Editing IBM System/370 (section)

===Dual address space===
In 1981, IBM added the dual-address-space facility to System/370.<ref name="dual-address-space-share-paper" />  This allows a program to have two address spaces; Control Register 1 contains the segment table origin (STO) for the primary address space and CR7 contains the STO for the secondary address space.  The processor can run in primary-space mode or secondary-space mode.  When in primary-space mode, instructions and data are fetched from the primary address space. When in secondary-space mode, operands whose addresses defined to be logical are fetched from the secondary address space; it is unpredictable whether instructions will be fetched from the  primary or secondary address space, so code must be mapped into both address spaces in the same address ranges in both address spaces.  The program can switch between primary-space and secondary-space mode with the SET ADDRESS SPACE CONTROL instruction; there are also MOVE TO PRIMARY and MOVE TO SECONDARY instructions that copy a range of bytes from an address range in one address space to an address range in the other address space.<ref name="GA22-7000-7">{{cite book |url=http://bitsavers.trailing-edge.com/pdf/ibm/370/princOps/GA22-7000-7_IBM_System_370_Principles_of_Operation_8th_ed_198103.pdf |title=IBM System/370 Principles of Operation |edition=Eighth |date=September 1981 |id=GA22-7000-7 |publisher=[[IBM]] |page=3{{hyp}}11-3{{hyp}}6,5{{hyp}}11-5{{hyp}}29}}</ref>

Address spaces are identified by an address-space number (ASN).  The ASN contains indices into a two-level table, structured similarly to a two-level page table, with entries containing a presence bit, various fields indicating permissions granted for access to the address space, the starting address and length of the segment table for the address space, and other information.  The SET SECONDARY ASN instruction makes the address space identified by a given ASN value the current secondary address space.<ref name="GA22-7000-7" />