Because the DRNG (Digital RNG) on Intel processors filter the data through a DRNG (Deterministic RNG) (i.e. NIST 800-90)[1] which is constantly reseeded. The r

Because the DRNG (Digital RNG) on Intel processors filter the data through a DRNG (Deterministic RNG) (i.e. NIST 800-90)[1] which is constantly reseeded. The rdseed instruction was added later to allow for accessing the hardware generator directly (or the bits from the AES CBC-MAC conditioner directly). I don't know why rdseed was added a generation after rdrand. Whatever the case is you can guarantee that rdrand's internal generator gets reseeded by calling it a certain number of times[2]. You can also wait some amount of time between calls but it's likely faster just to generate more data to force the reseed. [1] Like Windows it uses AES-256 (AFAIK) in CTR mode as specified in NIST 800-90 or whatever the latest version of that standard is. [2] the DRNG reseeds automatically after generating 511 128-bit samples (from Intel's drng_samples source package)