When using the pshufb opcode I cannot get it to work with mem as the source. Works ok when xmm* used. Any thoughts.

IntermittentP
The mask label should be made typeless (e.g. by putting a colon after its name), or the memory access type should be enforced with the dqword directive in the instruction.

Thanks I_inc
I've tried the your suggestions but to no avail, trying dqword on the source, dest (and even both). Another opcode, psignb, which like pshufb, can also operate on 64 bits also fails. movdqu which is only 128 bits works ok. I'm using 1.71.39.

IntermittentP
Please provide your code. Preferably a minimal example with the problem.

Using psignb produces ... 66 0F 38 08 05 08 20 40 00

66 0F 38 08 seems correct

05 must be the xxm register. I used xxm0 but have been unable to decode the 05

08 20 40 00 is the correct address for the mask.

This data is produced by listing.exe from the .fas file.

Apologies. Of course in this instance the 05 is the dest.

Will provide code soonest. Am summoned atm!

IntermittentP

05 is the ModR/M byte, which specifies both the source and the destination. 05 stands for an operation with a register AL/AX/EAX/MM0/XMM0 and memory addressed by a displacement that follows.

Thanks for that info I_inc, I'll try to find the info in the docs.
I minimised the code to present it here by bulk deletion of lines. As I was only experimenting I didn't store the original (Flatasm resaved on run). When the code was minimised the line ran with no problems which is a "good thing" but of course it would have been better to have found the problem. I'll play around with it and come back if I find anything interesting!

Thanks again I_inc.

IntermittentP: If you show the source code we can probably help very easily. Please reduce it to just the lines of interest.