Sorgelig wrote:I don't see how random per address values can be worse than patterns of fixed values.
i dont think its worse, but the fixed patterns are specific algorithems designed to produce errors by flipping bits next to each other to reproduce the scenerios that cause faulty memory to fail hard.
having said that, i dont think its a hard requirement for MiSTer to go overboard on memtest, very much a case of diminishing returns ..
here's a description of a popular software's two main patterns
MemTest86 uses two algorithms that provide a reasonable approximation of the ideal test strategy above. The first of these strategies is called moving inversions. The moving inversion test works as follows:
Fill memory with a pattern
Starting at the lowest address
check that the pattern has not changed
write the patterns complement
increment the address
repeat
Starting at the highest address
check that the pattern has not changed
write the patterns complement
decrement the address
repeat
This algorithm is a good approximation of an ideal memory test but there are some limitations. Most high density chips today store data 4 to 16 bits wide. With chips that are more than one bit wide it is impossible to selectively read or write just one bit. This means that we cannot guarantee that all adjacent cells have been tested for interaction. In this case the best we can do is to use some patterns to insure that all adjacent cells have at least been written with all possible one and zero combinations.
It can also be seen that caching, buffering and out of order execution will interfere with the moving inversions algorithm and make less effective. It is possible to turn off cache but the memory buffering in new high performance chips can not be disabled. To address this limitation a new algorithm I call Modulo-X was created. This algorithm is not affected by cache or buffering. The algorithm works as follows:
For starting offsets of 0 - 20 do
write every 20th location with a pattern
write all other locations with the patterns complement
repeat above one or more times
check every 20th location for the pattern
This algorithm accomplishes nearly the same level of adjacency testing as moving inversions but is not affected by caching or buffering. Since separate write passes (1a, 1b) and the read pass (1c) are done for all of memory we can be assured that all of the buffers and cache have been flushed between passes. The selection of 20 as the stride size was somewhat arbitrary. Larger strides may be more effective but would take longer to execute. The choice of 20 seemed to be a reasonable compromise between speed and thoroughness.
what else can i do aye )
So it looks like you was correct about it being a ram issue. i just got my replacement one and it's now all working perfectly!! Thanks for the advice. Its just a bit odd that it was showing perfectly fine on the mem test.
Thanks for the reminder.