The meaning of special ALETs 0 and 1 differs depending on whether the ALET is supplied for access register translation or specified on the BSG instruction.

A program running in AR ASC mode in a subspace or with full address space addressability can use special ALETs 0, 1, and 2 to access its primary, secondary, and home address spaces. This concept is described in detail in Special ALET values.

Likewise, a program running in a subspace or with full address space addressability can supply these ALETs on an ALESERV EXTRACT request to obtain the STOKEN for the address space they require. To obtain the STOKEN for the full address space, invoke ALESERV with the EXTRACTH parameter.

However, when specified on a BSG instruction, ALETs 0 and 1 have different meanings. Specifying BSG with ALET 0 causes a program to run with full address space addressability. Specifying BSG with ALET 1 causes a program to run in the subspace that most recently had control. If a program is running in a subspace when it issues BSG with ALET 1, the program will continue to run in that subspace.

If a program has never run in a subspace and it attempts to return to a subspace by issuing the BSG instruction with ALET 1, the program will abnormally end with system completion code X'0D3'.