the same address space or an address
space different from that of the caller.
In general, it ;s used to transfer
control to a program of higher
authority. PROGRAM TRANSFER permits a change of the instruction address and
address space. PROGRAM TRANSFER also
permits a reduction in PSW-key-mask
authority and a change from the supervi­
sor to the problem state. In general,
it is used to transfer control from one
program to another of equal or lower
authority. PROGRAM TRANSFER can be used
to return from a program called by
PROGRAM CALL.
The operation of PROGRAM CALL is
controlled by means of an entry-table
entry, which is located as part of a
table-lookup process during the
execution of the instruction. The
instruction causes the primary address space to be changed only when the ASN in
the entry-table entry ;s nonzero. When
the primary address space;s changed,
the operation ;s called PROGRAM CALL
with space switching (PC-ss). When the
primary address space is not changed,
the operation is called PROGRAM CALL to
current primary (PC-cp).
PROGRAM TRANSFER specifies the address
space which is to become the new primary
address space. When the primary address
space is changed, the operation is called PROGRAM TRANSFER with space switching (PT-ss). When the primary
address space is not changed, the opera­
tion is called PROGRAM TRANSFER to
current primary (PT-cp).
The linkage instructions provided and
the functions performed by each are
summarized ;n the figure "Linkage­
Instruction Summary."
Chapter 5. Program Execution 5-7

Instruction Problem PASN Address State CR4 PSW-Key Bi ts 40-63 PSW Bit 15 Bits 16-31 Mask
Changed
Instruction Format Save Set Save Set Save Set in CR3
BALR* RR Yes R2
1
- - - - - BAL* RX Yes Yes - - - - -
BASR RR Yes R2
1
- - - - -
BAS RX Yes Yes - - - - - MC"2 SI Yes Yes Yes Yes - - - PC-cp S Yes Yes Yes Yes - - "OR" EKM PC-ss S Yes Yes Yes Yes Yes Yes "OR" EKM PT-cp RRE -R2 - R2** - - "AND" R j PT-ss RRE -R2 - R2** - Yes "AND" R t SVC2 RR Yes Yes Yes Yes - - -
EXElanation:
- No * The instruction-length code, condition code, and program mask are
also saved. ** A change from the supervisor to the problem state is allowed; a privileged operation exception is recognized when a change from
the problem to the supervisor state is specified.
Monitor-mask bits provide a means of disallowing linkage, or en­
abling linkage, for selected classes of events.
The action takes place only if the R2 field in the instruction is
nonzero. 2 MC and SVC, as part of the interruption, save the entire current PSW and load a new PSW. Linkage-Instruction Summary INTERRUPTIONS Interruptions permit
state as a result of
to the system,
input/output (I/O) CPUs, or in the CPU to be found
"Interruptions."
the CPU to change
conditions external
in channels or
devices, in other
itself. Details are
in Chapter 6,
Six classes of interruption conditions
are pro v i ded: external, I/O, machi ne
check, program, restart, and supervisor
call. Each class has two related PSWs, called old and new, in permanently
assigned real storage locations. In all
classes, an interruption involves stor­
ing information identifying the cause of
the interruption, storing the current PSW at the old-PSW location, and fetch-
5-8 System/370 Principles of Operation ing the PSW at the new-PSW location,
which becomes the current PSW. The old PSW contains CPU-status informa­
tion necessary for resumption of the
interrupted program. At the conclusion
of the program invoked by the inter­
ruption, the instruction LOAD PSW may be
used to restore the current PSW to the
value of the old PSW. TYPES OF INSTRUCTION ENDING
Instruction
five ways:
suppression,
completion.
execution ends in one of
completion, nullification,
termination, and partial