To Concatenate or Not Concatenate RDRAND?

At the heart of Intel® Data Protection with Secure Key is the digital random number generator (DRNG), a NIST* SP800-90A compliant pseudorandom number generator which is accessed using the RDRAND instruction. Beginning with Intel CPU's code-named Broadwell, Secure Key will also include an SP800-90B and C compliant true random number generator, called an enhanced nondeterministic random number generator in the NIST specifications, that will be accessible via the RDSEED instruction.

RDRAND: Do I need to check the carry flag, or can I just check for zero?

One question I have been getting a lot lately is whether you have to check the status of the carry flag to see if a valid random number was returned by RDRAND. The reason why this question gets asked is because of this description of a RDRAND underflow condition, which appears in the DRNG Software Implementation Guide:

Intel® Trusted Execution Technology

Intel® Trusted Execution Technology (Intel® TXT) provides a hardware- based root of trust to ensure that a platform boots with a known good configuration of firmware, BIOS, virtual machine monitor, and operating system. For more information, please refer to

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