Test register
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Test register is a register used by processor, usually to do self-test. Most of these registers are undocumented, and used by specialised software. These registers are marked TR4 to TR7 (in x86 series) Usually, programs don't require these registers in work.
Two test registers, TR6 and TR7, are provided for the purpose of testing. TR6 is the test command register, and TR7 is the test data register. These registers are accessed by variants of the MOV instruction. A test register may be either the source operand or 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.
