To perform the emulation of privileged instructions, Disco additionally maintains the privileged registers and TLB contents of the virtual CPU in this structure.
Supervisor mode allows the operating system to use a protected portion of the address space (the supervisor segment), but does not give access to privileged instructions or physical memory.
To reduce this overhead, we patched the HAL of IRIX to convert these frequently used privileged instructions to use nontrapping load and store instructions to a special page of the address space that contains these registers.