PURPOSE: To access two virtual address spaces even when an STO stack is disconnected, by allocating ID for STO disconnection forcibly in accordance with contents of head addresses of a segment table set to two control registers when the STO stack is disconnected due to a fault or the like.

CONSTITUTION: When an STO stack 4 is disconnected and a disable signal goes to logic "1", disconnection ID allocating circuits 9 and 10 send disconnection ID, which is different from ID set to the first and the second ID registers 7 and 8, to an out-pointer 11. This disconnection ID is controlled in accordance with the output of a comparing circuit 1. Disconnection ID used for disconnection of the STO stack 4 is not registered in a table TBL. Consequently, two spaces beginning with head addresses of the segment table set to control registers CR1 and CR2 are accessed without hindrance.