Test register
A test register, in the Intel 80386 and Intel 80486 processor, was a register used by the processor, usually to do a self-test. Most of these registers were undocumented, and used by specialized software. The test registers were named TR3 to TR7. Regular programs don't usually require these registers to work. With the Pentium, the test registers were replaced by a variety of model-specific registers (MSRs). Two test registers, TR6 and TR7, were provided for the purpose of testing. TR6 was the test command register, and TR7 was the test data register. These registers were  accessed by variants of the MOV instruction. A test register may either be the source operand or the destination operand. The MOV instructions are defined in both real-address mode and protected mode. The test registers are privileged resources. In protected mode, the MOV instructions that access them can only be executed at privilege level 0. An attempt to read or write the test registers when executing at any other privilege level causes a general protection exception. Also, those instructions generate invalid opcode exception on any CPU newer than 80486. 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 codice_1 (with codice_2 being the GPR, and codice_3 being the test register). Moving data the other way (i.e. from the test register into a general purpose register) is encoded as codice_4 (with codice_2 being the GPR, and codice_3 being the test register). Only register-register moves are allowed; memory forms of the ModR/M byte are undefined. In other words, the codice_7 field (the two MSBs) must be set to codice_8.