Graphics Glitch During Boot

I put your small movie up on my 30-in display, and Brian Beisser's picture looks about the same.That line of "stuff' looks like un-initialized or garbage VRAM trying to be displayed.

Now I have a question for all participants. Is your card the genuine Mac card with 1/2GB of VRAM, or a flashed ATI card with 1GB of VRAM or a straight PC card with 1GB of VRAM, or something else?

I have both a Mac Pro 4,1 and a 5,1. The 4,1 has a flashed PC version of the 1GB 5770 while the 5,1 has the Apple version. Both are running Sierra 10.12.1 and both exhibit the exact same problem documented in this thread. One screen in a 20 inch Apple Cinema display (1680 X 1050) and the other is a 23 inch Samsung (1920 X 1080) and both display exactly the same line of garbled video (or uninitialized VRAM as Grant Bennet-Alder puts it) that is shown in the OP's message. Like all the others, I am have absolutely no other issues.

What do you mean by a stocked HD? Your experience seems to be the only deviation. I think you are onto something. Please elaborate on what you did.

Stocked HD = The HD shipped with the machine in 2010 🙂 Been testing for a while, and after a safe boot the artifacts are back again. Even more strange: I have a couple of months old Mac Pro, and after a safe boot, that machine also get the same graphics 😟 It´s time to test a clean install (again) and call Apple

I propose the following theory as to why this might be happening:

Some Memory Hardware on display cards does not anticipate the existence of really large display RAM memory. It may not fully decode all the possible Address bits that could potentially be supplied. When this happens, a reference to memory above a certain point -- say, the 1GB boundary (which does not exist on these cards) "should" throw a protection fault.

Instead, that reference will seem to "wrap around" back to the beginning of RAM and make a mistaken reference to a much lower address, because the card does not see the high-order bits. So that which was supposed to be to a very High address gets truncated into a reference to a very low address. It becomes a reference to memory below the 4GB boundary, because it is only looking at the low-order bits of the Address.

The possible problem introduced is that the simple display driver is defaulting to the much large display RAM sizes on newer cards -- and the programmer has forgotten to take into account that some older cards may "wrap" rather than "fault" on too-high addresses.

If all this sounds like word-salad, please post back and I will try to explain further.

I follow the theory, but no problems on my 2010 iMac, MB pro or MacBook Air. Use an "plain" and cheap Samsung 23" monitor on my old Mac Pro, and an Apple Display 27" on the new Mac Pro. Talked to Apple support to day , and the new machine is sent to repair.

I am having this issue on a 27" 11,3 mid 2010 iMac with an i7 and 5250 graphics card. I upgraded to Sierra and did not notice this issue at all until after I installed an SSD kit and re-installed the OS from time machine. Unlike some of you, I am having issues when using the computer. I get "beach ball of death" when in the OS and after forcing the computer down with the power button, it will not boot. Chime, white screen, no logo... If I come back a day later, it may boot and work for a random period of time, and then it will repeat. Not sure what the cause is here. I can see it being any number of things really. Tonight I am going to re-install the original spinning hard disk and see what happens.

Re-install of original SSD fixed my lock ups but the digital artifact is still there during boot up. SSD sent back to OWC for refund.

On my Mac Pro 2010 with AMD Radeon HD 7950 Mac Edition 3GB VRAM, Mac OS Sierra boot screen is flashing😠

Reset PRAM and SMC is not help me.

I'm getting this exact line from on my SO's 27" iMac (Mid 2011) 3.4/i7 32GB AMD Radeon HD 6970M.

Any chance it might be a RAM issue?

I'm getting this exact line from on my SO's 27" iMac (Mid 2011) 3.4/i7 32GB AMD Radeon HD 6970M.

Any chance it might be a RAM issue?

I forgot to mention in my post above, a clean install of Sierra will also cause the issue to occur. This is why you need to install El Capitan first, then upgrade install from within the app store.

Mine came back from service, and the service guy told me that this issue is quite common,

and nothing to worry about !