Supervisor-Call Interruption
This interruption occurs as a result of execution of the
instructionSUPERVISOR CALL. Eight bits from the in
struction format are placed in the interruption code
of the old psw, permitting a message to be associated
with the interruptions. A major use for the instructionSUPERVISOR CALL is to switch from the problem-state
to the supervisor state. This interruption may also be
used for other modes of status-switching.
External Interruption
The external interruption provides the means by
which theCPU responds to signals from the interrup
tion key on the system control panel, the timer, and
the external signals of the direct control feature
(Figure 17).Interruption External
Code BitI nterrupti on Cause Mask Bit
24 Timer
25Interrupt key 7
26 External signal 6 7
27 External signal 5 7
28 External signal 4 7
29 External signal 3 730 External signal 2 7 3'1 External signal 1 7
Figure 17. Interruption Code for External Interruption
An external interruption can occur only when the
system mask bit 7 is one.
The source of the interruption is identified by the
interruption code in bits 24-31 of the psw. Bits 16-23
of the interruption code are made zero.
Machine-Check Interruption
The occurence of a machine check (if not masked
off) terminates the current instruction, initiates a diag
nostic procedure, and subsequently causes the ma
chine-check interruption. A machine check cannot be
caused by invalid data or instructions. The diagnostic
scan is performed into the scan area starting at lo
cation 128.Proper execution of these steps depends
on the nature of the machine check.
Priority of Interruptions
During execution of an instruction, several interrup
tion requests may occur simultaneously. Simultaneous
interruption requests are honored in the following pre
determined order:
Machine CheckProgram or Supervisor Call
ExternalInput/Output The program and supervisor-call interruptions are
mutually exclusive and cannot occur at the same time.
When more than one interruption cause requests
service, the action consists of storing the old psw and
fetching the new psw belonging to the interruption
which is taken first. This new psw subsequently is
stored without any instruction execution and the next
interruption psw is fetched. This process continues
until no more interruptions are to be serviced. When
the last interruption request has been serviced, in
struction execution is resumed using the psw last
fetched. The order of execution of the interruption
subroutines is, therefore, the reverse of the order in
which the psw's are fetched.
Thus, the most important interruptions -I/O, ex
ternal, program or supervisor call -are actually serv
iced first. Machine check, when it occurs, does not al
low any other interruptions to be taken.
Program StatesOver-all CPU status is determined by four types of pro
gram-state alternatives, each of which can be changed
independently to its opposite and most of which are
indicated by a bit or bits in the psw. The program
state alternatives are named stopped or operating,
running or waiting, masked or interruptable, and sup
ervisor or problem state. These states differ in the way
they affect theCPU functions and the manner in which
their status is indicated and switched. All program
states are independent of each other in their functions,
indication, and status-switching.Stopped or Operating States: The stopped state is
entered and left by manual procedure. Instructions are
not executed, interruptions are not accepted, and the
timer is not updated. In the operating state, theCPU is capable of executing instructions and being inter
rupted.
Running or WaitingState: In the running state, in
struction fetching execution proceeds in the normal
manner. The wait state is normally entered by the
program to await an interruption, for example, anI/O interruption or operator intervention from the console.
In the wait state, no instructions are processed, the
timer is updated, and110 and external interruptions
are accepted, unless masked. Running or waiting state
is determined by the setting of bit 14 in the psw.Masked or Interruptable State: The CPU may be in
terruptable or masked forthe system, program, and
machine interruptions. \Vhen theCPU is interruptable
for a class of interruptions, these interruptions are ac
cepted. When theCPU is masked, the system inter
ruptions remain pending, while the program and ma
chine-check interruptions are ignored.The interrupt
able states of theCPU are changed by changing the
mask bits of the psw.
System Structure 17
This interruption occurs as a result of execution of the
instruction
struction format are placed in the interruption code
of the old psw, permitting a message to be associated
with the interruptions. A major use for the instruction
to the supervisor state. This interruption may also be
used for other modes of status-switching.
External Interruption
The external interruption provides the means by
which the
tion key on the system control panel, the timer, and
the external signals of the direct control feature
(Figure 17).
Code Bit
24 Timer
25
26 External signal 6 7
27 External signal 5 7
28 External signal 4 7
29 External signal 3 7
Figure 17. Interruption Code for External Interruption
An external interruption can occur only when the
system mask bit 7 is one.
The source of the interruption is identified by the
interruption code in bits 24-31 of the psw. Bits 16-23
of the interruption code are made zero.
Machine-Check Interruption
The occurence of a machine check (if not masked
off) terminates the current instruction, initiates a diag
nostic procedure, and subsequently causes the ma
chine-check interruption. A machine check cannot be
caused by invalid data or instructions. The diagnostic
scan is performed into the scan area starting at lo
cation 128.
on the nature of the machine check.
Priority of Interruptions
During execution of an instruction, several interrup
tion requests may occur simultaneously. Simultaneous
interruption requests are honored in the following pre
determined order:
Machine Check
External
mutually exclusive and cannot occur at the same time.
When more than one interruption cause requests
service, the action consists of storing the old psw and
fetching the new psw belonging to the interruption
which is taken first. This new psw subsequently is
stored without any instruction execution and the next
interruption psw is fetched. This process continues
until no more interruptions are to be serviced. When
the last interruption request has been serviced, in
struction execution is resumed using the psw last
fetched. The order of execution of the interruption
subroutines is, therefore, the reverse of the order in
which the psw's are fetched.
Thus, the most important interruptions -
ternal, program or supervisor call -are actually serv
iced first. Machine check, when it occurs, does not al
low any other interruptions to be taken.
Program States
gram-state alternatives, each of which can be changed
independently to its opposite and most of which are
indicated by a bit or bits in the psw. The program
state alternatives are named stopped or operating,
running or waiting, masked or interruptable, and sup
ervisor or problem state. These states differ in the way
they affect the
their status is indicated and switched. All program
states are independent of each other in their functions,
indication, and status-switching.
entered and left by manual procedure. Instructions are
not executed, interruptions are not accepted, and the
timer is not updated. In the operating state, the
rupted.
Running or Waiting
struction fetching execution proceeds in the normal
manner. The wait state is normally entered by the
program to await an interruption, for example, an
In the wait state, no instructions are processed, the
timer is updated, and
are accepted, unless masked. Running or waiting state
is determined by the setting of bit 14 in the psw.
terruptable or masked for
machine interruptions. \Vhen the
for a class of interruptions, these interruptions are ac
cepted. When the
ruptions remain pending, while the program and ma
chine-check interruptions are ignored.
able states of the
mask bits of the psw.
System Structure 17