I saw rasm2 giving different output each time for x64 Take a look at this:
C:\WINDOWS\system32>rasm2 -a x86 -b 64 -d "41f6c7FF"
test r15b, 0xff
C:\WINDOWS\system32>rasm2 -a x86 -b 64 "test r15b, 0xff"
f7ffff000000
C:\WINDOWS\system32>rasm2 -v
rasm2 3.5.1 1 @ windows-x86-64 git.3.5.1
commit: 4ec482ba2d4f554beeafb3aa47758e970bcab937 build: Wed 05/15/2019__ 7:58:44.85
Is this some unresolved bug or did I do something wrong?