Not sure what you’re trying to do, but the registry setting you mention will just set bit 12 of reg 0x3a to 0
(upper 16 bits is the mask for bits to set, and lower 16 bits is the value to be set). So you will see no difference from the reset value of bit 12 which is already 0.
maybe you wanted to set bit 12 to 1?
Then you should write:
Reg0x3a = dword:10001000