I have Mac Pro 3,1. I have just bought 2x Xeon E5450 SLBBM E0 stepping and found that there are unsupported by my Mac Pro.
I tried to google for this problem and found that only SLANQ C0 stepping works with it.
SLANQ has CPUID 10676; SLBBM has CPUID 1067A
The MacPro 3,1 firmware has the microcode for the SLANQ (CPUID 10676).
Is it possible to edit MacPro 3,1 firmware and add support of SLBBM (CPUID 1067A)?
As far as I know, no-one has ever done it. No E0 stepping 3,1s can be found anywhere except Geekbench - which are probably Hackintoshes.
There's a comprehensive compatibility list on MacRumors - Mac Pro CPU Compatibility List - excerpt from there
The 3,1 MP is not compatible with all steppings. Processors with a 5-digit sSpec starting with "SLB" do not work. Those that start with "SLA" work. The sSpec is marked on the top of the processor. sSpec numbers listed in this table should work.
For my money [I have one] the best bang for the buck is a pair of X5690 in a 4,1 with the Firmware upgrade. Put a flashed AMD HD7950 in it & it runs Mojave [with full Apple driver support & pre-login visibility], though it won't run Catalina, at time of writing.
