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The following terms are used within this section. For definitions
of other terms used in this document, see the glossary. - Address space control (ASC) mode: The mode (determined
by the PSW) that tells the system where to find the data it is to
reference. Two ASC modes are AR and primary. For each ASC mode, the
following table defines:
- The address space from which the system fetches instructions
- The address space or data space that the system accesses when
an instruction, other than an MVCP or MVCS instruction, references
data
- The address space the system accesses when an MVCP or MVCS instruction
references data
- AR ASC mode: The ASC mode in which the system uses both
the GPR (used as the base register) and the corresponding AR to resolve
an address in an address/data space.
ASC Mode |
Instruction Fetch |
Data Access |
MVCP/MVCS |
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Primary ASC mode |
Primary address space |
Primary address space |
Primary or secondary address space |
Secondary ASC mode |
Primary address space (see Note) |
Secondary address space |
Primary or secondary address space |
AR ASC mode |
Primary address space |
Address space indicated by the AR |
Unavailable. Causes an abend in AR ASC mode. |
Note: Prior to ESA/370 architecture,
the address space from which instructions are fetched is unpredictable
when a program is running in secondary ASC mode.
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- Basic PC: Transfers control to another program, the PC
routine. The basic PC requires the service provider to save and restore
the user's environment. The PC routine can be in the same address
space as the program that issues the PC instruction, or in a different
address space.
- Cross memory local (CML) lock: The LOCAL lock of an address
space other than the home address space.
- Cross memory mode: Cross memory mode exists when at least
one of the following conditions are true:
- The primary address space (PASN) and the home address space (HASN)
are different address spaces.
- The secondary address space (SASN) and the home address space
(HASN) are different address spaces.
- The ASC mode is secondary.
- Home address space: The address space in which MVS™ initially dispatches a TCB or SRB (work unit).
In the case of a TCB, the home address space contains the TCB. PSAAOLD
points to the home address space. When MVS initially
dispatches a work unit, the home address space, the primary address
space, and the secondary address space are all the same. During execution
of the work unit, the home address space remains the same. The primary
and secondary address spaces may be changed, however, through the
PC, PR, PT, or SSAR instructions.
- LX reuse facility: The LX reuse facility is available with z/OS® V1R6 to provide additional
LXs and improve reusability of LXs. It is enabled when running on
a z890 or z990 processor at driver level 55 or above, with APAR OA07708
installed. When the facility is enabled bit CVTALR in byte CVTFLAG2
of the CVT data area is 1.
- Primary address space: The address space whose segment
table is used to fetch instructions in primary, secondary, and AR
ASC modes. A program in primary mode fetches data from the primary
address space.
- Primary ASC mode: The ASC mode in which the system uses
the GPRs, but not the ARs, to resolve an address in an address space.
In primary ASC mode, the system fetches instructions and data from
the primary address space.
- PC number: A number that identifies a PC routine. The service
provider creates the number, by using MVS services,
and supplies it to the user. The user specifies the number in a PC
instruction to identify the PC routine that is to be invoked.
- PC routine: A program that receives control as the result
of a PC instruction's executing and performs a service for the caller.
- Secondary address space: The address space whose segment
table the system uses to access data in secondary ASC mode.
- Secondary ASC mode: The ASC mode in which the system fetches
instructions from the primary address space and data from the secondary
address space.
- Space switch routine: A program that issues a PC instruction
that causes the primary address space to change.
- Stacking PC: Transfers control to another program, the
PC routine. The stacking PC uses the linkage stack for storing the
caller's status. It provides more options and more automatic function
than the basic PC instruction. The PC routine can be in the same address
space as the program that issues the PC instruction, or a different
address space. IBM® recommends using
the stacking PC instead of the basic PC.
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