The instruction is encoded in two ways, depending on the flow of data. Moving data from a general purpose register into a test register is encoded as 0F 26 /r (with r/m being the GPR, and reg being the test register).
The encoding of this instruction varies based on data movement. To transfer data from a general-purpose register into a test register, use the code 0F 26 /r, where r/m denotes the source register and reg denotes the destination register.