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# hv_vcpu_set_sys_reg(_:_:_:)

Sets the value of a vCPU system register.

```
func hv_vcpu_set_sys_reg(_ vcpu: hv_vcpu_t, _ reg: hv_sys_reg_t, _ value: UInt64) -> hv_return_t
```

## Parameters

`vcpu`

The instance of the vCPU.

`reg`

The ID of the system register.

`value`

The new value of the register.

## Return Value

[`HV_SUCCESS`](/documentation/Hypervisor/HV_SUCCESS) if the operation was successful, otherwise an error code specified in [`hv_return_t`](/documentation/Hypervisor/hv_return_t).

## Discussion

> Important:
> This function must be called by the owning thread.

---

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