Boot ROM MP51.0084.B00 to xxxx.xxxx.B08

These are all the Mac Pro firmware updates listed by Apple.

With eight DIMMs installed you will probably only be able to run at 1066.

If you remove the DIMM from slot 4 on each side, and your mac then reports the 1333 speed, it is running as expected.

where do you propose to get this firmware, and what is the complete version number without xxx

Welcome in my world 🙂

I've got 4 2009 Mac Pros - in 4 of them I've done successfully update do 5,1 - and in 4 of them update to High Sierra (which means genuine Apple Firmware online download applied).

In 3 of them RAM is running on 1333MHz - in one...... 1066MHz :/

I've done RAM swap - nothing.

Obviously everywhere Xeon supports 1333MHz RAM - I even swapped whole CPU trays - nothing.

Crazy thing how this one is different is... the one which was dual socket tray (12 core) (MOTHERBOARD!!!) does not working under 1333MHz - I said motherboard as..... when I put that dual socket tray to..... genuine ONE socket.... MOTHERBOARD - it is showing.... 1333MHz.

The only thing what comes to my mind - Apple is releasing different firmwares (I know they are the same number) but as I KNOW - doing upgrade Mac Pro 1,1 to 2,1 - there are two the same - but DIFFERENT firmwares for..... refurbished Mac Pro - and non-refurbished!!!

Madness - but this is how Apple is doing.

I'm wonder what would happen when downgrade to 4,1 - BUT SINGLE CPU tray in dual CPU MOTHERBOARD then flash to 5,1 again - and flash again doing High Sierra update.

The thing is stopping me to try this it is that genuine Apple firmware applied when High Sierra update - if there would be no that update - it is safe to downgrade - with that different firmware.... well - who knows - anyone wants to try? 🙂

You are massively overthinking this.

1333 is used natively by the 6-core processors, and not the four-core processors.

The memory is organized as three independent channels, and slots 3 and 4 share a channel. If you plug a DIMM into the fourth slot, it changes the capacitive loading in slot 3-4, and may change the RAM timing back to 1066.

True Grant - but how You explain that if I put the same CPU tray into any other single CPU motherboard it is showing 1333MHz.

If I put any (single or dual) CPU tray (all of them with 1333MHz RAM to be specific) into dual CPU... motherboard - it is showing 1066MHz?

P.S. All my four CPU trays are occupied with all RAM slots

The Mac Pro with Error Correcting Code memory does a lot of testing at startup. It uses the error correction logic to assist in that testing.

Minor differences between the different Macs such as tiny Voltage differences may make one able to run at those speeds and a different combination NOT able to run that those speeds.

OWC, the biggest seller of replacement and add-on memory AND processor and shelf upgrades for these Macs, does not sell the higher speed DIMMs for all-slots-occupied. That indicates to me that they sometimes do not run RELIABLY at full speed with all slots occupied.

Grant - sorry - but with all due respect Your theory is wrong.

I've just checked with my two Mac Pros:

1. I've removed 2 of 4 RAM modules - still 1066MHz

2. Then removed all RAM modules (from slow RAM Mac Pro) and then I've put 2 out of 4 RAM modules (which they are even different brand) from my other Mac Pro (which shows 1333MHz) and put them into "slow RAM Mac Pro" - still 1066MHz 🙂

3. I even put 3 of of 4 RAM modules to eliminate any magic theory - still 1066MHz.

X-Files case?

As I've said - the only one thing what comes to my mind - it is dual CPU motherboard - same as that person's who started this tread.

The question is - why my three other single CPU motherboards work with full 1333MHz?

How did you decide you needed different firmware, and what version you needed?

How many RAM modules are installed?

Four DIMM is all eight slots, or four DIMMs on each of two sides?

that article and the information in it is quite old.

The firmware version you said you have is the one Installed in a 5,1 AFTER upgrade to Sierra. You are lacking nothing.