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From someone:

Some information about the Atom D510 After poweron/reset the cpus are not connected to jtag, there is only a device (i call it GMCHTapController) that can be used for standard jtag functions and must be configured to connect the cpus to the jtag chain: tdi->GMCHTapController->tdo GMCHTapController instruction register length is 6 bits, idcode instruction is 0x3 

To connect the cpus, GMCHTapController instruction 0xa must be written into the instruction register, after this the cpus are connected to the jtag chain (make sure the treset pin is at logic level high!): tdi->GMCHTapController->CPU1->CPU0->tdo 

Cpus instruction register length is 7 bits, idcode instruction is 0x2 

For example to read the idcode of the first cpu write 20 bits 0xfff82 into the instruction register, read 34 bits from the data register and consider the lower 32