A data processing system includes a host processor and a graphics processing unit operable to process data under the control of an operating system executing on the host processor. The graphics processing unit can be switched between a normal mode of operation in which the it has read and write access to data that is stored in non-protected memory regions 9 but no or write-only access to any protected memory regions 8, and a protected mode of operation in which it has read and write access to data that is stored in protected memory regions 8 but only has read-only access to any non-protected memory regions 9. The data processing system further comprises a mechanism for switching the graphics processing unit from its normal mode of operation to the protected mode of operation, and from its protected mode of operation to the normal mode of operation.