MemTest86 V10.2 (Free) displays invalid addresses

[ghewson]

On a machine with 64GB of memory, on most tests MemTest86 displays an address range of (using apostrophes for ease of reading) 0x1'0000'0000 - 0x10'2F38'0000. See photo. Also, it says under Address Range that the maximum system address is 0x10'3000'0000. This address and the high address displayed in testing exceed the actual maximum for 64GB of 0x10'0000'0000-1.

Most tests start off displaying a rapid sequence of addresses which look reasonable, but quickly settle on the above range (except test 10, which has the low address as 0x10'2C00'0000).

Test 13 is different. It displays reasonable-looking addresses, but the ranges are not consecutive. It starts as follows:

0x108'0000 - 0x9D1'F000
0x4B02'0000 - 0x6B02'0000
0x1'0000'0000 - 0x1'2000'0000
0x1'4000'0000 - 0x1'6000'0000

It settles into this last pattern, increasing the addresses by 0x1'4000'0000 at each step.

A machine with 32GB of memory displays similar problems. For instance, MemTest86 says the maximum system address is 0x8'B180'0000, when it should be 0x8'0000'0000-1. Most tests settle on the address range 0x1'0000'0000 - 0x8'B180'0000.

The 64GB machine has 4 x 16GB DIMMs with ECC, and the 32GB machine has 2 x 16GB DIMMs without ECC.

I'm sure MemTest86 is running the tests correctly, just not displaying the addresses properly.

[David (PassMark)]

This is all about the memory map.

And on some machines it looks pretty messed up.

BIOS and other hardware in the systems reserve memory addresses for their own use (video cards, network cards and dozens of other things). Making the free RAM available for testing fragmented, sometimes into tiny blocks. Some addresses are also used for memory mapped I/O.

MemTest86 can display the map from the main config menu, It looks like this (but is different for each machine).




So sometimes these big holes mean that the largest memory address has a number that appears bigger than the amount of physical RAM installed.

But from time to time we do see machines with errors in the memory map (e.g. free RAM and some hardware using the same address caused by a BIOS bug). This can result in system instability.