# hv_vcpu_set_sme_p_reg(_:_:_:_:) Sets the value of a vCPU P predicate register in streaming Scalable Vector Extension (SVE) mode. ``` func hv_vcpu_set_sme_p_reg(_ vcpu: hv_vcpu_t, _ reg: hv_sme_p_reg_t, _ value: UnsafePointer, _ length: Int) -> hv_return_t ``` ## Parameters `vcpu` The ID of the vCPU instance. `reg` The ID of the P predicate register. `value` The pointer to the register value to set. `length` The length of the P register value., in bytes. ## Return Value `HV_SUCCESS` on success, an error code otherwise. ## Discussion > Important: > You need to call this on the owning thread This method returns an error if `PSTATE.ZA` is `0` (For example, if.[`za_storage_enabled`](/documentation/Hypervisor/hv_vcpu_sme_state_t/za_storage_enabled) is `false`), or if the provided value storage isn’t [maximum SVL bytes x maximum SVL bytes]. This method doesn’t require streaming SVE mode to be in an enabled state. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)