Enabling WB can cause problems with DMA if not implemented correctly.

jakethompson1 wrote on 2024-07-26, 20:47:

The CPUID and internal cache mode are determined by sensing the WB/WT# pin on the CPU at reset. Nothing you change in the BIOS affects that. It has to be something with how it is jumpered. Check the jumper settings for recommendations for a P24D configuration (that's a 486DX2WB which predates the DX4WB). You can use those settings aside from voltage (obviously) and clock multiplier.

Thank you. I've used P24D settings in the booklet and WB cache is activated.

Glad you got it figured out!