Test register

Test register

Test register was a register used by the processor, usually to do a self-test. Most of these registers were undocumented, and used by specialized software. In the x86 series, these registers were marked TR4 to TR7. Usually, regular programs don't require these registers to work. In the newer CPU sets, the test registers are removed.

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.

ee also

*Control register
*Debug register

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